Protist locomotion

File:Cladogram for some unicellular eukaryotes.webp.jpg

Unicellular protists comprise a vast, diverse group of organisms that covers virtually all environments and habitats, displaying a menagerie of shapes and forms. Hundreds of species of the ciliate genus Paramecium{{hsp}}{{Cite book|url=https://books.google.com/books?id=JB_pBwAAQBAJ&pg=PA1|title = The Biology of Paramecium|isbn = 9781475703726|last1 = Wichterman|first1 = R.|date = 6 December 2012| publisher=Springer }} or flagellated Euglena{{hsp}}{{cite book |doi = 10.1002/9780470015902.a0001964.pub3|chapter = Euglena|title = eLS|year = 2011|last1 = Buetow|first1 = Dennis E.|isbn = 978-0470016176}} are found in marine, brackish, and freshwater reservoirs; the green algae Chlamydomonas is distributed in soil and fresh water world-wide;{{Cite book|url=https://books.google.com/books?id=xTjJGV5GWY0C&pg=PP1|title = The Chlamydomonas Sourcebook: Introduction to Chlamydomonas and Its Laboratory Use: Volume 1|isbn = 97-80080919553|last1 = Harris|first1 = Elizabeth H.|date = 7 March 2009| publisher=Academic Press }} parasites from the genus Giardia colonize intestines of several vertebrate species.{{cite journal |doi = 10.1128/CMR.14.3.447-475.2001|title = Biology of Giardia lamblia|year = 2001|last1 = Adam|first1 = Rodney D.|journal = Clinical Microbiology Reviews|volume = 14|issue = 3|pages = 447–475|pmid = 11432808|pmc = 88984}} One of the shared features of these organisms is their motility, crucial for nutrient acquisition and avoidance of danger.{{Cite book|url=https://books.google.com/books?id=1HBTDwAAQBAJ&pg=PP1|title = Cell Movements: From Molecules to Motility|isbn = 9781136844355|last1 = Bray|first1 = Dennis|date = 2 November 2000| publisher=Garland Science }} In the process of evolution, single-celled organisms have developed in a variety of directions, and thus their rich morphology results in a large spectrum of swimming modes.{{cite book |doi = 10.1007/978-94-009-5812-8_4|chapter = The movement of eukaryotic cells|title = Motility of Living Cells|year = 1980|last1 = Cappuccinelli|first1 = P.|pages = 59–74|isbn = 978-0-412-15770-7}}{{cite journal |doi = 10.7554/eLife.44907|title = Swimming eukaryotic microorganisms exhibit a universal speed distribution|year = 2019|last1 = Lisicki|first1 = Maciej|last2 = Velho Rodrigues|first2 = Marcos F.|last3 = Goldstein|first3 = Raymond E.|last4 = Lauga|first4 = Eric|journal = eLife|volume = 8|pmid = 31310238|pmc = 6634970|arxiv = 1907.00906 | doi-access=free }} 50px Material was copied from this source, which is available under a [https://creativecommons.org/licenses/by/4.0/ Creative Commons Attribution 4.0 International License].

Many swimming protists actuate tail-like appendages called flagella or cilia in order to generate the required thrust.Sleigh, M. A. (1974) Cilia and flagella. Academic Press. This is achieved by actively generating deformations along the flagellum, giving rise to a complex waveform. The flagellar axoneme itself is a bundle of nine pairs of microtubule doublets surrounding two central microtubules, termed the 9+2 axoneme,{{cite journal |doi = 10.1073/pnas.0508274102|title = 3D structure of eukaryotic flagella in a quiescent state revealed by cryo-electron tomography|year = 2005|last1 = Nicastro|first1 = D.|last2 = McIntosh|first2 = J. R.|last3 = Baumeister|first3 = W.|journal = Proceedings of the National Academy of Sciences|volume = 102|issue = 44|pages = 15889–15894|pmid = 16246999|pmc = 1276108|bibcode = 2005PNAS..10215889N|doi-access = free}} and cross-linking dynein motors, powered by ATP hydrolysis, perform mechanical work by promoting the relative sliding of filaments, resulting in bending deformations.

Although protist flagella have a diversity of forms and functions,{{cite journal |last1=Moran |first1=Jonathan |last2=McKean |first2=Paul G. |last3=Ginger |first3=Michael L. |title=Eukaryotic Flagella: Variations in Form, Function, and Composition during Evolution |journal=BioScience |volume=64 |issue=12 |year=2014 |pages=1103–1114 |issn=1525-3244 |doi=10.1093/biosci/biu175|doi-access=free }} two large families, flagellates and ciliates, can be distinguished by the shape and beating pattern of their flagella.

In the phylogenetic tree on the right, aquatic organisms (living in marine, brackish, or freshwater environments) have their branches drawn in blue while parasitic organisms have their branches drawn in red. Ciliates are indicated by an asterisk after their names. For each phylum marked in bold font, a representative organism has been sketched next to its name.

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File:Flagellum-beating.svg

Flagella are used in prokaryotes (archaea and bacteria) as well as protists. In addition, both flagella and cilia are widely used in eukaryotic cells (plant and animal) apart from protists.

The regular beat patterns of eukaryotic cilia and flagella generates motion on a cellular level. Examples range from the propulsion of single cells such as the swimming of spermatozoa to the transport of fluid along a stationary layer of cells such as in a respiratory tract. Though eukaryotic flagella and motile cilia are ultrastructurally identical, the beating pattern of the two organelles can be different. In the case of flagella, the motion is often planar and wave-like, whereas the motile cilia often perform a more complicated three-dimensional motion with a power and recovery stroke.

Eukaryotic flagella—those of animal, plant, and protist cells—are complex cellular projections that lash back and forth. Eukaryotic flagella are classed along with eukaryotic motile cilia as undulipodia[http://www.encyclopedia.com/doc/1O6-undulipodium.html A Dictionary of Biology], 2004, accessed 2011-01-01. to emphasize their distinctive wavy appendage role in cellular function or motility. Primary cilia are immotile, and are not undulipodia.

File:Marine flagellates.jpg, Abollifer, Bodo, Rhynchomonas, Kittoksia, Allas, and Metromonas{{hsp}}Patterson, David J. (2000) [http://tolweb.org/accessory/flagellates?acc_id=50 "Flagellates: Heterotrophic Protists With Flagella"] Tree of Life.}}]]

File:Chlamydomonas (10000x).jpg| Freshwater green algal flagellate (Chlamydomonas)

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Flagellates typically have a small number of long flagella distributed along the bodies, and they actuate them to generate thrust. The set of observed movement sequences includes planar undulatory waves and traveling helical waves, either from the base to the tip, or in the opposite direction.{{cite journal |doi = 10.1146/annurev.fl.04.010172.000521|title = Locomotion of Protozoa|year = 1972|last1 = Jahn|first1 = T. L.|last2 = Votta|first2 = J. J.|journal = Annual Review of Fluid Mechanics|volume = 4|pages = 93–116|bibcode = 1972AnRFM...4...93J}}{{cite journal |doi = 10.1146/annurev.fl.09.010177.002011|title = Fluid Mechanics of Propulsion by Cilia and Flagella|year = 1977|last1 = Brennen|first1 = C.|last2 = Winet|first2 = H.|journal = Annual Review of Fluid Mechanics|volume = 9|pages = 339–398|bibcode = 1977AnRFM...9..339B|url = https://authors.library.caltech.edu/58/}} Flagella attached to the same body might follow different beating patterns, leading to a complex locomotion strategy that often relies also on the resistance the cell body poses to the fluid.

File:Cillia1.png

In contrast to flagellates, propulsion of ciliates derives from the motion of a layer of densely-packed and collectively-moving cilia, which are short hair-like flagella covering their bodies. The seminal review paper of Brennen and Winet (1977) lists a few examples from both groups, highlighting their shape, beat form, geometric characteristics and swimming properties. Cilia may also be used for transport of the surrounding fluid, and their cooperativity can lead to directed flow generation. In higher organisms this can be crucial for internal transport processes, as in cytoplasmic streaming within plant cells,{{cite journal |doi = 10.1146/annurev.bb.07.060178.002433|title = Cytoplasmic Streaming in Green Plants|year = 1978|last1 = Allen|first1 = N. S.|last2 = Allen|first2 = R. D.|journal = Annual Review of Biophysics and Bioengineering|volume = 7|pages = 497–526|pmid = 352247}} or the transport of ova from the ovary to the uterus in female mammals.{{cite journal |doi = 10.1093/humupd/dml012|title = The reproductive significance of human Fallopian tube cilia|year = 2006|last1 = Lyons|first1 = R.A.|last2 = Saridogan|first2 = E.|last3 = Djahanbakhch|first3 = O.|journal = Human Reproduction Update|volume = 12|issue = 4|pages = 363–372|pmid = 16565155|doi-access = free}}

Ciliates generally have hundreds to thousands of cilia that are densely packed together in arrays. Like the flagella, the cilia are powered by specialised molecular motors. An efficient forward stroke is made with a stiffened flagellum, followed by an inefficient backward stroke made with a relaxed flagellum. During movement, an individual cilium deforms as it uses the high-friction power strokes and the low-friction recovery strokes. Since there are multiple cilia packed together on an individual organism, they display collective behaviour in a metachronal rhythm. This means the deformation of one cilium is in phase with the deformation of its neighbor, causing deformation waves that propagate along the surface of the organism. These propagating waves of cilia are what allow the organism to use the cilia in a coordinated manner to move. A typical example of a ciliated microorganism is the Paramecium, a one-celled, ciliated protozoan covered by thousands of cilia. The cilia beating together allow the Paramecium to propel through the water at speeds of 500 micrometers per second.{{cite journal|last=Lauga|first=Eric|author2=Thomas R Powers|title=The hydrodynamics of swimming microorganisms|journal=Reports on Progress in Physics|date=25 August 2009|volume=72|issue=9|pages=096601|doi=10.1088/0034-4885/72/9/096601|bibcode=2009RPPh...72i6601L|arxiv=0812.2887|s2cid=3932471}}

File:Cilia beating in a multi-ciliated microswimmer.webp

File:Инфузория туфелька поедает бактерии!.gif|Paramecium feeding on bacteria

File:Oxytricha trifallax.jpg|The ciliate Oxytricha trifallax with cilia clearly visible

File:Animation of swimming ciliate.webm| Animation of swimming ciliate{{hsp}}

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| video1 = [https://www.youtube.com/watch?v=KLi5bl-gdeQ Paramecium: The white rat of ciliates]

| video2 = [https://www.youtube.com/watch?v=utFxuTrTTeU Flinching saves lives in the microcosmos] – Journey to the Microcosmos

| video3 = [https://www.youtube.com/watch?v=08emOkUtHJI&ab_channel=JourneytotheMicrocosmos Microbes in slow motion] – Journey to the Microcosmos

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{{see also|Amoeboid movement|Pseudopodia}}

The third prevalent forms of protist cell motility is actin-dependent cell migration. The evolution of flagellar-based swimming has been well studied, and strong evidence suggests a single evolutionary origin for the eukaryotic flagellum occurred before the diversification of modern eukaryotes. On the other hand, actin-dependent crawling uses many different molecular mechanisms, and the study of how these evolved is only just beginning.{{cite journal |doi = 10.1016/j.cub.2020.03.026|title = The evolution of animal cell motility|year = 2020|last1 = Fritz-Laylin|first1 = Lillian K.|journal = Current Biology|volume = 30|issue = 10|pages = R477–R482|pmid = 32428485|s2cid = 218711237|doi-access = free| bibcode=2020CBio...30.R477F }}

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File:Geometry and locomotion of Gonium pectorale 2.png{{hsp}}{{cite journal |doi = 10.1103/PhysRevE.101.022416|title = Motility and phototaxis of Gonium, the simplest differentiated colonial alga|year = 2020|last1 = De Maleprade|first1 = Hélène|last2 = Moisy|first2 = Frédéric|last3 = Ishikawa|first3 = Takuji|last4 = Goldstein|first4 = Raymond E.|journal = Physical Review E|volume = 101|issue = 2|page = 022416|pmid = 32168596|arxiv = 1911.08837|bibcode = 2020PhRvE.101b2416D|s2cid = 211858528}} 50px Material was copied from this source, which is available under a [https://creativecommons.org/licenses/by/4.0/ Creative Commons Attribution 4.0 International License].}} (a) Sixteen-cell colony. Each cell has two flagella, 30–40 μm long. Scale bar is 10 μm.
(b) Schematic of a colony of radius a: sixteen cells (green) each with one eye spot (orange dot). The cis flagellum is closest to the eye spot, the trans flagellum is furthest.{{cite journal |doi = 10.1111/j.1529-8817.2012.01168.x|title = A Comparative Analysis of the Volvocaceae (Chlorophyta)1|year = 2012|last1 = Coleman|first1 = A. W.|journal = Journal of Phycology|volume = 48|issue = 3|pages = 491–513|pmid = 27011065| bibcode=2012JPcgy..48..491C |s2cid = 422091}} Flagella of the central cells beat in an opposing breaststroke, while the peripheral flagella beat in parallel. The pinwheel organization of the peripheral flagella leads to a left-handed body rotation at a rate ω3.]]

Gonium is a genus of colonial algae belonging to the family Volvocaceae. Typical colonies have 4 to 16 cells, all the same size, arranged in a flat plate, with no anterior-posterior differentiation. In a colony of 16 cells, four are in the center, and the other 12 are on the four sides, three each.{{cite book | last = Pennak | first = Robert W | title = Fresh-Water Invertebrates of the United States | publisher = John Wiley & Sons | edition = Second | pages = [https://archive.org/details/freshwaterinvert0000penn/page/43 43] | year = 1978 | isbn = 0-471-04249-8 | url = https://archive.org/details/freshwaterinvert0000penn/page/43 }}

Since the work of August Weismann on germ-plasm theory in biology{{hsp}}{{Cite web|url=https://books.google.com/books?id=R8XRAAAAMAAJ&q=Essays+on+Heredity+and+Kindred+Biological+Problems|title=Essays Upon Heredity and Kindred Biological Problems: By Dr. August Weismann ... Ed. By Edward B. Poulton ... Selmar Schönland ... And Arthur e. Shipley...Authorised Translation|last1=Weismann|first1=August|year=1889}} and of Julian Huxley on the nature of the individual in evolutionary theory,{{Cite book|url=https://books.google.com/books?id=d_F-dmJe0igC&q=The+Individual+in+the+Animal+Kingdom|title=The Individual in the Animal Kingdom|isbn=9781107606074|last1=Huxley|first1=Julian S.|date=22 March 2012|publisher=Cambridge University Press }} the various species of green algae belonging to the family Volvocaceae have been recognized as important ones in the study of evolutionary transitions from uni- to multicellular life. In a modern biological view,{{Cite book|url=https://books.google.com/books?id=1MdJS2086usC&q=Volvox.+Molecular-Genetic+Origins+of+Multicellularity+and+Cellular+Differentiation|title=Volvox: A Search for the Molecular and Genetic Origins of Multicellularity and Cellular Differentiation|isbn=9780521019149|last1=Kirk|first1=David L.|date=8 September 2005|publisher=Cambridge University Press }} this significance arises from a number of specific features of these algae, including the fact that they are an extant family (obviating the need to study microfossils), are readily obtainable in nature, have been studied from a variety of perspectives (biochemical, developmental, genetic), and have had significant ecological studies. From a fluid dynamical perspective,{{cite journal |doi = 10.1146/annurev-fluid-010313-141426|title = Green Algae as Model Organisms for Biological Fluid Dynamics|year = 2015|last1 = Goldstein|first1 = Raymond E.|journal = Annual Review of Fluid Mechanics|volume = 47|pages = 343–375|pmid = 26594068|pmc = 4650200}} their relatively large size and easy culturing conditions allow for precise studies of their motility, the flows they create with their flagella, and interactions between organisms, while their high degree of symmetry simplifies theoretical descriptions of those same phenomena.{{cite journal |doi = 10.1017/jfm.2016.586|title = Batchelor Prize Lecture Fluid dynamics at the scale of the cell|year = 2016|last1 = Goldstein|first1 = Raymond E.|journal = Journal of Fluid Mechanics|volume = 807|pages = 1–39|bibcode = 2016JFM...807....1G|s2cid = 55745525|doi-access = free}}

As they are photosynthetic, the ability of these algae to execute phototaxis is central to their life. Because the lineage spans from unicellular to large colonial forms, it can be used to study the evolution of multicellular coordination of motility. Motility and phototaxis of motile green algae have been the subjects of an extensive literature in recent years,{{cite journal |doi = 10.1007/BF01294499|title = Motility in the colonial and multicellular Volvocales: Structure, function, and evolution|year = 1997|last1 = Hoops|first1 = H. J.|journal = Protoplasma|volume = 199|issue = 3–4|pages = 99–112|s2cid = 22315728}}{{cite journal |doi = 10.1073/pnas.1000901107|title = Fidelity of adaptive phototaxis|year = 2010|last1 = Drescher|first1 = K.|last2 = Goldstein|first2 = R. E.|last3 = Tuval|first3 = I.|journal = Proceedings of the National Academy of Sciences|volume = 107|issue = 25|pages = 11171–11176|pmid = 20534560|pmc = 2895142|doi-access = free}}{{cite journal |doi = 10.1103/PhysRevLett.105.168101|title = Direct Measurement of the Flow Field around Swimming Microorganisms|year = 2010|last1 = Drescher|first1 = Knut|last2 = Goldstein|first2 = Raymond E.|last3 = Michel|first3 = Nicolas|last4 = Polin|first4 = Marco|last5 = Tuval|first5 = Idan|journal = Physical Review Letters|volume = 105|issue = 16|page = 168101|pmid = 21231017|arxiv = 1008.2681|bibcode = 2010PhRvL.105p8101D|s2cid = 8306079}}{{cite journal |doi = 10.1103/PhysRevLett.105.168102|title = Oscillatory Flows Induced by Microorganisms Swimming in Two Dimensions|year = 2010|last1 = Guasto|first1 = Jeffrey S.|last2 = Johnson|first2 = Karl A.|last3 = Gollub|first3 = J. P.|journal = Physical Review Letters|volume = 105|issue = 16|page = 168102|pmid = 21231018|arxiv = 1008.2535|bibcode = 2010PhRvL.105p8102G|s2cid = 9533722}}{{cite journal |doi = 10.1098/rsif.2014.1164|title = A steering mechanism for phototaxis in Chlamydomonas|year = 2015|last1 = Bennett|first1 = Rachel R.|last2 = Golestanian|first2 = Ramin|journal = Journal of the Royal Society Interface|volume = 12|issue = 104|pmid = 25589576|pmc = 4345482}}{{cite journal |doi = 10.1038/s41598-017-03618-8|title = Phototaxis beyond turning: Persistent accumulation and response acclimation of the microalga Chlamydomonas reinhardtii|year = 2017|last1 = Arrieta|first1 = Jorge|last2 = Barreira|first2 = Ana|last3 = Chioccioli|first3 = Maurizio|last4 = Polin|first4 = Marco|last5 = Tuval|first5 = Idan|journal = Scientific Reports|volume = 7|issue = 1|page = 3447|pmid = 28615673|pmc = 5471259|arxiv = 1611.08224|bibcode = 2017NatSR...7.3447A}}{{cite journal |doi = 10.1038/s41567-018-0277-7|title = Polygonal motion and adaptable phototaxis via flagellar beat switching in the microswimmer Euglena gracilis|year = 2018|last1 = Tsang|first1 = Alan C. H.|last2 = Lam|first2 = Amy T.|last3 = Riedel-Kruse|first3 = Ingmar H.|journal = Nature Physics|volume = 14|issue = 12|pages = 1216–1222|bibcode = 2018NatPh..14.1216T|s2cid = 126294173}} focusing primarily on the two extreme cases: unicellular Chlamydomonas and much larger Volvox, with species composed of 1000–50,000 cells. Chlamydomonas swims typically by actuation of its two flagella in a breast stroke, combining propulsion and slow body rotation. It possesses an eyespot, a small area highly sensitive to light,{{cite journal |doi = 10.1128/mr.44.4.572-630.1980|title = Light Antennas in phototactic algae|year = 1980|last1 = Foster|first1 = K. W.|last2 = Smyth|first2 = R. D.|journal = Microbiological Reviews|volume = 44|issue = 4|pages = 572–630|pmid = 7010112|pmc = 373196}}{{cite journal |doi = 10.1146/annurev.arplant.59.032607.092847|title = Algal Sensory Photoreceptors|year = 2008|last1 = Hegemann|first1 = Peter|journal = Annual Review of Plant Biology|volume = 59|pages = 167–189|pmid = 18444900}} which triggers the two flagella differently.{{cite journal |doi = 10.1083/jcb.98.1.97|title = Submicromolar levels of calcium control the balance of beating between the two flagella in demembranated models of Chlamydomonas|year = 1984|last1 = Kamiya|first1 = R.|last2 = Witman|first2 = G. B.|journal = Journal of Cell Biology|volume = 98|issue = 1|pages = 97–107|pmid = 6707098|pmc = 2112995}} Those responses are adaptive, on a timescale matched to the rotational period of the cell body,{{cite journal |doi = 10.1002/cm.20069|title = Ciliary behavior of a negatively phototactic Chlamydomonas reinhardtii|year = 2005|last1 = Josef|first1 = Keith|last2 = Saranak|first2 = Jureepan|last3 = Foster|first3 = Kenneth W.|journal = Cell Motility and the Cytoskeleton|volume = 61|issue = 2|pages = 97–111|pmid = 15849714}}{{cite journal |doi = 10.1002/cm.20158|title = Linear systems analysis of the ciliary steering behavior associated with negative-phototaxis in Chlamydomonas reinhardtii|year = 2006|last1 = Josef|first1 = Keith|last2 = Saranak|first2 = Jureepan|last3 = Foster|first3 = Kenneth W.|journal = Cell Motility and the Cytoskeleton|volume = 63|issue = 12|pages = 758–777|pmid = 16986140}}{{cite journal |doi = 10.1093/pcp/pce084|title = The Sensitivity of Chlamydomonas Photoreceptor is Optimized for the Frequency of Cell Body Rotation|year = 2001|last1 = Yoshimura|first1 = Kenjiro|last2 = Kamiya|first2 = Ritsu|journal = Plant and Cell Physiology|volume = 42|issue = 6|pages = 665–672|pmid = 11427687|doi-access = free}} and allow cells to scan the environment and swim toward light.{{cite journal |doi = 10.1101/254714|title = An Adaptive Flagellar Photoresponse Determines the Dynamics of Accurate Phototactic Steering in Chlamydomonas|year = 2018|last1 = Leptos|first1 = Kyriacos C.|last2 = Chioccioli|first2 = Maurizio|last3 = Furlan|first3 = Silvano|last4 = Pesci|first4 = Adriana I.|author4-link=Adriana Pesci|last5 = Goldstein|first5 = Raymond E.|s2cid = 90374721}} Multicellular Volvox shows a higher level of complexity, with differentiation between interior germ cells and somatic cells dedicated to propulsion. Despite lacking a central nervous system to coordinate its cells, Volvox exhibits accurate phototaxis. This is also achieved by an adaptive response to changing light levels, with a response time tuned to the colony rotation period which creates a differential response between the light and dark sides of the spheroid.{{cite journal |doi = 10.1016/j.cub.2004.07.034|title = Volvox|year = 2004|last1 = Kirk|first1 = David L.|journal = Current Biology|volume = 14|issue = 15|pages = R599–R600|pmid = 15296767|s2cid = 235312006|doi-access = free| bibcode=2004CBio...14.R599K }}

In light of the above, a natural questions is as follows: How does the simplest differentiated organism achieve phototaxis? In the Volvocine lineage the species of interest is Gonium. This 8- or 16-cell colony represents one of the first steps to true multicellularity,{{cite journal |doi = 10.1371/journal.pone.0081641|title = The Simplest Integrated Multicellular Organism Unveiled|year = 2013|last1 = Arakaki|first1 = Yoko|last2 = Kawai-Toyooka|first2 = Hiroko|last3 = Hamamura|first3 = Yuki|last4 = Higashiyama|first4 = Tetsuya|last5 = Noga|first5 = Akira|last6 = Hirono|first6 = Masafumi|last7 = Olson|first7 = Bradley J. S. C.|last8 = Nozaki|first8 = Hisayoshi|journal = PLOS ONE|volume = 8|issue = 12|pages = e81641|pmid = 24349103|pmc = 3859500|bibcode = 2013PLoSO...881641A|doi-access = free}} presumed to have evolved from the unicellular common ancestor earlier than other Volvocine algae.{{cite journal |doi = 10.1111/j.1558-5646.2007.00304.x|title = Evolution of Complexity in the Volvocine Algae: Transitions in Individuality Through Darwin's Eye|year = 2008|last1 = Herron|first1 = Matthew D.|last2 = Michod|first2 = Richard E.|journal = Evolution|volume = 62|issue = 2|pages = 436–451|pmid = 18031303|s2cid = 12139760}} It is also the first to show cell differentiation.

The 16-cell Gonium colony shown in the diagram on the right is organized into two concentric squares of respectively 4 and 12 cells, each biflagellated, held together by an extracellular matrix.{{cite journal |doi = 10.2216/i0031-8884-29-1-1.1|title = Ultrastructure of the extracellular matrix of Gonium (Volvocales, Chlorophyta)|year = 1990|last1 = Nozaki|first1 = Hisayoshi|journal = Phycologia|volume = 29| issue=1 |pages = 1–8| bibcode=1990Phyco..29....1N }} All flagella point out on the same side: It exhibits a much lower symmetry than Volvox, lacking anterior-posterior symmetry. Yet it performs similar functions to its unicellular and large colonies counterparts as it mixes propulsion and body rotation and swims efficiently toward light.{{cite journal |doi = 10.1002/jez.1400210306|title = The mechanism of orientation in Gonium|year = 1916|last1 = Moore|first1 = A. R.|journal = Journal of Experimental Zoology|volume = 21|issue = 3|pages = 431–432| bibcode=1916JEZ....21..431M |url = https://zenodo.org/record/1802064}}{{cite journal |doi = 10.1002/jez.1400200102|title = The process of orientation in the colonial organism, Gonium pectorale, and a study of the structure and function of the eye-spot|year = 1916|last1 = Mast|first1 = S. O.|journal = Journal of Experimental Zoology|volume = 20| issue=1 |pages = 1–17| bibcode=1916JEZ....20....1M |url = https://zenodo.org/record/2271270}} The flagellar organization of inner and peripheral cells deeply differs:{{cite journal |doi = 10.1111/j.0022-3646.1985.00358.x|title = Development of the Flagellar Apparatus and Flagellar Orientation in the Colonial Green Alga Gonium Pectorale (Volvocales)1|year = 1985|last1 = Greuel|first1 = Brian T.|last2 = Floyd|first2 = Gary L.|journal = Journal of Phycology|volume = 21|issue = 3|pages = 358–371| bibcode=1985JPcgy..21..358G |s2cid = 85760904}}{{cite journal |doi = 10.2307/3221328|jstor = 3221328|title = The Structure and Development of the Colony in Gonium|last1 = Harper|first1 = R. A.|journal = Transactions of the American Microscopical Society|year = 1912|volume = 31|issue = 2|pages = 65–83|url = https://www.biodiversitylibrary.org/part/90698}} Central cells are similar to Chlamydomonas, with the two flagella beating in an opposing breast stroke, and contribute mostly to the forward propulsion of the colony. Cells at the periphery, however, have flagella beating in parallel, in a fashion close to Volvox cells. This minimizes steric interactions and avoids flagella crossing each other. Moreover, these flagella are implanted with a slight angle and organized in a pinwheel fashion [see Fig. 1(b)]: Their beating induces a left-handed rotation of the colony, highlighted in Figs. 1(c) and 1(d) and in Supplemental Movie 1 [29]. Therefore, the flagella structure of Gonium reinforces its key position as intermediate in the evolution toward multicellularity and cell differentiation.

These small flat assemblies show intriguing swimming along helical trajectories—with their body plane almost normal to the swimming direction—that have attracted the attention of naturalists since the 18th century.Müller O. F. (1782) Kleine Schriften Aus Der Naturhistorie, Dessau, herausgegeben von JAE Goeze, pp. 15–21. Yet the way in which Gonium colonies bias their swimming toward the light remains unclear. Early microscopic observations have identified differential flagellar activity between the illuminated and the shaded sides of the colony as the source of phototactic reorientation. Yet a full fluid-dynamics description, quantitatively linking the flagellar response to light variations and the hydrodynamic forces and torques acting on the colony, is still lacking. From an evolutionary perspective, phototaxis in Gonium raises fundamental issues such as the extent to which the phototactic strategy of the unicellular ancestor is retained in the colonial form, how the phototactic flagella reaction adapted to the geometry and symmetry of the colony, and how it leads to effective reorientation.

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File:Distribution of three-dimensional phototaxis in the tree of eukaryotes.jpg

{{see also|Eyespot apparatus}}

File:Diversity of phototactic protists.jpg Material was copied from this source, which is available under a [https://creativecommons.org/licenses/by/4.0/ Creative Commons Attribution 4.0 International License].
(a) green alga (b) heterokont zoospore (c) cryptomonad alga
(d) dinoflagellate (e) Euglena}}]]

Some protists can move toward or away from a stimulus, a movement referred to as taxis. For example, movement toward light, termed phototaxis, is accomplished by coupling their locomotion strategy with a light-sensing organ.Clark, M.A., Choi, J. and Douglas, M. (2018) [https://opentextbc.ca/biology2eopenstax/chapter/characteristics-of-protists/ Characteristics of Protists] Biology 2e. OpenStax. {{ISBN|9781947172951}}. 50px Material was copied from this source, which is available under a [https://creativecommons.org/licenses/by/4.0/ Creative Commons Attribution 4.0 International License] Eukaryotes evolved for the first time in the history of life the ability to follow light direction in three dimensions in open water. The strategy of eukaryotic sensory integration, sensory processing and the speed and mechanics of tactic responses is fundamentally different from that found in prokaryotes.{{Cite book|url=https://books.google.com/books?id=2nevsljDiCYC&pg=PP1|title = Photomovement|isbn = 9780080538860|last1 = Häder|first1 = D. -P|last2 = Lebert|first2 = M.|date = 19 June 2001| publisher=Elsevier }}

Both single-celled and multi-cellular eukaryotic phototactic organisms have a fixed shape, are polarized, swim in a spiral and use cilia for swimming and phototactic steering. Signalling can happen via direct light-triggered ion currents, adenylyl cyclases or trimeric G-proteins. The photoreceptors used can also be very different (see below). However, signalling in all cases eventually modifies the beating activity of cilia. The mechanics of phototactic orientation is analogous in all eukaryotes. A photosensor with a restricted view angle rotates to scan the space and signals periodically to the cilia to alter their beating, which will change the direction of the helical swimming trajectory. Three-dimensional phototaxis can be found in five out of the six eukaryotic major groups (opisthokonts, Amoebozoa, plants, chromalveolates, excavates, rhizaria).

Pelagic phototaxis is present in green algae – it is not present in glaucophyte algae or red algae. Green algae have a "stigma" located in the outermost portion of the chloroplast, directly underneath the two chloroplast membranes. The stigma is made of tens to several hundreds of lipid globules, which often form hexagonal arrays and can be arranged in one or more rows. The lipid globules contain a complex mixture of carotenoid pigments, which provide the screening function and the orange-red colour,{{cite journal |doi = 10.1007/BF00191604|title = Carotenoids in the eyespot apparatus of the flagellate green alga Spermatozopsis similis: Adaptation to the retinal-based photoreceptor|year = 1994|last1 = Grung|first1 = Merete|last2 = Kreimer|first2 = Georg|last3 = Calenberg|first3 = Michael|last4 = Melkonian|first4 = Michael|last5 = Liaaen-Jensen|first5 = Synnøve|authorlink5=Synnøve Liaaen Jensen|journal = Planta|volume = 193|s2cid = 29443649}} as well as proteins that stabilize the globules.{{cite journal |doi = 10.1007/s004250000473|title = Subfractionation of eyespot apparatuses from the green alga Spermatozopsis similis : Isolation and characterization of eyespot globules|year = 2001|last1 = Renninger|first1 = S.|last2 = Backendorf|first2 = E.|last3 = Kreimer|first3 = G.|journal = Planta|volume = 213|issue = 1|pages = 51–63|pmid = 11523656| bibcode=2001Plant.213...51R |s2cid = 24880210}} The stigma is located laterally, in a fixed plane relative to the cilia, but not directly adjacent to the basal bodies.{{cite journal |doi = 10.1111/j.1550-7408.1967.tb02038.x|title = Ultrastructure of the Eyespot and its Possible Significance in Phototaxis of Tetracystis excentrica*†|year = 1967|last1 = Arnott|first1 = Howard J.|last2 = Brown|first2 = R. Malcolm|journal = The Journal of Protozoology|volume = 14|issue = 4|pages = 529–539}}{{cite journal |doi = 10.1007/BF01283929|title = The eyespot of the flagellate Tetraselmis cordiformis stein (Chlorophyceae): Structural spezialization of the outer chloroplast membrane and its possible significance in phototaxis of green algae|year = 1979|last1 = Melkonian|first1 = M.|last2 = Robenek|first2 = H.|journal = Protoplasma|volume = 100|issue = 2|pages = 183–197|s2cid = 24606055}} The fixed position is ensured by the attachment of the chloroplast to one of the ciliary roots.{{cite journal |doi = 10.1007/BF00982810|title = Structure and significance of cruciate flagellar root systems in green algae: Comparative investigations in species of Chlorosarcinopsis (Chlorosarcinales)|year = 1978|last1 = Melkonian|first1 = Michael|journal = Plant Systematics and Evolution|volume = 130|issue = 3–4|pages = 265–292| bibcode=1978PSyEv.130..265M |s2cid = 22938771}} The pigmented stigma is not to be confused with the photoreceptor. The stigma only provides directional shading for the adjacent membrane-inserted photoreceptors (the term "eyespot" is therefore misleading). Stigmata can also reflect and focus light like a concave mirror, thereby enhancing sensitivity.

In the best-studied green alga, Chlamydomonas reinhardtii, phototaxis is mediated by a rhodopsin pigment, as first demonstrated by the restoration of normal photobehaviour in a blind mutant by analogues of the retinal chromophore.{{cite journal |doi = 10.1038/311756a0|title = A rhodopsin is the functional photoreceptor for phototaxis in the unicellular eukaryote Chlamydomonas|year = 1984|last1 = Foster|first1 = Kenneth W.|last2 = Saranak|first2 = Jureepan|last3 = Patel|first3 = Nayana|last4 = Zarilli|first4 = Gerald|last5 = Okabe|first5 = Masami|last6 = Kline|first6 = Toni|last7 = Nakanishi|first7 = Koji|journal = Nature|volume = 311|issue = 5988|pages = 756–759|pmid = 6493336|bibcode = 1984Natur.311..756F|s2cid = 4263301}} Two archaebacterial-type rhodopsins, channelrhodopsin-1 and -2,{{cite journal |doi = 10.1126/science.1072068|title = Channelrhodopsin-1: A Light-Gated Proton Channel in Green Algae|year = 2002|last1 = Nagel|first1 = G.|last2 = Ollig|first2 = D.|last3 = Fuhrmann|first3 = M.|last4 = Kateriya|first4 = S.|last5 = Musti|first5 = A. M.|last6 = Bamberg|first6 = E.|last7 = Hegemann|first7 = P.|journal = Science|volume = 296|issue = 5577|pages = 2395–2398|pmid = 12089443|bibcode = 2002Sci...296.2395N|s2cid = 206506942}}{{cite journal |doi = 10.1073/pnas.1936192100|title = Channelrhodopsin-2, a directly light-gated cation-selective membrane channel|year = 2003|last1 = Nagel|first1 = G.|last2 = Szellas|first2 = T.|last3 = Huhn|first3 = W.|last4 = Kateriya|first4 = S.|last5 = Adeishvili|first5 = N.|last6 = Berthold|first6 = P.|last7 = Ollig|first7 = D.|last8 = Hegemann|first8 = P.|last9 = Bamberg|first9 = E.|journal = Proceedings of the National Academy of Sciences|volume = 100|issue = 24|pages = 13940–13945|pmid = 14615590|pmc = 283525|bibcode = 2003PNAS..10013940N|doi-access = free}} were identified as phototaxis receptors in Chlamydomonas.{{cite journal |doi = 10.1073/pnas.122243399|title = Two rhodopsins mediate phototaxis to low- and high-intensity light in Chlamydomonas reinhardtii|year = 2002|last1 = Sineshchekov|first1 = O. A.|last2 = Jung|first2 = K.-H.|last3 = Spudich|first3 = J. L.|journal = Proceedings of the National Academy of Sciences|volume = 99|issue = 13|pages = 8689–8694|pmid = 12060707|pmc = 124360|doi-access = free}} Both proteins have an N-terminal 7-transmembrane portion, similar to archaebacterial rhodopsins, followed by an approximately 400 residue C-terminal membrane-associated portion. CSRA and CSRB act as light-gated cation channels and trigger depolarizing photocurrents.{{cite journal |doi = 10.1105/tpc.108.057919|title = Channelrhodopsin-1 Initiates Phototaxis and Photophobic Responses in Chlamydomonas by Immediate Light-Induced Depolarization|year = 2008|last1 = Berthold|first1 = Peter|last2 = Tsunoda|first2 = Satoshi P.|last3 = Ernst|first3 = Oliver P.|last4 = Mages|first4 = Wolfgang|last5 = Gradmann|first5 = Dietrich|last6 = Hegemann|first6 = Peter|journal = The Plant Cell|volume = 20|issue = 6|pages = 1665–1677|pmid = 18552201|pmc = 2483371}} CSRA was shown to localize to the stigma region using immunofluorescence analysis (Suzuki et al. 2003). Individual RNAi depletion of both CSRA and CSRB modified the light-induced currents and revealed that CSRA mediates a fast, high-saturating current while CSRB a slow, low-saturating one. Both currents are able to trigger photophobic responses and can have a role in phototaxis,{{cite journal |doi = 10.1016/S0006-3495(04)74291-5|title = Chlamydomonas Sensory Rhodopsins a and B: Cellular Content and Role in Photophobic Responses|year = 2004|last1 = Govorunova|first1 = Elena G.|last2 = Jung|first2 = Kwang-Hwan|last3 = Sineshchekov|first3 = Oleg A.|last4 = Spudich|first4 = John L.|journal = Biophysical Journal|volume = 86|issue = 4|pages = 2342–2349|pmid = 15041672|pmc = 1304083|bibcode = 2004BpJ....86.2342G}} although the exact contribution of the two receptors is not yet clear.

As in all bikonts (plants, chromalveolates, excavates, rhizaria), green algae have two cilia, which are not identical. The anterior cilium is always younger than the posterior one.{{cite journal |doi = 10.1099/00207713-52-2-297|title = The phagotrophic origin of eukaryotes and phylogenetic classification of Protozoa|year = 2002|last1 = Cavalier-Smith|first1 = T.|journal = International Journal of Systematic and Evolutionary Microbiology|volume = 52|issue = 2|pages = 297–354|pmid = 11931142}}{{cite journal |doi = 10.1111/j.1550-7408.2008.00373.x|title = Megaphylogeny, Cell Body Plans, Adaptive Zones: Causes and Timing of Eukaryote Basal Radiations|year = 2009|last1 = Cavalier-Smith|first1 = Thomas|journal = Journal of Eukaryotic Microbiology|volume = 56|issue = 1|pages = 26–33|pmid = 19340985|s2cid = 10205240|doi-access = free}} In every cell cycle, one daughter cell receives the anterior cilium and transforms it into a posterior one. The other daughter inherits the posterior, mature cilium. Both daughters then grow a new anterior cilium.

As all other ciliary swimmers, green algae always swim in a spiral. The handedness of the spiral is robust and is guaranteed by the chirality of the cilia. The two cilia of green algae have different beat patterns and functions. In Chlamydomonas, the phototransduction cascade alters the stroke pattern and beating speed of the two cilia differentially in a complex pattern. This results in the reorientation of the helical swimming trajectory as long as the helical swimming axis is not aligned with the light vector.

File:Chlamydomonas (10000x).jpg, a genus of unicellular green algae with two flagella each as long as the other,Harris, Elizabeth H. (2009) [https://books.google.com/books?id=xTjJGV5GWY0C&q=%22The+Chlamydomonas+Sourcebook%22 "The Genus Chlamydomonas"] In The Chlamydomonas Sourcebook (Second Edition), chapter 1, volume 1, pages 1-24. {{ISBN|9780080919553}} {{doi|10.1016/B978-0-12-370873-1.00001-0}} exhibit both phototaxis and thermotaxis.]]

Temperature is a key environmental factor for living organisms because chemical reaction rates and physical characteristics of biological materials can change substantially with temperature. Living organisms acclimate to cold and heat stress using acquired mechanisms, including the ability to migrate to an environment with temperatures suitable for inhabitation. One of the simplest forms of the behavior to migrate to a suitable thermal environment is thermotaxis. Thermotaxis has been found in multicellular organisms, such as Caenorhabditis elegans and Drosophila melanogaster, as well as in unicellular organisms, such as Paramecium caudatum, Dictyostelium discoideum, Physarum polycephalum, and Escherichia coli.Jennings H. S. (1907) [https://www.jstor.org/stable/2455119 "Behavior of the Lower Organisms"] The American Naturalist, 41(481): 42-44. Individual cells within multicellular organisms also show thermotaxis. For example, mammalian sperm migrate through the oviduct to the fertilization site guided by a rise in temperature.{{cite journal |doi = 10.1038/nm0203-149|title = Thermotaxis of mammalian sperm cells: A potential navigation mechanism in the female genital tract|year = 2003|last1 = Bahat|first1 = Anat|last2 = Tur-Kaspa|first2 = Ilan|last3 = Gakamsky|first3 = Anna|last4 = Giojalas|first4 = Laura C.|last5 = Breitbart|first5 = Haim|last6 = Eisenbach|first6 = Michael|journal = Nature Medicine|volume = 9|issue = 2|pages = 149–150|pmid = 12563318|s2cid = 36538049|hdl = 11336/66658|hdl-access = free}}{{cite journal |doi = 10.1038/s41598-018-34487-4|title = Thermotaxis in Chlamydomonas is brought about by membrane excitation and controlled by redox conditions|year = 2018|last1 = Sekiguchi|first1 = Masaya|last2 = Kameda|first2 = Shigetoshi|last3 = Kurosawa|first3 = Satoshi|last4 = Yoshida|first4 = Megumi|last5 = Yoshimura|first5 = Kenjiro|journal = Scientific Reports|volume = 8|issue = 1|page = 16114|pmid = 30382191|pmc = 6208428|bibcode = 2018NatSR...816114S}} 50px Material was copied from this source, which is available under a [https://creativecommons.org/licenses/by/4.0/ Creative Commons Attribution 4.0 International License].

The investigation of how unicellular organisms migrate toward preferred temperatures began more than 100 years ago. In particular, the thermotactic behavior of Paramecium cells has been well studied. Paramecium cells accumulate at sites that are close to the cultivation temperature, i. e. the temperature at which cells are grown. Accumulation at these sites occurs because cells frequently reverse their swimming direction when they encounter a temperature change that deviates from the cultivation temperature and increase their swimming velocity when they experience a temperature change that approaches the cultivation temperature.{{cite journal |doi = 10.1111/j.1550-7408.1972.tb03412.x|title = Responses of Parameciumto Temperature Change|year = 1972|last1 = Tawada|first1 = K.|last2 = Oosawa|first2 = F.|journal = The Journal of Protozoology|volume = 19|issue = 1|pages = 53–57|pmid = 5008849}}{{cite journal |doi = 10.1111/j.1550-7408.1977.tb01018.x|title = Temperature-Sensitive Behavior of Paramecium caudatum|year = 1977|last1 = Nakaoka|first1 = Yasuo|last2 = Oosawa|first2 = Fumio|journal = The Journal of Protozoology|volume = 24|issue = 4|pages = 575–580}} The reversal in swimming direction is induced by a depolarizing receptor potential, which triggers an action potential in the cilia.{{cite journal |doi = 10.1007/BF00610340|title = A heat-induced depolarization of Paramecium and its relationship to thermal avoidance behavior|year = 1983|last1 = Hennessey|first1 = Todd M.|last2 = Saimi|first2 = Yoshiro|last3 = Kung|first3 = Ching|journal = Journal of Comparative Physiology A|volume = 153|pages = 39–46|s2cid = 7152549}} These studies on Paramecium cells highlighted the thermotaxis in unicellular organisms more than 30 years ago, but the molecular mechanisms for thermoreception and signal transduction are not yet understood.

The understanding of the molecular mechanisms for thermotaxis has progressed greatly in recent years, from investigations of mammalian sperm. Human sperm migrates toward warmer temperatures, ranging from 29 °C to 41 °C. Sperm can detect a temperature gradient as small as 0.014 °C/mm, suggesting that sperm detect temporal changes in temperature rather than spatial differences. Several molecules have been proposed to be sensor molecules, including opsin and transient receptor potential (TRP) channels such as TRPV1, TRPV4, and TRPM8.{{cite journal |doi = 10.1038/srep16146|title = Involvement of opsins in mammalian sperm thermotaxis|year = 2015|last1 = Pérez-Cerezales|first1 = Serafín|last2 = Boryshpolets|first2 = Sergii|last3 = Afanzar|first3 = Oshri|last4 = Brandis|first4 = Alexander|last5 = Nevo|first5 = Reinat|last6 = Kiss|first6 = Vladimir|last7 = Eisenbach|first7 = Michael|journal = Scientific Reports|volume = 5|page = 16146|pmid = 26537127|pmc = 4633616|bibcode = 2015NatSR...516146P}}{{cite journal |doi = 10.1262/jrd.2015-106|title = Involvement of Transient Receptor Potential Vanilloid (TRPV) 4 in mouse sperm thermotaxis|year = 2016|last1 = Hamano|first1 = Koh-Ichi|last2 = Kawanishi|first2 = Tae|last3 = Mizuno|first3 = Atsuko|last4 = Suzuki|first4 = Makoto|last5 = Takagi|first5 = Yuji|journal = Journal of Reproduction and Development|volume = 62|issue = 4|pages = 415–422|pmid = 27180924|pmc = 5004798}}{{cite journal |doi = 10.1371/journal.pone.0006095|title = TRPM8, a Versatile Channel in Human Sperm|year = 2009|last1 = De Blas|first1 = Gerardo A.|last2 = Darszon|first2 = Alberto|last3 = Ocampo|first3 = Ana Y.|last4 = Serrano|first4 = Carmen J.|last5 = Castellano|first5 = Laura E.|last6 = Hernández-González|first6 = Enrique O.|last7 = Chirinos|first7 = Mayel|last8 = Larrea|first8 = Fernando|last9 = Beltrán|first9 = Carmen|last10 = Treviño|first10 = Claudia L.|journal = PLOS ONE|volume = 4|issue = 6|pages = e6095|pmid = 19582168|pmc = 2705237|bibcode = 2009PLoSO...4.6095D|doi-access = free}} TRP channels are multimodal sensor for thermal, chemical and mechanical stimuli, but the function of opsins as a thermosensor awaits to be established.

Temperature is a critical environmental factor also for Chlamydomonas cells, which produce small heat shock proteins, chaperonins, and HSP70 heat shock proteins, and also undergo other heat shock responses to cope with heat stress.{{cite journal |doi = 10.1002/j.1460-2075.1985.tb03869.x|title = Synthesis, transport and localization of a nuclear coded 22-kd heat-shock protein in the chloroplast membranes of peas and Chlamydomonas reinhardi|year = 1985|last1 = Kloppstech|first1 = Klaus|last2 = Meyer|first2 = Gabriele|last3 = Schuster|first3 = Gadi|last4 = Ohad|first4 = Itzhak|journal = The EMBO Journal|volume = 4|issue = 8|pages = 1901–1909|pmid = 16453628|pmc = 554439}}{{cite journal |doi = 10.1104/pp.102.018325|title = Identification of Novel Mitochondrial Protein Components of Chlamydomonas reinhardtii. A Proteomic Approach|year = 2003|last1 = Van Lis|first1 = Robert|last2 = Atteia|first2 = Ariane|last3 = Mendoza-HernáNdez|first3 = Guillermo|last4 = GonzáLez-Halphen|first4 = Diego|journal = Plant Physiology|volume = 132|issue = 1|pages = 318–330|pmid = 12746537|pmc = 166977}}{{cite journal |doi = 10.1128/mcb.9.9.3911-3918.1989|title = Three light-inducible heat shock genes of Chlamydomonas reinhardtii|year = 1989|last1 = von Gromoff|first1 = E. D.|last2 = Treier|first2 = U.|last3 = Beck|first3 = C. F.|journal = Molecular and Cellular Biology|volume = 9|issue = 9|pages = 3911–3918|pmid = 2779571|pmc = 362453}}{{cite journal |doi = 10.1111/tpj.12816|title = The Chlamydomonasheat stress response|year = 2015|last1 = Schroda|first1 = Michael|last2 = Hemme|first2 = Dorothea|last3 = Mühlhaus|first3 = Timo|journal = The Plant Journal|volume = 82|issue = 3|pages = 466–480|pmid = 25754362|doi-access = free}} In response to a cold shock of 4 °C, cells halt proliferation and accumulate starch and sugar.{{cite journal |doi = 10.1074/mcp.M112.026765|title = Systemic Cold Stress Adaptation of Chlamydomonas reinhardtii|year = 2013|last1 = Valledor|first1 = Luis|last2 = Furuhashi|first2 = Takeshi|last3 = Hanak|first3 = Anne-Mette|last4 = Weckwerth|first4 = Wolfram|journal = Molecular & Cellular Proteomics|volume = 12|issue = 8|pages = 2032–2047| doi-access=free |pmid = 23564937|pmc = 3734567}} Behavioral responses to avoid stressful warm or cold environments are expected to be present in Chlamydomonas. Although C. moewusii cells are reported to migrate toward warmer temperatures in a 10 °C to 15 °C gradient,{{cite journal |doi = 10.1080/0967026031000121697|title = Behavioural responses of freshwater phytoplanktonic flagellates to a temperature gradient|year = 2003|last1 = Clegg|first1 = Mark R.|last2 = Maberly|first2 = Stephen C.|last3 = Jones|first3 = Roger I.|journal = European Journal of Phycology|volume = 38|issue = 3|pages = 195–203|s2cid = 85353895|doi-access = free| bibcode=2003EJPhy..38..195C }} there has been no report in which the temperature range was systematically manipulated to examine a relationship with cultivation temperature. A 2019 study demonstrated thermotaxis in Chlamydomonas reinhardtii, and found that between 10 °C and 30 °C Chlamydomonas cells migrated toward lower temperatures independent of cultivation temperature.

In contrast to thermotaxis, phototaxis has been extensively studied in Chlamydomonas. Two flagella of Chlamydomonas beat in a breast-stroke like pattern during forward swimming and, during phototaxis, Chlamydomonas cells make by a turn toward or away from a light source by controlling the balance of the propulsive forces generated by the two flagella.{{cite journal |doi = 10.2108/zsj.17.1261|title = Dominance between the Two Flagella during Phototactic Turning in Chlamydomonas|year = 2000|last1 = Isogai|first1 = Nahoko|last2 = Kamiya|first2 = Ritsu|last3 = Yoshimura|first3 = Kenjiro|journal = Zoological Science|volume = 17|issue = 9|pages = 1261–1266|s2cid = 84890095|doi-access = free}} The balance depends on the intraflagellar calcium ion concentration; thus, loss of calcium-dependent control in ptx1 mutants results in a phototaxis defect.{{cite journal |doi = 10.1083/jcb.120.3.733|title = Ptx1, a nonphototactic mutant of Chlamydomonas, lacks control of flagellar dominance|year = 1993|last1 = Horst|first1 = C. J.|last2 = Witman|first2 = G. B.|journal = Journal of Cell Biology|volume = 120|issue = 3|pages = 733–741|pmid = 8425899|pmc = 2119553}}{{cite journal |doi = 10.1242/jcs.01633|title = Phototactic activity in Chlamydomonas non-phototactic' mutants deficient in Ca2+-dependent control of flagellar dominance or in inner-arm dynein|year = 2005|last1 = Okita|first1 = Noriko|last2 = Isogai|first2 = Nahoko|last3 = Hirono|first3 = Masafumi|last4 = Kamiya|first4 = Ritsu|last5 = Yoshimura|first5 = Kenjiro|journal = Journal of Cell Science|volume = 118|issue = 3|pages = 529–537|pmid = 15657081|s2cid = 2379702|doi-access = free}} The direction of phototaxis in Chlamydomonas depends on the light intensity, but is also affected by intracellular reduction-oxidation (redox) conditions.{{cite journal |doi = 10.1073/pnas.1100592108|title = Reduction-oxidation poise regulates the sign of phototaxis in Chlamydomonas reinhardtii|year = 2011|last1 = Wakabayashi|first1 = K.-i.|last2 = Misawa|first2 = Y.|last3 = Mochiji|first3 = S.|last4 = Kamiya|first4 = R.|journal = Proceedings of the National Academy of Sciences|volume = 108|issue = 27|pages = 11280–11284|pmid = 21690384|pmc = 3131381|bibcode = 2011PNAS..10811280W|doi-access = free}} Cells migrate toward a light source when the light intensity is weak, but the direction reverses under reducing conditions. In contrast, cells swim away from light sources with strong intensity, but the direction reverses under oxidizing conditions.

File:Swimming speeds of flagellates and ciliates.jpg

File:Normalized curves for flagellate and ciliate swimming speeds.jpg

{{clear}}

File:Fast ciliary reversal.jpg Material was copied from this source, which is available under a [https://creativecommons.org/licenses/by/4.0/ Creative Commons Attribution 4.0 International License].}} Ultrafast escape response and reversal of ciliary beating by action potential]]

In flagellate algae, abrupt changes in light intensity or intense photic stimuli induce rapid flagellar reversal and transient backward swimming.{{cite journal |doi = 10.1128/MR.44.4.572-630.1980|title = Light Antennas in phototactic algae|year = 1980|last1 = Foster|first1 = K. W.|last2 = Smyth|first2 = R. D.|journal = Microbiological Reviews|volume = 44|issue = 4|pages = 572–630|pmid = 7010112|pmc = 373196}}{{cite journal |doi = 10.1016/S0006-3495(97)78171-2|title = Control of phobic behavioral responses by rhodopsin-induced photocurrents in Chlamydomonas|year = 1997|last1 = Holland|first1 = E.M.|last2 = Harz|first2 = H.|last3 = Uhl|first3 = R.|last4 = Hegemann|first4 = P.|journal = Biophysical Journal|volume = 73|issue = 3|pages = 1395–1401|pmid = 9284306|bibcode = 1997BpJ....73.1395H|pmc = 1181038}} In green algae, this action may be mediated by the contractile root fibre which alters the angle between basal bodies.{{cite journal |doi = 10.1002/(SICI)1097-0169(1998)41:1<49::AID-CM4>3.0.CO;2-A|title = Real-time observation of Ca2+-induced basal body reorientation in Chlamydomonas|year = 1998|last1 = Hayashi|first1 = Masahito|last2 = Yagi|first2 = Toshiki|last3 = Yoshimura|first3 = Kenjiro|last4 = Kamiya|first4 = Ritsu|journal = Cell Motility and the Cytoskeleton|volume = 41|issue = 1|pages = 49–56|pmid = 9744298}} Cells can also react at speed to unexpected mechanical stimuli. All-or-none contractions in the stalked ciliate Vorticella can occur at rates of 8 cm/s.{{cite journal |doi = 10.1016/s0006-3495(98)77806-3|title = High-Speed Video Cinematographic Demonstration of Stalk and Zooid Contraction of Vorticella convallaria|year = 1998|last1 = Moriyama|first1 = Yasushige|last2 = Hiyama|first2 = Shigeo|last3 = Asai|first3 = Hiroshi|journal = Biophysical Journal|volume = 74|issue = 1|pages = 487–491|pmid = 9449349|pmc = 1299401|bibcode = 1998BpJ....74..487M}} In some species of heliozoa, axopods can completely retract within 20 ms in order to draw in trapped prey for phagocytosis.{{cite journal |doi = 10.1002/cm.970140214|title = Structure and function of the cytoskeleton in heliozoa: I. Mechanism of rapid axopodial contraction in Echinosphaerium|year = 1989|last1 = Ando|first1 = Motonori|last2 = Shigenaka|first2 = Yoshinobu|journal = Cell Motility and the Cytoskeleton|volume = 14|issue = 2|pages = 288–301}}

These fast reactions are usually induced by action potentials — unidirectional electrical pulses involving fast, regenerative changes in membrane potential. While all cells display some electrical activity, phylogenetic evidence suggests that the capacity to propagate action potentials may have been an ancestral eukaryotic trait supported by the last eukaryotic common ancestor. These may have emerged in response to accidental membrane damage and sudden calcium influx.{{cite journal |doi = 10.1098/rstb.2015.0043|title = From damage response to action potentials: Early evolution of neural and contractile modules in stem eukaryotes|year = 2016|last1 = Brunet|first1 = Thibaut|last2 = Arendt|first2 = Detlev|journal = Philosophical Transactions of the Royal Society B: Biological Sciences|volume = 371|issue = 1685|pmid = 26598726|pmc = 4685582}} Bioelectrical signalling in the form of action potentials occurs orders of magnitude faster than any other signalling modalities, e.g. chemical diffusion, protein phosphorylation etc.

In order to initiate fast escape responses, these may have been coupled directly to the motility apparatus—particularly to flexible, membrane-continuous structures such as cilia and pseudopodia. Loss of voltage-gated sodium/calcium channels is further correlated with loss of cilia in many taxa. In protists, all-or-none action potentials occur almost exclusively in association with ciliary membranes,{{cite journal |doi = 10.1146/annurev.bb.08.060179.002033|title = Ionic Mechanisms of Excitation in Paramecium|year = 1979|last1 = Eckert|first1 = R.|last2 = Brehm|first2 = P.|journal = Annual Review of Biophysics and Bioengineering|volume = 8|pages = 353–383|pmid = 383005}}{{cite journal |doi = 10.1111/j.1550-7408.1972.tb03444.x|title = Bioelectric Control of Locomotion in the Ciliates*†|year = 1972|last1 = Eckert|first1 = Roger|last2 = Naitoh|first2 = Yutaka|journal = The Journal of Protozoology|volume = 19|issue = 2|pages = 237–243|pmid = 4624297}}{{cite journal |doi = 10.1007/bf00609450|title = Membrane permeabilities determining resting, action and mechanoreceptor potentials in Stentor coeruleus|year = 1982|last1 = Wood|first1 = David C.|journal = Journal of Comparative Physiology A|volume = 146|issue = 4|pages = 537–550|s2cid = 21083419}} with the exception of some non-ciliated diatoms.{{cite journal |doi = 10.1371/journal.pone.0004966|title = A Fast Na+/Ca2+-Based Action Potential in a Marine Diatom|year = 2009|last1 = Taylor|first1 = Alison R.|journal = PLOS ONE|volume = 4|issue = 3|pages = e4966|pmid = 19305505|pmc = 2654917|bibcode = 2009PLoSO...4.4966T|doi-access = free}}{{cite journal |doi = 10.1016/j.cub.2019.03.041|title = Alternative Mechanisms for Fast Na+/Ca2+ Signaling in Eukaryotes via a Novel Class of Single-Domain Voltage-Gated Channels|year = 2019|last1 = Helliwell|first1 = Katherine E.|last2 = Chrachri|first2 = Abdul|last3 = Koester|first3 = Julie A.|last4 = Wharam|first4 = Susan|last5 = Verret|first5 = Frédéric|last6 = Taylor|first6 = Alison R.|last7 = Wheeler|first7 = Glen L.|last8 = Brownlee|first8 = Colin|journal = Current Biology|volume = 29|issue = 9|pages = 1503–1511.e6|pmid = 31006567|pmc = 6509283}} Graded potentials occur in amoebae, also for movement control.{{cite journal |doi = 10.1016/s0022-5193(62)80024-1|title = Bioelectric potentials in relation to movement in amoebae|year = 1962|last1 = Bingley|first1 = M.S.|last2 = Thompson|first2 = C.M.|journal = Journal of Theoretical Biology|volume = 2|issue = 1|pages = 16–32|bibcode = 1962JThBi...2...16B}}

File:Tintinnid ciliate Favella.jpg ciliate Favella]]

In Chlamydomonas, action-potential-like flagellar currents induce photophobic responses and flagella reversal (via the voltage-gated calcium channel Cav2), while photoreceptor currents elicit much milder responses.{{cite journal |doi = 10.1038/351489a0|title = Rhodopsin-regulated calcium currents in Chlamydomonas|year = 1991|last1 = Harz|first1 = Hartmann|last2 = Hegemann|first2 = Peter|journal = Nature|volume = 351|issue = 6326|pages = 489–491|bibcode = 1991Natur.351..489H|s2cid = 4309593}} Here, a mechanosensory channel of the transient receptor potential family is localized to the ciliary base, while Cav2 is localized only to the distal regions of cilia.{{cite journal |doi = 10.1038/ncb2214|title = Mechanoreception in motile flagella of Chlamydomonas|year = 2011|last1 = Fujiu|first1 = Kenta|last2 = Nakayama|first2 = Yoshitaka|last3 = Iida|first3 = Hidetoshi|last4 = Sokabe|first4 = Masahiro|last5 = Yoshimura|first5 = Kenjiro|journal = Nature Cell Biology|volume = 13|issue = 5|pages = 630–632|pmid = 21478860|s2cid = 19883187}}{{cite journal |doi = 10.1016/j.cub.2008.11.068|title = Chlamydomonas CAV2 Encodes a Voltage- Dependent Calcium Channel Required for the Flagellar Waveform Conversion|year = 2009|last1 = Fujiu|first1 = Kenta|last2 = Nakayama|first2 = Yoshitaka|last3 = Yanagisawa|first3 = Ayaka|last4 = Sokabe|first4 = Masahiro|last5 = Yoshimura|first5 = Kenjiro|journal = Current Biology|volume = 19|issue = 2|pages = 133–139|pmid = 19167228|s2cid = 14063142|doi-access = free| bibcode=2009CBio...19..133F }} In Paramecium, hyperpolarizations increase ciliary beat frequency, while depolarizations have the opposite effect and eventually lead to a ciliary reversal. Depolarizations above a certain threshold result in action potentials, owing to opening of Cav channels located exclusively in the ciliary membrane.Umbach JA (1981) "pH and membrane excitability in Paramecium caudatum". Los Angeles, CA: University of California.{{cite journal |doi = 10.1113/jphysiol.1977.sp011993|title = Localization of calcium channels in Paramecium caudatum|year = 1977|last1 = Dunlap|first1 = K.|journal = The Journal of Physiology|volume = 271|issue = 1|pages = 119–133|pmid = 915829|pmc = 1353610}} Potassium channels — also residing in the membrane — help restore the resting membrane potential.

Eukaryotes manipulate their membrane potential to achieve transitions between different behaviours. Complex bioelectric sequences have been recorded in association with integrated feeding and predation behaviours in Favella.{{cite journal |doi = 10.1242/jeb.121871|title = Feast or flee: Bioelectrical regulation of feeding and predator evasion behaviors in the planktonic alveolate Favella sp. (Spirotrichia)|year = 2015|last1 = Echevarria|first1 = Michael L.|last2 = Wolfe|first2 = Gordon V.|last3 = Taylor|first3 = Alison R.|journal = Journal of Experimental Biology|volume = 219|issue = Pt 3|pages = 445–456|pmid = 26567352|s2cid = 37255456|doi-access = free}} Repetitive behaviours arise from rhythmic spiking. In ciliates, rhythmic depolarizations control fast and slow walking by tentacle-like compound cilia called cirri,{{cite journal |doi = 10.1016/s0932-4739(96)80038-1|title = Rhythmic spontaneous depolarizations determine a slow-and-fast rhythm in walking of the marine hypotrich Euplotes vannus|year = 1996|last1 = Lueken|first1 = Wolfgang|last2 = Ricci|first2 = Nicola|last3 = Krüppel|first3 = Thomas|journal = European Journal of Protistology|volume = 32|pages = 47–54}} enabling escape from dead ends{{hsp}}{{cite journal |doi = 10.3389/fmicb.2014.00270|doi-access = free|title = Attempts to retreat from a dead-ended long capillary by backward swimming in Paramecium|year = 2014|last1 = Kunita|first1 = Itsuki|last2 = Kuroda|first2 = Shigeru|last3 = Ohki|first3 = Kaito|last4 = Nakagaki|first4 = Toshiyuki|journal = Frontiers in Microbiology|volume = 5|page = 270|pmid = 24966852|pmc = 4052044}} and courtship rituals in conjugating gametes.Stock C, KrÜPpel T, Key G, Lueken W (1999) "Sexual behaviour in Euplotes raikovi is accompanied by pheromone-induced modifications of ionic currents". J Exp Biol, 202 (4): 475–483. {{PMID|9914154}}.Kimball RF (1942) "The Nature and Inheritance of Mating Types in Euplotes Patella". Genetics, 27(3): 269–285. {{PMID|17247040}}, {{PMCID|PMC1209158}}. In Stentor, action potentials produce whole-body contractions.{{cite journal |doi = 10.1523/JNEUROSCI.08-07-02254.1988|title = Habituation in Stentor: Produced by mechanoreceptor channel modification|year = 1988|last1 = Wood|first1 = DC|journal = The Journal of Neuroscience|volume = 8|issue = 7|pages = 2254–2258|pmid = 3249223|pmc = 6569508}} Finally, excitable systems operating close to bifurcations may admit limit cycles, which manifest as repetitive or rhythmic electrical spiking and repetitive behaviours. Ultimately, this may lead to habituation.{{cite journal |doi = 10.1086/277256|title = Studies on Reactions to Stimuli in Unicellular Organisms. III Reactions to Localized Stimuli in Spirostomum and Stentor|year = 1899|last1 = Jennings|first1 = H. S.|journal = The American Naturalist|volume = 33|issue = 389|pages = 373–389|s2cid = 85272784}}{{cite journal |doi = 10.1016/j.cub.2019.10.059|title = A Complex Hierarchy of Avoidance Behaviors in a Single-Cell Eukaryote|year = 2019|last1 = Dexter|first1 = Joseph P.|last2 = Prabakaran|first2 = Sudhakaran|last3 = Gunawardena|first3 = Jeremy|journal = Current Biology|volume = 29|issue = 24|pages = 4323–4329.e2|pmid = 31813604|s2cid = 208652463|doi-access = free| bibcode=2019CBio...29E4323D }}

File:Biohybrid Chlamydomonas reinhardtii microswimmers 2.jpg microswimmers{{hsp}}{{cite journal |doi = 10.1002/advs.202001256|title = High-Yield Production of Biohybrid Microalgae for On-Demand Cargo Delivery|year = 2020|last1 = Akolpoglu|first1 = Mukrime Birgul|last2 = Dogan|first2 = Nihal Olcay|last3 = Bozuyuk|first3 = Ugur|last4 = Ceylan|first4 = Hakan|last5 = Kizilel|first5 = Seda|last6 = Sitti|first6 = Metin|journal = Advanced Science|volume = 7|issue = 16|pmid = 32832367|pmc = 7435244}} 50px Material was copied from this source, which is available under a [https://creativecommons.org/licenses/by/4.0/ Creative Commons Attribution 4.0 International License].}} Top: Schematics of production steps for biohybrid C. reinhardtii.
Bottom: SEM images of bare microalgae (left) and biohybrid microalgae (right) coated with chitosan-coated iron oxide nanoparticles (CSIONPs). Images were pseudocolored. A darker green color on the right SEM image represents chitosan coating on microalgae cell wall. Orange-colored particles represents CSIONPs.]]

{{main|Biohybrid microswimmers}}

Biohybrid microswimmers can be defined as microswimmers that consist of both biological and artificial constituents, for instance, one or several living microorganisms attached to one or various synthetic parts.{{cite journal |doi = 10.1063/1.4993441|title = Hybrid Bio Micromotors|year = 2017|last1 = Schwarz|first1 = Lukas|last2 = Medina-Sánchez|first2 = Mariana|last3 = Schmidt|first3 = Oliver G.|journal = Applied Physics Reviews|volume = 4|issue = 3|page = 031301|bibcode = 2017ApPRv...4c1301S|doi-access = free}}{{cite journal |doi = 10.3389/frobt.2018.00097|title = Bacterial Biohybrid Microswimmers|year = 2018|last1 = Bastos-Arrieta|first1 = Julio|last2 = Revilla-Guarinos|first2 = Ainhoa|last3 = Uspal|first3 = William E.|last4 = Simmchen|first4 = Juliane|journal = Frontiers in Robotics and AI|volume = 5|page = 97|pmid = 33500976|pmc = 7805739|doi-access = free}} 50px Material was copied from this source, which is available under a [https://creativecommons.org/licenses/by/4.0/ Creative Commons Attribution 4.0 International License]. In 1999, Montemagno and Bachand published an article identifying specific attachment strategies of biological molecules to nanofabricated substrates, enabling the preparation of hybrid inorganic/organic nanoelectromechanical systems (NEMS).{{cite journal |doi = 10.1088/0957-4484/10/3/301|title = Constructing nanomechanical devices powered by biomolecular motors|year = 1999|last1 = Montemagno|first1 = Carlo|last2 = Bachand|first2 = George|journal = Nanotechnology|volume = 10|issue = 3|pages = 225–231|bibcode = 1999Nanot..10..225M| s2cid=250910730 }} They described the production of large amounts of F1-ATPase from the thermophilic bacteria Bacillus PS3 for the preparation of F1-ATPase biomolecular motors immobilized on a nanoarray pattern of gold, copper or nickel produced by electron beam lithography. These proteins were attached to one micron microspheres tagged with a synthetic peptide. Consequently, they accomplished the preparation of a platform with chemically active sites and the development of biohybrid devices capable of converting energy of biomolecular motors into useful work.

Over the past decade, biohybrid microrobots, in which living mobile microorganisms are physically integrated with untethered artificial structures, have gained growing interest to enable the active locomotion and cargo delivery to a target destination.{{cite journal |doi = 10.1126/scirobotics.aaq0495|title = Biohybrid actuators for robotics: A review of devices actuated by living cells|year = 2017|last1 = Ricotti|first1 = Leonardo|last2 = Trimmer|first2 = Barry|last3 = Feinberg|first3 = Adam W.|last4 = Raman|first4 = Ritu|last5 = Parker|first5 = Kevin K.|last6 = Bashir|first6 = Rashid|last7 = Sitti|first7 = Metin|last8 = Martel|first8 = Sylvain|last9 = Dario|first9 = Paolo|last10 = Menciassi|first10 = Arianna|journal = Science Robotics|volume = 2|issue = 12|pages = eaaq0495|pmid = 33157905|s2cid = 29776467|doi-access = free}}{{cite journal |doi = 10.1146/annurev-control-053018-023803|title = Microrobotics and Microorganisms: Biohybrid Autonomous Cellular Robots|year = 2019|last1 = Alapan|first1 = Yunus|last2 = Yasa|first2 = Oncay|last3 = Yigit|first3 = Berk|last4 = Yasa|first4 = I. Ceren|last5 = Erkoc|first5 = Pelin|last6 = Sitti|first6 = Metin|journal = Annual Review of Control, Robotics, and Autonomous Systems|volume = 2|pages = 205–230|s2cid = 139819519}}{{cite journal |doi = 10.1002/adma.201706245|title = Neutrophil-Based Drug Delivery Systems|year = 2018|last1 = Chu|first1 = Dafeng|last2 = Dong|first2 = Xinyue|last3 = Shi|first3 = Xutong|last4 = Zhang|first4 = Canyang|last5 = Wang|first5 = Zhenjia|journal = Advanced Materials|volume = 30|issue = 22|pages = e1706245|pmid = 29577477|pmc = 6161715| bibcode=2018AdM....3006245C }}{{cite journal |doi = 10.1002/smll.201400384|title = Bio-Hybrid Cell-Based Actuators for Microsystems|year = 2014|last1 = Carlsen|first1 = Rika Wright|last2 = Sitti|first2 = Metin|journal = Small|volume = 10|issue = 19|pages = 3831–3851|pmid = 24895215}} In addition to the motility, the intrinsic capabilities of sensing and eliciting an appropriate response to artificial and environmental changes make cell-based biohybrid microrobots appealing for transportation of cargo to the inaccessible cavities of the human body for local active delivery of diagnostic and therapeutic agents.{{cite journal |doi = 10.1016/j.snb.2015.09.034|title = Active tumor-therapeutic liposomal bacteriobot combining a drug (Paclitaxel)-encapsulated liposome with targeting bacteria (Salmonella Typhimurium)|year = 2016|last1 = Nguyen|first1 = Van Du|last2 = Han|first2 = Ji-Won|last3 = Choi|first3 = Young Jin|last4 = Cho|first4 = Sunghoon|last5 = Zheng|first5 = Shaohui|last6 = Ko|first6 = Seong Young|last7 = Park|first7 = Jong-Oh|last8 = Park|first8 = Sukho|journal = Sensors and Actuators B: Chemical|volume = 224|pages = 217–224}}{{cite journal |doi = 10.1038/nnano.2016.137|title = Magneto-aerotactic bacteria deliver drug-containing nanoliposomes to tumour hypoxic regions|year = 2016|last1 = Felfoul|first1 = Ouajdi|last2 = Mohammadi|first2 = Mahmood|last3 = Taherkhani|first3 = Samira|last4 = De Lanauze|first4 = Dominic|last5 = Zhong Xu|first5 = Yong|last6 = Loghin|first6 = Dumitru|last7 = Essa|first7 = Sherief|last8 = Jancik|first8 = Sylwia|last9 = Houle|first9 = Daniel|last10 = Lafleur|first10 = Michel|last11 = Gaboury|first11 = Louis|last12 = Tabrizian|first12 = Maryam|last13 = Kaou|first13 = Neila|last14 = Atkin|first14 = Michael|last15 = Vuong|first15 = Té|last16 = Batist|first16 = Gerald|last17 = Beauchemin|first17 = Nicole|last18 = Radzioch|first18 = Danuta|last19 = Martel|first19 = Sylvain|journal = Nature Nanotechnology|volume = 11|issue = 11|pages = 941–947|pmid = 27525475|pmc = 6094936|bibcode = 2016NatNa..11..941F}}{{cite journal |doi = 10.1002/adma.201804130|title = Microalga-Powered Microswimmers toward Active Cargo Delivery|year = 2018|last1 = Yasa|first1 = Oncay|last2 = Erkoc|first2 = Pelin|last3 = Alapan|first3 = Yunus|last4 = Sitti|first4 = Metin|journal = Advanced Materials|volume = 30|issue = 45|pages = e1804130|pmid = 30252963| bibcode=2018AdM....3004130Y |s2cid = 52823884}} Active locomotion, targeting and steering of concentrated therapeutic and diagnostic agents embedded in mobile microrobots to the site of action can overcome the existing challenges of conventional therapies.{{cite journal |doi = 10.1039/C7LC00064B|title = Mobile microrobots for bioengineering applications|year = 2017|last1 = Ceylan|first1 = Hakan|last2 = Giltinan|first2 = Joshua|last3 = Kozielski|first3 = Kristen|last4 = Sitti|first4 = Metin|journal = Lab on a Chip|volume = 17|issue = 10|pages = 1705–1724|pmid = 28480466|doi-access = free}}{{cite journal |doi = 10.1126/scirobotics.aam6431|title = Micro/Nanorobots for biomedicine: Delivery, surgery, sensing, and detoxification|year = 2017|last1 = Li|first1 = Jinxing|last2 = Esteban-Fernández De Ávila|first2 = Berta|last3 = Gao|first3 = Wei|last4 = Zhang|first4 = Liangfang|last5 = Wang|first5 = Joseph|journal = Science Robotics|volume = 2|issue = 4|pages = eaam6431|pmid = 31552379|pmc = 6759331}}{{cite journal |doi = 10.1002/adtp.201800064|title = Mobile Microrobots for Active Therapeutic Delivery|year = 2019|last1 = Erkoc|first1 = Pelin|last2 = Yasa|first2 = Immihan C.|last3 = Ceylan|first3 = Hakan|last4 = Yasa|first4 = Oncay|last5 = Alapan|first5 = Yunus|last6 = Sitti|first6 = Metin|journal = Advanced Therapeutics|volume = 2|s2cid = 88204894|doi-access = free}} To this end, bacteria have been commonly used with attached beads and ghost cell bodies.{{cite journal |doi = 10.1021/acsnano.7b03207|title = Multifunctional Bacteria-Driven Microswimmers for Targeted Active Drug Delivery|year = 2017|last1 = Park|first1 = Byung-Wook|last2 = Zhuang|first2 = Jiang|last3 = Yasa|first3 = Oncay|last4 = Sitti|first4 = Metin|journal = ACS Nano|volume = 11|issue = 9|pages = 8910–8923|pmid = 28873304}}{{cite journal |doi = 10.1063/1.2431454|title = Bacterial flagella-based propulsion and on/Off motion control of microscale objects|year = 2007|last1 = Behkam|first1 = Bahareh|last2 = Sitti|first2 = Metin|journal = Applied Physics Letters|volume = 90|issue = 2|page = 023902|bibcode = 2007ApPhL..90b3902B}}{{cite journal |doi = 10.1063/1.3040318|title = Effect of quantity and configuration of attached bacteria on bacterial propulsion of microbeads|year = 2008|last1 = Behkam|first1 = Bahareh|last2 = Sitti|first2 = Metin|journal = Applied Physics Letters|volume = 93|issue = 22|page = 223901|bibcode = 2008ApPhL..93v3901B}}{{cite journal |doi = 10.1002/advs.201700058|title = Bioadhesive Bacterial Microswimmers for Targeted Drug Delivery in the Urinary and Gastrointestinal Tracts|year = 2017|last1 = Mostaghaci|first1 = Babak|last2 = Yasa|first2 = Oncay|last3 = Zhuang|first3 = Jiang|last4 = Sitti|first4 = Metin|journal = Advanced Science|volume = 4|issue = 6|pmid = 28638787|pmc = 5473323}}{{cite journal |doi = 10.1038/s41598-018-28102-9|title = Motility and chemotaxis of bacteria-driven microswimmers fabricated using antigen 43-mediated biotin display|year = 2018|last1 = Schauer|first1 = Oliver|last2 = Mostaghaci|first2 = Babak|last3 = Colin|first3 = Remy|last4 = Hürtgen|first4 = Daniel|last5 = Kraus|first5 = David|last6 = Sitti|first6 = Metin|last7 = Sourjik|first7 = Victor|journal = Scientific Reports|volume = 8|issue = 1|page = 9801|pmid = 29955099|pmc = 6023875|bibcode = 2018NatSR...8.9801S}}{{cite journal |doi = 10.1021/acsnano.7b02082|title = Microemulsion-Based Soft Bacteria-Driven Microswimmers for Active Cargo Delivery|year = 2017|last1 = Singh|first1 = Ajay Vikram|last2 = Hosseinidoust|first2 = Zeinab|last3 = Park|first3 = Byung-Wook|last4 = Yasa|first4 = Oncay|last5 = Sitti|first5 = Metin|journal = ACS Nano|volume = 11|issue = 10|pages = 9759–9769|pmid = 28858477}}{{cite journal |doi = 10.1002/smll.201603679|title = Biohybrid Microtube Swimmers Driven by Single Captured Bacteria|year = 2017|last1 = Stanton|first1 = Morgan M.|last2 = Park|first2 = Byung-Wook|last3 = Miguel-López|first3 = Albert|last4 = Ma|first4 = Xing|last5 = Sitti|first5 = Metin|last6 = Sánchez|first6 = Samuel|journal = Small|volume = 13|issue = 19|pmid = 28299891|hdl = 2445/123481|hdl-access = free}}{{cite journal |doi = 10.1021/acsnano.7b04128|title = Magnetotactic Bacteria Powered Biohybrids TargetE. 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Chlamydomonas reinhardtii is a unicellular green microalga. The wild-type C. reinhardtii has a spherical shape that averages about 10 μm in diameter.{{cite journal |doi = 10.1146/annurev.arplant.52.1.363|title = Chlamydomonasas Amodelorganism|year = 2001|last1 = Harris|first1 = Elizabeth H.|journal = Annual Review of Plant Physiology and Plant Molecular Biology|volume = 52|pages = 363–406|pmid = 11337403}} This microorganism can perceive the visible light and be steered by it (i.e., phototaxis) with high swimming speeds in the range of 100–200 μm/s. It has natural autofluorescence that permits label-free fluorescent imaging. C. reinhardtii has been actively explored as the live component of biohybrid microrobots for the active delivery of therapeutics. They are biocompatible with healthy mammalian cells, leave no known toxins, mobile in the physiologically relevant media, and allow for surface modification to carry cargo on the cell wall.{{cite journal |doi = 10.1073/pnas.0505481102|title = Microoxen: Microorganisms to move microscale loads|year = 2005|last1 = Weibel|first1 = D. B.|last2 = Garstecki|first2 = P.|last3 = Ryan|first3 = D.|last4 = Diluzio|first4 = W. R.|last5 = Mayer|first5 = M.|last6 = Seto|first6 = J. E.|last7 = Whitesides|first7 = G. M.|journal = Proceedings of the National Academy of Sciences|volume = 102|issue = 34|pages = 11963–11967|pmid = 16103369|pmc = 1189341|bibcode = 2005PNAS..10211963W|doi-access = free}}{{cite journal |doi = 10.1016/j.actbio.2013.12.055|title = Development of photosynthetic biomaterials for in vitro tissue engineering|year = 2014|last1 = Hopfner|first1 = Ursula|last2 = Schenck|first2 = Thilo-Ludwig|last3 = Chávez|first3 = Myra-Noemi|last4 = Machens|first4 = Hans-Günther|last5 = Bohne|first5 = Alexandra-Viola|last6 = Nickelsen|first6 = Jörg|last7 = Giunta|first7 = Riccardo-Enzo|last8 = Egaña|first8 = José-Tomás|journal = Acta Biomaterialia|volume = 10|issue = 6|pages = 2712–2717|pmid = 24406198}}{{cite journal |doi = 10.1016/j.actbio.2018.09.060|title = Development of photosynthetic sutures for the local delivery of oxygen and recombinant growth factors in wounds|year = 2018|last1 = Centeno-Cerdas|first1 = Carolina|last2 = Jarquín-Cordero|first2 = Montserrat|last3 = Chávez|first3 = Myra Noemi|last4 = Hopfner|first4 = Ursula|last5 = Holmes|first5 = Christopher|last6 = Schmauss|first6 = Daniel|last7 = Machens|first7 = Hans-Günther|last8 = Nickelsen|first8 = Jörg|last9 = Egaña|first9 = José Tomás|journal = Acta Biomaterialia|volume = 81|pages = 184–194|pmid = 30287280|s2cid = 52922420}}{{cite journal |doi = 10.1016/j.actbio.2014.12.012|title = Photosynthetic biomaterials: A pathway towards autotrophic tissue engineering|year = 2015|last1 = Schenck|first1 = Thilo Ludwig|last2 = Hopfner|first2 = Ursula|last3 = Chávez|first3 = Myra Noemi|last4 = Machens|first4 = Hans-Günther|last5 = Somlai-Schweiger|first5 = Ian|last6 = Giunta|first6 = Riccardo Enzo|last7 = Bohne|first7 = Alexandra Viola|last8 = Nickelsen|first8 = Jörg|last9 = Allende|first9 = Miguel L.|last10 = Egaña|first10 = José Tomás|journal = Acta Biomaterialia|volume = 15|pages = 39–47|pmid = 25536030}} Alternative attachment strategies for C. reinhardtii have been proposed for the assembly through modifying the interacting surfaces by electrostatic interactions and covalent bonding.{{cite journal |doi = 10.1021/acs.langmuir.8b01210|title = Artificial Magnetotaxis of Microbot: Magnetophoresis versus Self-Swimming|year = 2018|last1 = Ng|first1 = Wei Ming|last2 = Che|first2 = Hui Xin|last3 = Guo|first3 = Chen|last4 = Liu|first4 = Chunzhao|last5 = Low|first5 = Siew Chun|last6 = Chieh Chan|first6 = Derek Juinn|last7 = Mohamud|first7 = Rohimah|last8 = Lim|first8 = Jitkang|journal = Langmuir|volume = 34|issue = 27|pages = 7971–7980|pmid = 29882671|s2cid = 46953567}}

• Aquatic locomotion
• Bacterial motility
• Cytoskeleton
• Spirostomum
• Squirmer
• Vorticella

{{reflist}}

• {{cite book | last1=Cohn | first1=Stanley | last2=Manoylov | first2=Kalina | last3=Gordon | first3=Richard | title=Diatom gliding motility : biology and applications | publisher=Scrivener Publishing | publication-place=Beverly, MA | date=2021 | isbn=978-1-119-52648-3 | oclc=1262966612}}

Category:Microswimmers

Category:Protista