halorespiration

The process of organohalide respiration, uses reductive dehalogenation to produce energy that can be used by the respiring microorganism to carry out its growth and metabolism.{{Cite journal|last1=Futagami|first1=Taiki|last2=Goto|first2=Masatoshi|last3=Furukawa|first3=Kensuke|date=2008-01-01|title=Biochemical and genetic bases of dehalorespiration|journal=The Chemical Record|language=en|volume=8|issue=1|pages=1–12|doi=10.1002/tcr.20134|pmid=18302277|issn=1528-0691|doi-access=free}} Halogenated organic compounds are used as the terminal electron acceptor, which results in their dehalogenation. Reductive dehalogenation is the process by which this occurs. It involves the reduction of halogenated compounds by removing the halogen substituents, while simultaneously adding electrons to the compound.{{Cite journal|last1=Mohn|first1=W. W.|last2=Tiedje|first2=J. M.|date=September 1992|title=Microbial reductive dehalogenation|journal=Microbiological Reviews|volume=56|issue=3|pages=482–507|issn=0146-0749|pmid=1406492|pmc=372880|doi=10.1128/mmbr.56.3.482-507.1992}} Hydrogenolysis and vicinal reduction are the two known processes of this mechanism that have been identified. In both processes, the removed halogen substituents are released as anions. Reductive dehalogenation is catalyzed by reductive dehalogenases, which are membrane-associated enzymes.{{cite journal|last1=Scheutz|first1=Charlotte|last2=Durant|first2=Neal d.|last3=Dennis|first3=Philip|last4=Hansen|first4=Maria Heisterberg|last5=Jørgensen|first5=Torben|last6=Jakobsen|first6=Rasmus|last7=Cox|first7=Evan e.|last8=Bjerg|first8=Poul L.|title=Concurrent Ethene Generation and Growth of Dehalococcoides Containing Vinyl Chloride Reductive Dehalogenase Genes During an Enhanced Reductive Dechlorination Field Demonstration|journal=Environmental Science & Technology|language=en|volume=42|issue=24|pages=9302–9309|doi=10.1021/es800764t|pmid=19174908|year=2008|bibcode=2008EnST...42.9302S}} A number of not only membrane-associated but also cytoplasmic hydrogenases, in some cases as part of the protein complexes, are predicted to play roles in the organohalide respiration process.{{Cite journal|last1=Jugder|first1=Bat-Erdene|last2=Ertan|first2=Haluk|last3=Wong|first3=Yie Kuan|last4=Braidy|first4=Nady|last5=Manefield|first5=Michael|last6=Marquis|first6=Christopher P.|last7=Lee|first7=Matthew|date=2016-08-10|title=Genomic, transcriptomic and proteomic analyses of Dehalobacter UNSWDHB in response to chloroform|journal=Environmental Microbiology Reports|volume=8|issue=5|pages=814–824|doi=10.1111/1758-2229.12444|pmid=27452500|bibcode=2016EnvMR...8..814J |issn=1758-2229|hdl=1959.4/unsworks_46701|hdl-access=free}} Most of these enzymes contain iron-sulfur (Fe-S) clusters, and a corrinoid cofactor at their active sites. Although the exact mechanism is unknown, research suggests that these two components of the enzyme may be involved in the reduction.

Common substrates that are used as terminal electron acceptors in organohalide respiration are organochloride pesticides, aryl halides and alkyl solvents. Many of these are persistent pollutants that can only be degraded anaerobically by organohalide respiration, either partially or completely. Trichloroethylene (TCE) and tetrachloroethylene (PCE) are two examples of such pollutants, and their degradation has been a focus of research.{{Cite journal|last1=Maymó-Gatell|first1=X.|last2=Chien|first2=Y.|last3=Gossett|first3=J. M.|last4=Zinder|first4=S. H.|date=1997-06-06|title=Isolation of a bacterium that reductively dechlorinates tetrachloroethene to ethene|journal=Science|volume=276|issue=5318|pages=1568–1571|issn=0036-8075|pmid=9171062|doi=10.1126/science.276.5318.1568}} PCE is a chlorinated solvent that is widely used in dry cleaning, degreasing machinery and other applications. It remains a common contaminant of groundwater. Bacteria that are capable of completely degrading PCE to ethene, a gaseous chemical, have been isolated. They have been found to belong to the genus Dehalococcoides and to use H₂ as their electron donor. The process of organohalide respiration has been applied to in situ bioremediation of PCE and TCE in the past. For example, enhanced reductive dechlorination has been used to treat contaminated groundwater by introducing electron donors and dehalorespiring bacteria into the contaminated site, to create conditions that stimulate bacterial growth and organohalide respiration. In enhanced reductive dechlorination, the pollutants act as the electron acceptors and are completely reduced to ultimately produce ethene in a series of reactions.

An ecologically significant aspect of bacterial organohalide respiration is the reduction of the two anthropogenic pollutants tetrachloroethylene (PCE) and trichloroethylene (TCE).{{Cite journal|last=Ruder|first=AM|date=September 2006|title=Potential health effects of occupational chlorinated solvent exposure.|journal=Annals of the New York Academy of Sciences|volume=1076|issue=1|pages=207–227|doi=10.1196/annals.1371.050|pmid=17119204|bibcode=2006NYASA1076..207R|s2cid=43678533}} Their presence as environmental pollutants arose from their common industrial use as metal-degreasing agents from the 1920s - 1970.{{Cite journal|last1=Bakke|first1=Berit|last2=Stewart|first2=Patricia A.|last3=Waters|first3=Martha A.|date=November 2007|title=Uses of an Exposure to Trichloroethylene in U.S Industry: A Systematic Literature Review|journal=Journal of Occupational and Environmental Hygiene|volume=4|issue=5|pages=375–390|doi=10.1080/15459620701301763|pmid=17454505|s2cid=32801149}} These xenobiotic compounds tend to form partially insoluble layers called dense non-aqueous phase liquids (DNAPLs) at the bottom of groundwater aquifers, which solubilize in a slow, reservoir-like manner, making TCE and PCE among the most common groundwater pollutants.{{Cite journal|last=Dugat-Bony|first=Eric|date=March 2012|title=In situ TCE degradation mediated by complex dehalorespiring communities during biostimulation processes|journal=Microbial Biotechnology|volume=5|issue=5|pages=642–653|doi=10.1111/j.1751-7915.2012.00339.x|pmid=22432919|pmc=3815876}}

A commonly used strategy for the removal of TCE and PCE from groundwater is the use of bioremediation via enhanced reductive dechlorination (ERD).{{Cite journal|last=Scheutz|first=Charlotte|date=November 2008|title=Concurrent ethene generation and growth of Dehalococcoides containing vinyl chloride reductive dehalogenase genes during an enhanced reductive dechlorination field demonstration.|journal=Environmental Science & Technology|volume=42|issue=24|pages=9302–9309|doi=10.1021/es800764t|pmid=19174908|bibcode=2008EnST...42.9302S}} ERD involves in situ injections of dehalorespiring bacteria, among fermentable organic substrates serving as electron donors, while the two pollutants, TCE and PCE, act as the electron acceptors. This facilitates the sequential dechlorination of PCE and TCE into noxious cis-1,2-Dichloroethylene (DCE) and Vinyl chloride (VC), which then suit as electron acceptors for the full dechlorination into ethylene.

A wide array of bacteria across different genera have the capacity to partially dechlorinate PCE and TCE into cis-DCE and VC. One such example of this is the Magnetospirillum bacterium, strain MS-1, which can reduce PCE into cis-DCE under aerobic conditions.{{Cite journal|last=Sharma|first=Pramod K|date=March 1996|title=Isolation and Characterization of a Facultatively Aerobic Bacterium That Reductively Dehalogenates Tetrachloroethene to cis-1,2-Dichloroethene|journal=Applied and Environmental Microbiology|volume=62|issue=3|pages=761–765|pmc=1388792|pmid=16535267|doi=10.1128/aem.62.3.761-765.1996|bibcode=1996ApEnM..62..761S}} However, these daughter substrates have higher toxicity profiles than their parent compounds. As such, effective dechlorination of cis-DCE and VC into innocuous ethene is crucial for bioremediation of PCE and TCE-contaminated aquifers. Currently, bacteria of the Dehalococcoides genera are the only known organisms that can fully dechlorinate PCE into ethylene. This is due to their specific transmembrane reductive dehalogenases (RDases) that metabolize the chlorine atoms on the xenobiotic pollutants for cellular energy.{{Cite journal|last=Khoshnood|first=Behrang|date=August 2015|title=Genome Closing and Transcription Kinetics for RDase Genes in Dehalococcoides and Their Prevalence in a Wastewater Treatment Plant|url=http://scholarbank.nus.sg/handle/10635/122574|journal=National University of Singapore Libraries|via=Proquest|access-date=2017-11-19|archive-date=2017-12-01|archive-url=https://web.archive.org/web/20171201042115/http://scholarbank.nus.sg/handle/10635/122574|url-status=dead}} In particular, Dehalococcoides isolates VS and BAV1 encode Vinyl Chloride RDases, which metabolize VC into innocuous ethene, making them required species in ERD systems used in bioremediation of PCE and TCE.

• Bioaugmentation
• Reductive dechlorination
• Chloroflexota
• Dehalococcoides
• Dehalobacter

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• {{cite journal|last1=Jugder|first1=J.|title=Reductive dehalogenases come of age in biological destruction of organohalides|journal=Trends in Microbiology|volume=33|issue=10|year=2015|pages=595–610|doi=10.1016/j.tibtech.2015.07.004|pmid=26409778}}
• {{cite journal|last1=Leys|first1=D.|last2=Adrian|first2=L.|last3=Smidt|first3=H.|title=Organohalide respiration: microbes breathing chlorinated molecules|journal=Philosophical Transactions of the Royal Society B: Biological Sciences|volume=368|issue=1616|year=2013|pages=20120316|issn=0962-8436|doi=10.1098/rstb.2012.0316|pmid=23479746|pmc=3638457}}
• {{cite book|last1=Futagami|first1=Taiki|last2=Goto|first2=Masatoshi|last3=Furukawa|first3=Kensuke|title=Biodegradative Bacteria |chapter=Genetic System of Organohalide-Respiring Bacteria |year=2014|pages=59–81|doi=10.1007/978-4-431-54520-0_4|isbn=978-4-431-54519-4}}
• {{cite journal|last1=Hug|first1=L. A.|last2=Maphosa|first2=F.|last3=Leys|first3=D.|last4=Loffler|first4=F. E.|last5=Smidt|first5=H.|last6=Edwards|first6=E. A.|last7=Adrian|first7=L.|title=Overview of organohalide-respiring bacteria and a proposal for a classification system for reductive dehalogenases|journal=Philosophical Transactions of the Royal Society B: Biological Sciences|volume=368|issue=1616|year=2013|pages=20120322|issn=0962-8436|doi=10.1098/rstb.2012.0322|pmid=23479752|pmc=3638463}}
• {{cite journal|last1=Maphosa|first1=Farai|last2=de Vos|first2=Willem M.|last3=Smidt|first3=Hauke|title=Exploiting the ecogenomics toolbox for environmental diagnostics of organohalide-respiring bacteria|url=https://zenodo.org/record/3422980|journal=Trends in Biotechnology|volume=28|issue=6|year=2010|pages=308–316|issn=0167-7799|doi=10.1016/j.tibtech.2010.03.005|pmid=20434786}}

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Category:Electrochemistry

Category:Respiration