Microbial oxidation of sulfur

The oxidation of hydrogen sulfide is a significant environmental process, particularly in the context of Earth's history, during which oceanic conditions were often characterized by very low oxygen and high sulfidic concentrations. The modern analog ecosystems are deep marine basins, for instance in the Black Sea, near the Cariaco trench and the Santa Barbara basin. Other zones of the ocean that experience periodic anoxic and sulfidic conditions are the upwelling zones off the coasts of Chile and Namibia, and hydrothermal vents, which are a key source of H₂S to the ocean.{{Cite journal|last1=Luther|first1=George W.|last2=Findlay|first2=Alyssa J.|last3=Macdonald|first3=Daniel J.|last4=Owings|first4=Shannon M.|last5=Hanson|first5=Thomas E.|last6=Beinart|first6=Roxanne A.|author7-link=Peter Girguis|last7=Girguis|first7=Peter R.|date=2011|title=Thermodynamics and kinetics of sulfide oxidation by oxygen: a look at inorganically controlled reactions and biologically mediated processes in the environment|journal=Frontiers in Microbiology|volume=2|pages=62|doi=10.3389/fmicb.2011.00062|issn=1664-302X|pmc=3153037|pmid=21833317|doi-access=free}} Sulfur oxidizing microorganisms (SOM) are thus restricted to upper sediment layers in these environments, where oxygen and nitrate are more readily available. The SOM may play an important yet unconsidered role in carbon sequestration,{{Cite journal|last1=Hawley|first1=Alyse K.|last2=Brewer|first2=Heather M.|last3=Norbeck|first3=Angela D.|last4=Paša-Tolić|first4=Ljiljana|last5=Hallam|first5=Steven J.|date=2014-08-05|title=Metaproteomics reveals differential modes of metabolic coupling among ubiquitous oxygen minimum zone microbes|journal=Proceedings of the National Academy of Sciences|language=en|volume=111|issue=31|pages=11395–11400|doi=10.1073/pnas.1322132111|issn=0027-8424|pmc=4128106|pmid=25053816|bibcode=2014PNAS..11111395H|doi-access=free}} since some models{{Cite journal|last=Middelburg|first=Jack J.|date=2011-12-23|title=Chemoautotrophy in the ocean|journal=Geophysical Research Letters|language=en|volume=38|issue=24|pages=n/a|doi=10.1029/2011gl049725|bibcode=2011GeoRL..3824604M|hdl=1874/248832|s2cid=131612365 |issn=0094-8276|hdl-access=free}} and experiments with Gammaproteobacteria{{Cite journal|last1=Boschker|first1=Henricus T. S.|last2=Vasquez-Cardenas|first2=Diana|last3=Bolhuis|first3=Henk|last4=Moerdijk-Poortvliet|first4=Tanja W. C.|last5=Moodley|first5=Leon|date=2014-07-08|title=Chemoautotrophic Carbon Fixation Rates and Active Bacterial Communities in Intertidal Marine Sediments|journal=PLOS ONE|language=en|volume=9|issue=7|pages=e101443|doi=10.1371/journal.pone.0101443|issn=1932-6203|pmc=4086895|pmid=25003508|bibcode=2014PLoSO...9j1443B|doi-access=free}}{{Cite journal|last1=Dyksma|first1=Stefan|last2=Bischof|first2=Kerstin|last3=Fuchs|first3=Bernhard M|last4=Hoffmann|first4=Katy|last5=Meier|first5=Dimitri|last6=Meyerdierks|first6=Anke|last7=Pjevac|first7=Petra|last8=Probandt|first8=David|last9=Richter|first9=Michael|date=2016-02-12|title=Ubiquitous Gammaproteobacteria dominate dark carbon fixation in coastal sediments|journal=The ISME Journal|language=En|volume=10|issue=8|pages=1939–1953|doi=10.1038/ismej.2015.257|issn=1751-7362|pmc=4872838|pmid=26872043|bibcode=2016ISMEJ..10.1939D }} have suggested that sulfur-dependent carbon fixation in marine sediments could be responsible for almost half of total dark carbon fixation in the oceans. Further, they may have been critical to the evolution of eukaryotic organisms, given that sulfur metabolism is hypothesized to have driven the formation of the symbiotic associations that sustained eukaryotes (see below).{{Cite journal|last1=Overmann|first1=Jörg|last2=van Gemerden|first2=Hans|date=2000|title=Microbial interactions involving sulfur bacteria: implications for the ecology and evolution of bacterial communities|journal=FEMS Microbiology Reviews|language=en|volume=24|issue=5|pages=591–599|doi=10.1111/j.1574-6976.2000.tb00560.x|pmid=11077152|issn=1574-6976|doi-access=free}}

Although the biological oxidation of reduced sulfur compounds competes with abiotic chemical reactions (e.g. the iron-mediated oxidation of sulfide to iron sulfide (FeS) or pyrite (FeS₂)),{{Cite book|title=Sulfide oxidation in marine sediments: Geochemistry meets microbiology. In: Sulfur Biogeochemistry - Past and Present|last1=Jørgensen|first1=Bo Barker|last2=Nelson|first2=Douglas C.|date=2004|publisher=Geological Society of America Special Paper 379|isbn=978-0813723792|pages=63–81|language=en|doi=10.1130/0-8137-2379-5.63}} thermodynamic and kinetic considerations suggest that biological oxidation far exceeds the chemical oxidation of sulfide in most environments. Experimental data from the anaerobic phototroph Chlorobaculum tepidum indicates that microorganisms may enhance sulfide oxidation by three or more orders of magnitude. However, the general contribution of microorganisms to total sulfur oxidation in marine sediments is still unknown. The SOM of Alphaproteobacteria, Gammaproteobacteria and Campylobacterota account for average cell abundances of 10⁸ cells/m³ in organic-rich marine sediments.{{Cite journal|last1=Ravenschlag|first1=Katrin|last2=Sahm|first2=Kerstin|last3=Amann|first3=Rudolf|date=2001-01-01|title=Quantitative Molecular Analysis of the Microbial Community in Marine Arctic Sediments (Svalbard)|journal=Applied and Environmental Microbiology|language=en|volume=67|issue=1|pages=387–395|doi=10.1128/AEM.67.1.387-395.2001|issn=0099-2240|pmc=92590|pmid=11133470|bibcode=2001ApEnM..67..387R}} Considering that these organisms have a very narrow range of habitats, as explained below, a major fraction of sulfur oxidation in many marine sediments may be accounted for by these groups.{{Cite journal|last1=Wasmund|first1=Kenneth|last2=Mußmann|first2=Marc|last3=Loy|first3=Alexander|date=August 2017|title=The life sulfuric: microbial ecology of sulfur cycling in marine sediments|journal=Environmental Microbiology Reports|volume=9|issue=4|pages=323–344|doi=10.1111/1758-2229.12538|issn=1758-2229|pmc=5573963|pmid=28419734|bibcode=2017EnvMR...9..323W }}

Given that the maximal concentrations of oxygen, nitrate and sulfide are usually separated in depth profiles, many SOM cannot directly access their hydrogen or electron sources (reduced sulfur species) and energy sources (O₂ or nitrate) simultaneously. This limitation has led SOM to develop different morphological adaptations. The large sulfur bacteria (LSB) of the family Beggiatoaceae (Gammaproteobacteria) have been used as model organisms for benthic sulfur oxidation. They are known as 'gradient organisms,' species that are indicative of hypoxic (low oxygen) and sulfidic (rich in reduced sulfur species) conditions. They internally store large amounts of nitrate and elemental sulfur to overcome the spatial gap between oxygen and sulfide. Some species of Beggiatoaceae are filamentous and can thus glide between oxic/suboxic and sulfidic environments, while the non-motile species rely on nutrient suspensions, fluxes, or attach themselves to larger particles. Some aquatic, non-motile LSB are the only known free-living bacteria that utilize two distinct carbon fixation pathways: the Calvin-Benson cycle (used by plants and other photosynthetic organisms) and the reverse tricarboxylic acid cycle.{{Cite journal|last1=Winkel|first1=Matthias|last2=Salman-Carvalho|first2=Verena|last3=Woyke|first3=Tanja|last4=Richter|first4=Michael|last5=Schulz-Vogt|first5=Heide N.|last6=Flood|first6=Beverly E.|last7=Bailey|first7=Jake V.|last8=Mußmann|first8=Marc|date=2016|title=Single-cell Sequencing of Thiomargarita Reveals Genomic Flexibility for Adaptation to Dynamic Redox Conditions|journal=Frontiers in Microbiology|language=en|volume=7|pages=964|doi=10.3389/fmicb.2016.00964|issn=1664-302X|pmc=4914600|pmid=27446006|doi-access=free}}

Another evolutionary strategy of SOM is form mutualistic relationships with motile eukaryotic organisms. The symbiotic SOM provides carbon and, in some cases, bioavailable nitrogen to the host, and receives enhanced access to resources and shelter in return. This lifestyle has evolved independently in sediment-dwelling ciliates, oligochaetes, nematodes, flatworms and bivalves.{{Cite journal|last1=Dubilier|first1=Nicole|last2=Bergin|first2=Claudia|last3=Lott|first3=Christian|date=2008|title=Symbiotic diversity in marine animals: the art of harnessing chemosynthesis|journal=Nature Reviews. Microbiology|volume=6|issue=10|pages=725–740|doi=10.1038/nrmicro1992|issn=1740-1534 |pmid=18794911 |s2cid=3622420}} Recently, a new mechanism for sulfur oxidation was discovered in filamentous bacteria. This mechanism, called electrogenic sulfur oxidation (e-SOx), involves the formation of multicellular bridges that connect the oxidation of sulfide in anoxic sediment layers with the reduction of oxygen or nitrate in oxic surface sediments, generating electric currents over centimeter-long distances. The so-called cable bacteria are widespread in shallow marine sediments,{{Cite journal|last1=Risgaard-Petersen|first1=Nils|last2=Revil|first2=André|last3=Meister|first3=Patrick|last4=Nielsen|first4=Lars Peter|date=2012|title=Sulfur, iron-, and calcium cycling associated with natural electric currents running through marine sediment|journal=Geochimica et Cosmochimica Acta|volume=92|pages=1–13|doi=10.1016/j.gca.2012.05.036|bibcode=2012GeCoA..92....1R|issn=0016-7037}} and are believed to conduct electrons through structures inside a common periplasm of the multicellular filament.{{Cite journal|last1=Nielsen|first1=Lars Peter|last2=Risgaard-Petersen|first2=Nils|last3=Fossing|first3=Henrik|last4=Christensen|first4=Peter Bondo|last5=Sayama|first5=Mikio|date=2010|title=Electric currents couple spatially separated biogeochemical processes in marine sediment|journal=Nature|language=En|volume=463|issue=7284|pages=1071–1074|doi=10.1038/nature08790|pmid=20182510|bibcode=2010Natur.463.1071N|s2cid=205219761|issn=0028-0836}} This process may influence the cycling of elements at aquatic sediment surfaces, for instance, by altering iron speciation.{{Cite journal|last1=Seitaj|first1=Dorina|last2=Schauer|first2=Regina|last3=Sulu-Gambari|first3=Fatimah|last4=Hidalgo-Martinez|first4=Silvia|last5=Malkin|first5=Sairah Y.|last6=Burdorf|first6=Laurine D. W.|last7=Slomp|first7=Caroline P.|author-link7=Caroline Slomp|last8=Meysman|first8=Filip J. R.|date=2015-10-27|title=Cable bacteria generate a firewall against euxinia in seasonally hypoxic basins|journal=Proceedings of the National Academy of Sciences|language=en|volume=112|issue=43|pages=13278–13283|doi=10.1073/pnas.1510152112|issn=0027-8424|pmc=4629370|pmid=26446670|bibcode=2015PNAS..11213278S|doi-access=free}} The LSB and cable bacteria are hypothesized to be restricted to undisturbed sediments with stable hydrodynamic conditions,{{Cite journal|last1=Malkin|first1=Sairah Y|last2=Rao|first2=Alexandra MF|last3=Seitaj|first3=Dorina|last4=Vasquez-Cardenas|first4=Diana|last5=Zetsche|first5=Eva-Maria|last6=Hidalgo-Martinez|first6=Silvia|last7=Boschker|first7=Henricus TS|last8=Meysman|first8=Filip JR|date=2014-03-27|title=Natural occurrence of microbial sulphur oxidation by long-range electron transport in the seafloor|journal=The ISME Journal|language=En|volume=8|issue=9|pages=1843–1854|doi=10.1038/ismej.2014.41|issn=1751-7362|pmc=4139731|pmid=24671086|bibcode=2014ISMEJ...8.1843M }} while symbiotic SOM and their hosts have mainly been identified in permeable coastal sediments.

The oxidation of reduced sulfur compounds is performed exclusively by bacteria and archaea. Archaea involved in this process are aerobic and belong to the order Sulfolobales,{{cite journal |last1=Fuchs|first1=Tanja |last2=Huber|first2=Harald|last3=Burggraf|first3=Siegfried|last4=Stetter|first4=Karl O. | name-list-style = vanc |date=1996|title=16S rDNA-based Phylogeny of the Archaeal Order Sulfolobales and Reclassification of Desulfurolobus ambivalens as Acidianus ambivalens comb. nov.|journal=Systematic and Applied Microbiology|volume=19|issue=1|pages=56–60|doi=10.1016/s0723-2020(96)80009-9|bibcode=1996SyApM..19...56F |issn=0723-2020}}{{cite book|title=Astrobiology|last=Stetter|first=Karl O. | name-list-style = vanc |date=2002|publisher=Springer, Berlin, Heidelberg|isbn=9783642639579|pages=169–184|doi=10.1007/978-3-642-59381-9_12|chapter = Hyperthermophilic Microorganisms|chapter-url = https://epub.uni-regensburg.de/11505/1/ubr04693_ocr.pdf}} characterized by acidophiles (extremophiles that require low pHs to grow) and thermophiles (extremophiles that require high temperatures to grow). The most studied have been the genera Sulfolobus, an aerobic archaea, and Acidianus, a facultative anaerobe (i.e. an organism that can obtain energy either by aerobic or anaerobic respiration).

Sulfur oxidizing bacteria (SOB) are aerobic, anaerobic or facultative, with most of them being obligate (capable of metabolizing only a specific compound) or facultative (capable of metabolizing a secondary compound when primary compound is absent) autotrophs that can utilize either carbon dioxide or organic compounds as a source of carbon (mixotrophs).{{cite book |title=Geomicrobiology of sulfur |vauthors=Fike DA, Bradley AS, Leavitt WD |publisher=Ehrlich's Geomicrobiology |year=2016 |edition=Sixth}} The most abundant and studied SOB are in the family Thiobacilliaceae, found in terrestrial environments, and in the family Beggiatoaceae, found in aquatic environments. Aerobic sulfur oxidizing bacteria are mainly mesophilic, growing optimally at moderate ranges of temperature and pH, although some are thermophilic and/or acidophilic. Outside of these families, other SOB described belong to the genera Acidithiobacillus, Aquaspirillum,{{cite journal|vauthors=Friedrich CG, Mitrenga G|date=1981|title=Oxidation of thiosulfate by Paracoccus denitrificans and other hydrogen bacteria|journal=FEMS Microbiology Letters|volume=10|issue=2|pages=209–212|doi=10.1111/j.1574-6968.1981.tb06239.x|doi-access=free}} Aquifex,{{cite book|title=The Prokaryotes|last1=Huber|first1=Robert|last2=Eder|first2=Wolfgang | name-list-style = vanc |date=2006|publisher=Springer, New York, NY|isbn=9780387254975|pages=925–938|doi=10.1007/0-387-30747-8_39}} Bacillus,{{cite book | vauthors = Aragno M | date = 1992 | chapter = The aerobic, chemolithoautotrophic, thermophilic bacteria | title = Thermophilic Bacteria | veditors = Kristjansson JK | pages = 77–104 | publisher = CRC Press | location = Boca Raton, Fla. }} Methylobacterium,{{cite book|title=Advances in Microbial Ecology|last1=Kelly|first1=Don P.|last2=Smith|first2=Neil A. | name-list-style = vanc |date=1990|publisher=Springer, Boston, MA|isbn=9781468476149|pages=345–385|doi=10.1007/978-1-4684-7612-5_9}} Paracoccus, Pseudomonas Starkeya,{{cite journal | vauthors = Kelly DP, McDonald IR, Wood AP | author-link3=Ann P. Wood|title = Proposal for the reclassification of Thiobacillus novellus as Starkeya novella gen. nov., comb. nov., in the alpha-subclass of the Proteobacteria | journal = International Journal of Systematic and Evolutionary Microbiology | volume = 50 Pt 5 | issue = 5 | pages = 1797–802 | date = September 2000 | pmid = 11034489 | doi = 10.1099/00207713-50-5-1797 | doi-access = free }} Thermithiobacillus,{{cite journal | vauthors = Kelly DP, Wood AP | author-link=Ann P. Wood|title = Reclassification of some species of Thiobacillus to the newly designated genera Acidithiobacillus gen. nov., Halothiobacillus gen. nov. and Thermithiobacillus gen. nov | journal = International Journal of Systematic and Evolutionary Microbiology | volume = 50 Pt 2 | issue = 2| pages = 511–16 | date = March 2000 | pmid = 10758854 | doi = 10.1099/00207713-50-2-511 | doi-access = free }} and Xanthobacter. On the other hand, the cable bacteria belong to the family Desulfobulbaceae of the Deltaproteobacteria and are currently represented by two candidate genera, "Candidatus Electronema" and "Candidatus Electrothrix."{{Cite journal|last1=Trojan|first1=Daniela|last2=Schreiber|first2=Lars|last3=Bjerg|first3=Jesper T.|last4=Bøggild|first4=Andreas|last5=Yang|first5=Tingting|last6=Kjeldsen|first6=Kasper U.|last7=Schramm|first7=Andreas|date=2016|title=A taxonomic framework for cable bacteria and proposal of the candidate genera Electrothrix and Electronema|journal=Systematic and Applied Microbiology|volume=39|issue=5|pages=297–306|doi=10.1016/j.syapm.2016.05.006|issn=0723-2020|pmc=4958695|pmid=27324572|bibcode=2016SyApM..39..297T }}

Anaerobic SOB (AnSOB) are mainly neutrophilic/mesophilic photosynthetic autotrophs, obtaining energy from sunlight but using reduced sulfur compounds instead of water as hydrogen or electron donors for photosynthesis. AnSOB include some purple sulfur bacteria (Chromatiaceae){{cite journal | vauthors = Imhoff JF, Süling J, Petri R | title = Phylogenetic relationships among the Chromatiaceae, their taxonomic reclassification and description of the new genera Allochromatium, Halochromatium, Isochromatium, Marichromatium, Thiococcus, Thiohalocapsa and Thermochromatium | journal = International Journal of Systematic Bacteriology | volume = 48 Pt 4 | issue = 4 | pages = 1129–43 | date = October 1998 | pmid = 9828415 | doi = 10.1099/00207713-48-4-1129 | doi-access = free }} such as Allochromatium,{{cite journal | vauthors = Imhoff JF, Suling J, Petri R | title = Phylogenetic relationships among the Chromatiaceae, their taxonomic reclassification and description of the new genera Allochromatium, Halochromatium, Isochromatium, Marichromatium, Thiococcus, Thiohalocapsa and Thermochromatium|journal=International Journal of Systematic Bacteriology|date=1 October 1998|volume=48|issue=4|pages=1129–1143|doi=10.1099/00207713-48-4-1129| pmid = 9828415|doi-access=free}} and green sulfur bacteria (Chlorobiaceae), as well as the purple non-sulfur bacteria (Rhodospirillaceae){{cite journal | vauthors = Brune DC | title = Sulfur oxidation by phototrophic bacteria | journal = Biochimica et Biophysica Acta (BBA) - Bioenergetics | volume = 975 | issue = 2 | pages = 189–221 | date = July 1989 | pmid = 2663079 | doi = 10.1016/S0005-2728(89)80251-8 }} and some Cyanobacteria. The AnSOB Cyanobacteria are only able to oxidize sulfide to elemental sulfur and have been identified as Oscillatoria, Lyngbya, Aphanotece, Microcoleus, and Phormidium.{{Citation |last1=Pathak |first1=Jainendra |title=Evolution and Distribution of Cyanobacteria |date=2021 |work=Ecophysiology and Biochemistry of Cyanobacteria |pages=1–30 |editor-last=Rastogi |editor-first=Rajesh Prasad |url=https://link.springer.com/chapter/10.1007/978-981-16-4873-1_1 |access-date=2025-04-24 |place=Singapore |publisher=Springer Nature |language=en |doi=10.1007/978-981-16-4873-1_1 |isbn=978-981-16-4873-1 |last2=Singh |first2=Prashant R. |last3=Sinha |first3=Rajeshwar P. |last4=Rastogi |first4=Rajesh P.}} Some AnSOB, such as the facultative anaerobes Thiobacillus spp., and Thermothrix sp., are chemolithoautotrophs, meaning that they obtain energy from the oxidation of reduced sulfur species, which is then used to fix CO₂. Others, such as some filamentous gliding green bacteria (Chloroflexaceae), are mixotrophs. From all of the SOB, the only group that directly oxidize sulfide to sulfate in an abundance of oxygen without accumulating elemental sulfur are the Thiobacilli. The other groups accumulate elemental sulfur, which they may oxidize to sulfate when sulfide is limited or depleted.

SOB have prospective use in environmental and industrial settings for detoxifying hydrogen sulfide, soil bioremediation, and wastewater treatment. In highly basic and ionic environments, Thiobacillus thiooxidans has been observed to increase the pH of soil from 1.5pH to a neutral 7.0pH.{{Cite journal |last1=Bao |last2=Wang |last3=Bao |last4=Li |last5=Wang |date=2016-09-02 |title=Biological treatment of saline-alkali soil by Sulfur-oxidizing bacteria |journal=Bioengineered |volume=7 |issue=5 |pages=372–375 |doi=10.1080/21655979.2016.1226664 |issn=2165-5979 |pmc=5060977 |pmid=27558517}} The use of SOB in the detoxification of hydrogen sulfide can circumvent detrimental effects from the conventional oxidation methods of hydrogen peroxide (H₂O₂), chlorine gas (Cl₂), and hypochlorite (NaClO) usage.{{Cite journal |last1=Nguyen |first1=Phuong Minh |last2=Do |first2=Phuc Thi |last3=Pham |first3=Yen Bao |last4=Doan |first4=Thi Oanh |last5=Nguyen |first5=Xuan Cuong |last6=Lee |first6=Woo Kul |last7=Nguyen |first7=D. Duc |last8=Vadiveloo |first8=Ashiwin |last9=Um |first9=Myoung-Jin |last10=Ngo |first10=Huu Hao |date=2022-12-15 |title=Roles, mechanism of action, and potential applications of sulfur-oxidizing bacteria for environmental bioremediation |url=https://www.sciencedirect.com/science/article/abs/pii/S0048969722053025 |journal=Science of the Total Environment |volume=852 |pages=158203 |doi=10.1016/j.scitotenv.2022.158203 |pmid=36044953 |bibcode=2022ScTEn.85258203N |issn=0048-9697}} SOB of the Beggiotoa genera oxidize sulfur compounds in microaerophilic up-flow sludge beds during wastewater treatment, and can be combined with nitrogen-reducing bacteria to effectively remove chemical build-ups in industrial settings.{{Cite journal |last1=Pokorna |first1=Dana |last2=Zabranska |first2=Jana |date=2015-11-01 |title=Sulfur-oxidizing bacteria in environmental technology |url=https://www.sciencedirect.com/science/article/abs/pii/S0734975015000324 |journal=Biotechnology Advances |series=BioTech 2014 and 6th Czech-Swiss Biotechnology Symposium |volume=33 |issue=6, Part 2 |pages=1246–1259 |doi=10.1016/j.biotechadv.2015.02.007 |pmid=25701621 |issn=0734-9750}}

The chemolithotrophic subset of SOB are gram-negative, rod-shaped bacteria, which abide in a wide range of environments—from anoxic to oxic, 4 to 90°C, and 1 to 9pH.{{Cite journal |last1=Ranadev |first1=Praveen |last2=Revanna |first2=Ashwin |last3=Bagyaraj |first3=Davis Joseph |last4=Shinde |first4=Ambika H |date=2023-08-01 |title=Sulfur oxidizing bacteria in agro ecosystem and its role in plant productivity—a review |url=https://academic.oup.com/jambio/article/134/8/lxad161/7231082#412995560 |journal=Journal of Applied Microbiology |volume=134 |issue=8 |pages=lxad161 |doi=10.1093/jambio/lxad161 |pmid=37491695 |issn=1364-5072}} Chemolithotrophic SOB play a key role in agricultural ecosystems by oxidizing reduced sulfur fertilizers into available forms, such as sulfate, for plants. SOB are often present in agricultural ecosystems at low densities, creating the opportunity for inoculation to increase nutrient availability. Presence of Thiobacillus thiooxidans has been shown to increase phosphorus availability in addition to the oxidation of sulfur.{{Cite journal |last1=Awad |first1=Nemat M. |last2=Abd El-Kader |first2=A. A. |last3=Attia |first3=M. |last4=Alva |first4=A. K. |date=2011 |title=Effects of Nitrogen Fertilization and Soil Inoculation of Sulfur-Oxidizing or Nitrogen-Fixing Bacteria on Onion Plant Growth and Yield |journal=International Journal of Agronomy |volume=2011 |pages=1–6 |doi=10.1155/2011/316856 |doi-access=free |issn=1687-8159}} Utilization of SOB in treating alkaline and low available-sulfur soils, such as those in Iran, could directly increase crop yields in many ecosystems around the world.{{Cite journal |last1=Mirzaei |first1=Mahnaz |last2=Zand |first2=Eskandar |last3=Rastgoo |first3=Mehdi |last4=Hasanfard |first4=Alireza |last5=Kudsk |first5=Per |date=2023 |title=Effects and mitigation of poor water quality on herbicide performance: A review |url=https://onlinelibrary.wiley.com/doi/abs/10.1111/wre.12573 |journal=Weed Research |language=en |volume=63 |issue=3 |pages=139–152 |doi=10.1111/wre.12573 |bibcode=2023WeedR..63..139M |issn=1365-3180}}

Certain SOB have the potential to serve as biotic pesticides and anti-infectious agents for the control of crops.{{Cite journal |last1=Ranadev |first1=Praveen |last2=Revanna |first2=Ashwin |last3=Bagyaraj |first3=Davis Joseph |last4=Shinde |first4=Ambika H |date=2023-08-01 |title=Sulfur oxidizing bacteria in agro ecosystem and its role in plant productivity—a review |url=https://academic.oup.com/jambio/article/doi/10.1093/jambio/lxad161/7231082 |journal=Journal of Applied Microbiology |language=en |volume=134 |issue=8 |doi=10.1093/jambio/lxad161 |issn=1365-2672}} The benefits of utilization have been demonstrated through the outcomes of sulfur-oxidation, including balancing sodium content as well as increasing sulfur and phosphorus availability in the soil. Increased levels of reduced sulfur compounds in acidic soil permits the growth of Streptomyces scabies and S. ipomea, both pathogens of potato plants. Presence of SOB such as Thiobacillus have decreased the growth of these bacteria, as well as root pathogens such as Rhizoctonia solani. An additional impact of SOB on crop protection includes a collateral effect of increased sulfur content in plants, resulting in resistance to Rhizoctonia.

SOB such as Hallothiobacillus and Thiobacillus have been shown to play a role in regulating the pH of mining impoundment waters in an oscillating cycle over the course of several years.{{Cite journal |last1=Whaley-Martin |first1=Kelly J. |last2=Chen |first2=Lin-Xing |last3=Nelson |first3=Tara Colenbrander |last4=Gordon |first4=Jennifer |last5=Kantor |first5=Rose |last6=Twible |first6=Lauren E. |last7=Marshall |first7=Stephanie |last8=McGarry |first8=Sam |last9=Rossi |first9=Laura |last10=Bessette |first10=Benoit |last11=Baron |first11=Christian |last12=Apte |first12=Simon |last13=Banfield |first13=Jillian F. |last14=Warren |first14=Lesley A. |date=2023-04-10 |title=O2 partitioning of sulfur oxidizing bacteria drives acidity and thiosulfate distributions in mining waters |url=https://www.nature.com/articles/s41467-023-37426-8 |journal=Nature Communications |language=en |volume=14 |issue=1 |pages=2006 |doi=10.1038/s41467-023-37426-8 |issn=2041-1723}} In the presence of oxygen, Halothiobacillus drives the ecosystem into a low pH, down to 4.3, and significantly decreases thiosulfate (S2O32-) levels through the sulfur oxidation (Sox) pathway. In the absence of oxygen, Thiobacillus dominates, leading to increased thiosulfate without a shift in pH. The increase in thiosulfate results from an incomplete Sox pathway coupled with the oxidation of sulfide to sulfite in the reverse dissimilatory sulfite reduction (rDsr) pathway. These opposing pathways result in adverse events for downstream environments by blocking the discharge of sulfur compounds.

File:Sulfide oxidation pathways.jpg There are two described pathways for the microbial oxidation of sulfide:

• The sulfide:quinone oxidorreductase pathway (SQR), widespread in green sulfur bacteria, that involves the formation of intermediate compounds such as sulfite (SO₃^2-) and adenosine 5'-phosphosulfate (APS),{{cite journal | vauthors = Beller HR, Chain PS, Letain TE, Chakicherla A, Larimer FW, Richardson PM, Coleman MA, Wood AP, Kelly DP | title = The genome sequence of the obligately chemolithoautotrophic, facultatively anaerobic bacterium Thiobacillus denitrificans | journal = Journal of Bacteriology | volume = 188 | issue = 4 | pages = 1473–88 | date = February 2006 | pmid = 16452431 | pmc = 1367237 | doi = 10.1128/JB.188.4.1473-1488.2006 }} which are known to have a significant oxygen isotope exchange.{{cite journal|last1=Turchyn|first1=Alexandra V.|last2=Brüchert|first2=Volker|last3=Lyons|first3=Timothy W.|last4=Engel|first4=Gregory S.|last5=Balci|first5=Nurgul|last6=Schrag|first6=Daniel P.|last7=Brunner|first7=Benjamin | name-list-style = vanc |date=2010|title=Kinetic oxygen isotope effects during dissimilatory sulfate reduction: A combined theoretical and experimental approach |journal=Geochimica et Cosmochimica Acta|volume=74|issue=7|pages=2011–2024|doi=10.1016/j.gca.2010.01.004|bibcode=2010GeCoA..74.2011T|issn=0016-7037}} The step catalyzed by SQR can also be mediated by a membrane-bound flavocytochrome c-sulfide dehydrogenase (FCSD).{{Cite journal |last1=Sousa |first1=Filipe M. |last2=Pereira |first2=Juliana G. |last3=Marreiros |first3=Bruno C. |last4=Pereira |first4=Manuela M. |date=2018-09-01 |title=Taxonomic distribution, structure/function relationship and metabolic context of the two families of sulfide dehydrogenases: SQR and FCSD |url=https://linkinghub.elsevier.com/retrieve/pii/S0005272818300781 |journal=Biochimica et Biophysica Acta (BBA) - Bioenergetics |series=20th European Bioenergetics Conference |volume=1859 |issue=9 |pages=742–753 |doi=10.1016/j.bbabio.2018.04.004 |pmid=29684324 |issn=0005-2728}}
• The Sox pathway,{{cite journal | vauthors = Sievert SM, Scott KM, Klotz MG, Chain PS, Hauser LJ, Hemp J, Hügler M, Land M, Lapidus A, Larimer FW, Lucas S, Malfatti SA, Meyer F, Paulsen IT, Ren Q, Simon J | title = Genome of the epsilonproteobacterial chemolithoautotroph Sulfurimonas denitrificans | journal = Applied and Environmental Microbiology | volume = 74 | issue = 4 | pages = 1145–56 | date = February 2008 | pmid = 18065616 | pmc = 2258580 | doi = 10.1128/AEM.01844-07 | bibcode = 2008ApEnM..74.1145S }} or Kelly-Friedrich pathway as established in the Alphaproteobacteria Paracoccus spp., mediated by the thiosulfate-oxidizing multi-enzyme (TOMES) complex, in which sulfide or elemental sulfur form a complex with the enzyme SoxY and remain bound to it until its final conversion to sulfate.{{Cite book|last=Kelly DP|date=1989|chapter=Physiology and biochemistry of unicellular sulfur bacteria|title=Autotrophic Bacteria|editor1=Schlegel HG|editor2=Bowien B|publisher=Springer-Verlag|series=FEMS Symposium|pages=193–217}}{{Cite journal|last=Kelly DP, Shergill JK, Lu WP & Wood AP|author-link4=Ann P. Wood|date=1997|title=Oxidative metabolism of inorganic sulfur compounds by bacteria.|journal=Antonie van Leeuwenhoek|volume=71|issue=1–2|pages=95–107|pmid=9049021|doi=10.1023/A:1000135707181|s2cid=2057300}}

Similarly, two pathways for the oxidation of sulfite (SO₃^2-) have been identified:

• The rDsr pathway, used by some microorganisms in the Chlorobiota (green sulfur bacteria), Alpha, Beta and Gammaproteobacteria, in which sulfide is oxidized to sulfite by means of a reverse operation of the dissimilatory sulfite reduction (Dsr) pathway. The sulfite generated by rDsr is then oxidized to sulfate by other enzymes.{{cite book|title=Microbial Sulfur Metabolism|vauthors=Grimm F, Franz B, Dahl C|publisher=Springer-Verlag GmbH|year=2008|veditors=Friedrich C, Dahl C|location=Berlin Heidelberg|pages=101–116|chapter=Thiosulfate and sulfur oxidation in purple sulfur bacteria.}}
• The direct oxidation of sulfite to sulfate by a type of mononuclear molybdenum enzyme known as sulfite oxidoreductase. Three different groups of these enzymes are recognized (the xanthine oxidase, sulfite oxidase (SO) and dimethyl sulfoxide reductase families), and they are present in the three domains of life.{{Cite journal|last1=Kappler|first1=U.|last2=Dahl|first2=C.|date=2001-09-11|title=Enzymology and molecular biology of prokaryotic sulfite oxidation|journal=FEMS Microbiology Letters|volume=203|issue=1|pages=1–9|issn=0378-1097|pmid=11557133|doi=10.1111/j.1574-6968.2001.tb10813.x|doi-access=free}}

On the other hand, at least three pathways exist for the oxidation of thiosulfate (S₂O₃^2-):

• The aforementioned Sox pathway, through which both sulfur atoms in thiosulfate are oxidized to sulfate without the formation of any free intermediate.
• The oxidation of thiosulfate (S₂O₃^2-) via the formation of tetrathionate (S₄O₆^2-) intermediate, that is present in several obligate chemolithotrophic Gamma and Betaproteobacteria as well as in facultative chemolithotrophic Alphaproteobacteria.{{Cite journal|last1=Ghosh|first1=Wriddhiman|last2=Dam|first2=Bomba|date=2009|title=Biochemistry and molecular biology of lithotrophic sulfur oxidation by taxonomically and ecologically diverse bacteria and archaea|journal=FEMS Microbiology Reviews|volume=33|issue=6|pages=999–1043|doi=10.1111/j.1574-6976.2009.00187.x|issn=1574-6976|pmid=19645821|doi-access=free}}
• The branched thiosulfate oxidation pathway, a mechanism in which water-insoluble globules of intermediate sulfur are formed during the oxidation of thiosulfate and sulfide. It is present in all the anoxygenic photolithotrophic green and purple sulfur bacteria, and the free-living as well as symbiotic strains of certain sulfur-chemolithotrophic bacteria.{{Cite journal|last=Dahl, C., Rákhely, G., Pott-Sperling, A. S., Fodor, B., Takács, M., Tóth, A., Kraeling, M., Győrfi, K., Kovács, A., Tusz, J. & Kovács, K. L.|date=1999|title=Genes involved in hydrogen and sulfur metabolism in phototrophic sulfur bacteria|journal=FEMS Microbiology Letters|language=en|volume=180|issue=2|pages=317–324|doi=10.1111/j.1574-6968.1999.tb08812.x|pmid=10556728|issn=0378-1097|doi-access=free}}

In any of these pathways, oxygen is the preferred electron acceptor, but in oxygen-limited environments, nitrate, oxidized forms of iron and even organic matter are used instead.{{cite journal | vauthors = Rivett MO, Buss SR, Morgan P, Smith JW, Bemment CD | title = Nitrate attenuation in groundwater: a review of biogeochemical controlling processes | journal = Water Research | volume = 42 | issue = 16 | pages = 4215–32 | date = October 2008 | pmid = 18721996 | doi = 10.1016/j.watres.2008.07.020 | bibcode = 2008WatRe..42.4215R }}

Cyanobacteria normally perform oxygenic photosynthesis by utilizing water as an electron donor. However, in the presence of sulfide, oxygenic photosynthesis is inhibited, and some cyanobacteria can perform anoxygenic photosynthesis by the oxidation of sulfide to thiosulfate by using Photosystem I with sulfite as a possible intermediate sulfur compound.{{cite journal|last1=Wit|first1=Rutger|last2=Gemerden|first2=Hans | name-list-style = vanc |date=1987|title=Oxidation of sulfide to thiosulfate by Microcoleus chtonoplastes |journal=FEMS Microbiology Letters|volume=45|issue=1|pages=7–13|doi=10.1111/j.1574-6968.1987.tb02332.x |doi-access=free}}{{cite journal | vauthors = Rabenstein A, Rethmeier J, Fischer U | date=2014|title=Sulphite as Intermediate Sulphur Compound in Anaerobic Sulphide Oxidation to Thiosulphate by Marine Cyanobacteria |journal=Zeitschrift für Naturforschung C|volume=50 | issue = 11–12 | pages = 769–774 | doi=10.1515/znc-1995-11-1206| doi-access=free }}

Sulfide oxidation can proceed under aerobic or anaerobic conditions. Aerobic sulfide-oxidizing bacteria usually oxidize sulfide to sulfate and are obligate or facultative chemolithoautotrophs. The latter can grow as heterotrophs, obtaining carbon from organic sources, or as autotrophs, using sulfide as the electron donor (energy source) for CO₂ fixation. The oxidation of sulfide can proceed aerobically by two different mechanisms: substrate-level phosphorylation, which is dependent on adenosine monophosphate (AMP), and oxidative phosphorylation independent of AMP,{{cite book | chapter = Chemolithotrophy | vauthors = Wood P | author-link1=Ann P. Wood|veditors = Anthony C | title = Bacterial Energy Transduction | location = London, UK | publisher = Academic Press | pages = 183–230 |year=1988 }} which has been detected in several Thiobacilli (T. denitrificans, T. thioparus, T. novellus and T. neapolitanus), as well as in Acidithiobacillus ferrooxidans.{{Citation |last=Kelly |first=Donovan P. |title=Microbial Inorganic Sulfur Oxidation: The APS Pathway |date=2003 |work=Biochemistry and Physiology of Anaerobic Bacteria |pages=205–219 |editor-last=Ljungdahl |editor-first=Lars G. |url=https://link.springer.com/chapter/10.1007/0-387-22731-8_15 |access-date=2025-04-24 |place=New York, NY |publisher=Springer |language=en |doi=10.1007/0-387-22731-8_15 |isbn=978-0-387-22731-3 |editor2-last=Adams |editor2-first=Michael W. |editor3-last=Barton |editor3-first=Larry L. |editor4-last=Ferry |editor4-first=James G.}} The archaeon Acidianus ambivalens appears to possess both an ADP-dependent and an ADP-independent pathway for the oxidation of sulfide.{{cite journal | vauthors = Zimmermann P, Laska S, Kletzin A | title = Two modes of sulfite oxidation in the extremely thermophilic and acidophilic archaeon acidianus ambivalens | journal = Archives of Microbiology | volume = 172 | issue = 2 | pages = 76–82 | date = August 1999 | pmid = 10415168 | doi = 10.1007/s002030050743 | bibcode = 1999ArMic.172...76Z | s2cid = 9216478 }} Similarly, both mechanisms operate in the chemoautotroph Thiobacillus denitrificans,{{cite journal | vauthors = Aminuddin M | title = Substrate level versus oxidative phosphorylation in the generation of ATP in Thiobacillus denitrificans | journal = Archives of Microbiology | volume = 128 | issue = 1 | pages = 19–25 | date = November 1980 | pmid = 7458535 | doi = 10.1007/BF00422300 | bibcode = 1980ArMic.128...19A | s2cid = 13042589 }} which can oxidize sulfide to sulfate anaerobically by utilizing nitrate—which is reduced to dinitrogen (N₂)—as a terminal electron acceptor.{{Cite journal |last1=Beller |first1=Harry R. |last2=Chain |first2=Patrick S. G. |last3=Letain |first3=Tracy E. |last4=Chakicherla |first4=Anu |last5=Larimer |first5=Frank W. |last6=Richardson |first6=Paul M. |last7=Coleman |first7=Matthew A. |last8=Wood |first8=Ann P. |last9=Kelly |first9=Donovan P. |date=2006-02-15 |title=The Genome Sequence of the Obligately Chemolithoautotrophic, Facultatively Anaerobic Bacterium Thiobacillus denitrificans |journal=Journal of Bacteriology |volume=188 |issue=4 |pages=1473–1488 |doi=10.1128/jb.188.4.1473-1488.2006 |pmc=1367237 |pmid=16452431}} Two other anaerobic strains that can perform a similar process were identified as similar to Thiomicrospira denitrificans and Arcobacter.{{cite journal | vauthors = Gevertz D, Telang AJ, Voordouw G, Jenneman GE | title = Isolation and characterization of strains CVO and FWKO B, two novel nitrate-reducing, sulfide-oxidizing bacteria isolated from oil field brine | journal = Applied and Environmental Microbiology | volume = 66 | issue = 6 | pages = 2491–501 | date = June 2000 | pmid = 10831429 | pmc = 110567 | doi = 10.1128/AEM.66.6.2491-2501.2000 | bibcode = 2000ApEnM..66.2491G }}

Among the heterotrophic SOB are included species of Beggiatoa that can grow mixotrophically, using sulfide to obtain energy (autotrophic metabolism) or to eliminate metabolically formed hydrogen peroxide in the absence of catalase (heterotrophic metabolism).{{Citation |last1=Teske |first1=Andreas |title=The Genera Beggiatoa and Thioploca |date=2006 |work=The Prokaryotes: A Handbook on the Biology of Bacteria Volume 6: Proteobacteria: Gamma Subclass |pages=784–810 |editor-last=Dworkin |editor-first=Martin |url=https://link.springer.com/referenceworkentry/10.1007/0-387-30746-X_27 |access-date=2025-04-24 |place=New York, NY |publisher=Springer |language=en |doi=10.1007/0-387-30746-x_27 |isbn=978-0-387-30746-6 |last2=Nelson |first2=Douglas C. |editor2-last=Falkow |editor2-first=Stanley |editor3-last=Rosenberg |editor3-first=Eugene |editor4-last=Schleifer |editor4-first=Karl-Heinz}} Other organisms, such as the Bacteria Sphaerotilus natans{{Cite journal |last1=Seder-Colomina |last2=Guillaume |last3=Benzerara |last4=Ona-Nguema |last5=Pernelle |last6=Esposito |last7=Hullebusch |date=2014-01-02 |title=Sphaerotilus natans, a Neutrophilic Iron-Related Sheath-Forming Bacterium: Perspectives for Metal Remediation Strategies |url=https://www.tandfonline.com/doi/abs/10.1080/01490451.2013.806611 |journal=Geomicrobiology Journal |volume=31 |issue=1 |pages=64–75 |doi=10.1080/01490451.2013.806611 |bibcode=2014GmbJ...31...64S |issn=0149-0451}} and the yeast Alternaria{{Cite journal |last1=Gai |first1=Yunpeng |last2=Li |first2=Lei |last3=Ma |first3=Haijie |last4=Riely |first4=Brendan K. |last5=Liu |first5=Bing |last6=Li |first6=Hongye |date=2021-01-29 |title=Critical Role of MetR/MetB/MetC/MetX in Cysteine and Methionine Metabolism, Fungal Development, and Virulence of Alternaria alternata |journal=Applied and Environmental Microbiology |volume=87 |issue=4 |pages=e01911–20 |doi=10.1128/AEM.01911-20 |pmc=7851696 |pmid=33277273|bibcode=2021ApEnM..87E1911G }} are able to oxidize sulfide to elemental sulfur by means of the rDsr pathway.{{cite journal | vauthors = Belousova EV, Chernousova EI, Dubinina GA, Turova TP, Grabovich MI | title = [Detection and analysis of sulfur metabolism genes in Sphaerotilus natans subsp. sulfidivorans representatives] | journal = Mikrobiologiia | volume = 82 | issue = 5 | pages = 579–87 | date = 2013 | pmid = 25509396 }}

Some Bacteria and Archaea can aerobically oxidize elemental sulfur to sulfuric acid. Acidithiobacillus ferrooxidans and Thiobacillus thioparus can oxidize sulfur to sulfite by means of an oxygenase enzyme, although it is hypothesized that an oxidase could also serve as an energy saving mechanism.{{Cite journal |last1=Wang |first1=Rui |last2=Lin |first2=Jian-Qiang |last3=Liu |first3=Xiang-Mei |last4=Pang |first4=Xin |last5=Zhang |first5=Cheng-Jia |last6=Yang |first6=Chun-Long |last7=Gao |first7=Xue-Yan |last8=Lin |first8=Chun-Mao |last9=Li |first9=Ya-Qing |last10=Li |first10=Yang |last11=Lin |first11=Jian-Qun |last12=Chen |first12=Lin-Xu |date=2019-01-10 |title=Sulfur Oxidation in the Acidophilic Autotrophic Acidithiobacillus spp. |journal=Frontiers in Microbiology |language=English |volume=9 |doi=10.3389/fmicb.2018.03290 |doi-access=free |issn=1664-302X |pmc=6335251 |pmid=30687275}} In the anaerobic oxidation of elemental sulfur, it is hypothesized that the Sox pathway plays an significant role, although the complexity of this pathway is not yet thoroughly understood.{{cite journal | vauthors = Friedrich CG, Rother D, Bardischewsky F, Quentmeier A, Fischer J | title = Oxidation of reduced inorganic sulfur compounds by bacteria: emergence of a common mechanism? | journal = Applied and Environmental Microbiology | volume = 67 | issue = 7 | pages = 2873–82 | date = July 2001 | pmid = 11425697 | pmc = 92956 | doi = 10.1128/AEM.67.7.2873-2882.2001 | bibcode = 2001ApEnM..67.2873F }} Thiobacillus denitrificans uses oxidized forms of nitrogen as an energy source and terminal electron acceptor instead of oxygen.{{Cite journal |last1=Zhou |first1=Xiaofang |last2=Huang |first2=Shaofu |last3=Chen |first3=Xiangyu |last4=Jianxiong Zeng |first4=Raymond |last5=Zhou |first5=Shungui |last6=Chen |first6=Man |date=2023-07-15 |title=Mechanisms of extracellular photoelectron uptake by a Thiobacillus denitrificans-cadmium sulfide biosemiconductor system |url=https://linkinghub.elsevier.com/retrieve/pii/S1385894723023987 |journal=Chemical Engineering Journal |volume=468 |pages=143667 |doi=10.1016/j.cej.2023.143667 |bibcode=2023ChEnJ.46843667Z |issn=1385-8947}}

Most of the chemosynthetic autotrophic bacteria that can oxidize elemental sulfur to sulfate are also able to oxidize thiosulfate to sulfate as a source of reducing power for carbon dioxide assimilation. However, the mechanisms that these bacteria utilize may vary, since some species, such as the photosynthetic purple bacteria, transiently accumulate extracellular elemental sulfur during the oxidation of tetrathionate, while other species, such as the green sulfur bacteria, do not. A direct oxidation reaction (T. versutus {{cite journal|last=Lu W-P.|date=1986|title=A periplasmic location for the bisulfiteoxidizing multienzyme system from Thiobacillus versutus.|journal=FEMS Microbiol Lett|volume=34|issue=3|pages=313–317|doi=10.1111/j.1574-6968.1986.tb01428.x|doi-access=free}}), as well as others that involve sulfite (T. denitrificans) and tetrathionate (A. ferrooxidans, A. thiooxidans, and Acidiphilum acidophilum {{cite journal|last=Pronk|first=J|date=1990|title=Oxidation of reduced inorganic sulphur compounds by acidophilic thiobacilli|journal=FEMS Microbiology Letters|volume=75|issue=2–3|pages=293–306|doi=10.1111/j.1574-6968.1990.tb04103.x|issn=0378-1097|doi-access=free}}) as intermediate compounds, have been proposed. Some mixotrophic bacteria only oxidize thiosulfate to tetrathionate.

The mechanism of bacterial oxidation of tetrathionate is still unclear and may involve sulfur disproportionation, during which both sulfide and sulfate are produced from reduced sulfur species and hydrolysis reactions.

The fractionation of sulfur and oxygen isotopes during microbial sulfide oxidation (MSO) has been studied to assess its potential as a proxy to differentiate it from the abiotic oxidation of sulfur.{{cite journal|vauthors=Knöller K, Vogt C, Feisthauer S, Weise SM, Weiss H, Richnow HH|date=November 2008|title=Sulfur cycling and biodegradation in contaminated aquifers: insights from stable isotope investigations|journal=Environmental Science & Technology|volume=42|issue=21|pages=7807–12|pmid=19031864|doi=10.1021/es800331p|bibcode=2008EnST...42.7807V}} The light isotopes of the elements that are most commonly found in organic molecules, such as ¹²C, ¹⁶O, ¹H, ¹⁴N and ³²S, form bonds that are broken slightly more easily than bonds between the corresponding heavy isotopes, ¹³C, ¹⁸O, ²H, ¹⁵N and ³⁴S. Because there is a lower energetic cost associated with the use of light isotopes, enzymatic processes usually discriminate against the heavy isotopes, and, as a consequence, biological fractionations of isotopes are expected between the reactants and the products. A normal kinetic isotope effect is that in which the products are depleted significantly in the heavy isotope relative to the reactants (low heavy isotope to light isotope ratio), and although this is not always the case, the study of isotope fractionations between enzymatic processes may enable tracing of the source of the product.

The formation of sulfate in aerobic conditions entails the incorporation of four oxygen atoms from water, and when coupled with dissimilatory nitrate reduction (DNR)—the preferential reduction pathway under anoxic conditions—this process can involve an additional contribution of oxygen atoms from nitrate. The δ¹⁸O value of the newly formed sulfate thus depends on the δ¹⁸O value of the water, the isotopic fractionation associated with the incorporation of oxygen atoms from water to sulfate and a potential exchange of oxygen atoms between sulfur and nitrogen intermediates and water.{{cite journal | vauthors = Poser A, Vogt C, Knöller K, Ahlheim J, Weiss H, Kleinsteuber S, Richnow HH | title = Stable sulfur and oxygen isotope fractionation of anoxic sulfide oxidation by two different enzymatic pathways | journal = Environmental Science & Technology | volume = 48 | issue = 16 | pages = 9094–102 | date = August 2014 | pmid = 25003498 | doi = 10.1021/es404808r | bibcode = 2014EnST...48.9094P }} MSO has been found to produce small fractionations in ¹⁸O compared to water (~5‰). Given the very small fractionation of ¹⁸O that usually accompanies MSO, the relatively higher depletions in ¹⁸O of the sulfate produced by MSO coupled to DNR (-1.8 to -8.5 ‰) suggest a kinetic isotope effect in the incorporation of oxygen from water to sulfate and the role of nitrate as a potential alternative source of light oxygen. The fractionations of oxygen produced by sulfur disproportionation from elemental sulfur have been found to be higher, with reported values from 8 to 18.4‰, which suggests a kinetic isotope effect in the pathways involved in the oxidation of elemental sulfur to sulfate, although more studies are necessary to determine what are the specific steps and conditions that favor this fractionation. The table below summarizes the reported fractionations of oxygen isotopes from MSO in different organisms and conditions.

Aerobic MSO generates depletions in the ³⁴S of sulfate that have been found to be as small as −1.5‰ and as large as -18‰. For most microorganisms and oxidation conditions, only small fractionations accompany either the aerobic or anaerobic oxidation of sulfide, elemental sulfur, thiosulfate and sulfite to elemental sulfur or sulfate. The phototrophic oxidation of sulfide to thiosulfate under anoxic conditions also generates negligible fractionations. Although the change in sulfur isotopes is usually small during MSO, MSO oxidizes reduced forms of sulfur which are usually depleted in ³⁴S compared to seawater sulfate. Therefore, large-scale MSO can also significantly affect the sulfur isotopes of a reservoir. It has been proposed that the observed global average S-isotope fractionation is around −50‰, instead of the theoretically predicted value of -70‰, because of MSO.{{Cite journal |last1=Tsang |first1=Man-Yin |last2=Wortmann |first2=Ulrich G. |date=2022-07-06 |title=Sulfur isotope fractionation derived from reaction-transport modelling in the Eastern Equatorial Pacific |url=https://doi.org/10.1144/jgs2021-068 |journal=Journal of the Geological Society |volume=179 |issue=5 |doi=10.1144/jgs2021-068 |bibcode=2022JGSoc.179...68T |s2cid=248647580 |issn=0016-7649}}

In the chemolithotrophs Thiobacillus denitrificans and Sulfurimonas denitrificans, MSO coupled with DNR has the effect of inducing the SQR and Sox pathways, respectively. In both cases, a small fractionation in the ³⁴S of the sulfate, lower than -4.3‰, has been measured. Sulfate depletion in ³⁴S from MSO could be used to trace sulfide oxidation processes in the environment, although a distinction between the SQR and Sox pathways is not currently possible. The depletion produced by MSO coupled to DNR is similar to up to -5‰ depletion estimated for the ³⁴S in the sulfide produced from rDsr.{{cite journal|last1=Brunner|first1=Benjamin|last2=Bernasconi|first2=Stefano M.|last3=Kleikemper|first3=Jutta|last4=Schroth|first4=Martin H. | name-list-style = vanc |date=2005|title=A model for oxygen and sulfur isotope fractionation in sulfate during bacterial sulfate reduction processes |journal=Geochimica et Cosmochimica Acta|volume=69|issue=20|pages=4773–4785|doi=10.1016/j.gca.2005.04.017|bibcode=2005GeCoA..69.4773B|issn=0016-7037}}{{cite journal|last1=Brunner|first1=Benjamin|last2=Bernasconi|first2=Stefano M.| name-list-style = vanc |date=2005|title=A revised isotope fractionation model for dissimilatory sulfate reduction in sulfate reducing bacteria |journal=Geochimica et Cosmochimica Acta|volume=69|issue=20|pages=4759–4771|doi=10.1016/j.gca.2005.04.015|bibcode=2005GeCoA..69.4759B|issn=0016-7037}} In contrast, disproportionation under anaerobic conditions generates sulfate enriched in ³⁴S up to 9‰ and ~34‰ from sulfide and elemental sulfur, respectively. The isotope effect of disproportionation is, however, limited by the rates of sulfate reduction and MSO.{{Cite journal |last1=Tsang |first1=Man-Yin |last2=Böttcher |first2=Michael Ernst |last3=Wortmann |first3=Ulrich Georg |date=2023-08-20 |title=Estimating the effect of elemental sulfur disproportionation on the sulfur-isotope signatures in sediments |url=https://www.sciencedirect.com/science/article/pii/S0009254123002334 |journal=Chemical Geology |language=en |volume=632 |pages=121533 |doi=10.1016/j.chemgeo.2023.121533 |s2cid=258600480 |issn=0009-2541}} Similar to the fractionation of oxygen isotopes, the larger fractionations in sulfate from the disproportionation of elemental sulfur point to a key step or pathway critical for inducing this large kinetic isotope effect. The table below summarizes the reported fractionations of sulfur isotopes from MSO in different organisms and conditions.

• Isotope fractionation
• Microbial metabolism
• Sulfur metabolism
• Sulfate-reducing microorganisms
• Dissimilatory sulfate reduction
• Dissimilatory nitrate reduction

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{{Microorganisms}}

Category:Sulfur metabolism

Category:Trophic ecology

Category:Bacterial substances