sulfate are also able to oxidize thiosulfate to sulfate as a source of reducing power for carbon dioxide assimilation. however, the mechanisms that they utilize may vary, since some of them, such as the photosynthetic purple bacteria, transiently accumulate extracellular elemental sulfur during the oxidation of tetrathionate before oxidizing it to sulfate, while the green sulfur bacteria do not. a direct oxidation reaction (t. versutus ), as well as others that involve sulfite (t. denitrificans) and tetrathionate (a. ferrooxidans, a. thiooxidans and acidiphilum acidophilum ) 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. isotope fractionations 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. 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 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 allow tracing the source of the product. fractionation of oxygen isotopes 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- can have a contribution of oxygen atoms from nitrate as well. 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. 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 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. starting compound (reactant) intermediate or end compounds(products) organism average 18o fractionation (product/reactant) details reference sulfide sulfate a. ferrooxidans (chemolithotroph) 4.1‰ (30 °c) aerobic taylor et al. (1984) a. ferrooxidans (chemolithotroph) 6.4‰ 3.8‰ (no temperature provided) aerobic anaerobic thurston et al. (2010) thiomicrospira sp. strain cvo (chemolithotroph) 0‰ (no temperature provided) anaerobic, coupled to dnr hubert et al. (2009) t. denitrificans (chemolithotroph)sulfurimonas denitrificans (chemolithotroph) −6 to −1.8‰ (30 °c) −8.5 to −2.1‰ (21 °c) anaerobic, coupled to dnr, sqr pathwayanaerobic, coupled to dnr, sox pathway poser et al. (2014) elemental sulfur sulfate desulfocapsa thiozymogenes (chemolithotroph; "cable bacteria") enrichment culture 11.0 to 18.4‰ (28 °c) 12.7 to 17.9‰ (28 °c) disproportionation, in the presence of iron scavengers böttcher et al. (2001) desulfocapsa thiozymogenes (chemolithotroph; "cable bacteria") enrichment culture 8 to 12 ‰ (28 °c) disproportionation, attenuated isotope effect due to reoxidation by manganese oxides böttcher & thamdrup (2001) fractionation of sulfur isotopes 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 change 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 -70‰ because of mso. in the chemolithotrophs thiobacillus denitrificans and sulfurimonas denitrificans, mso coupled to 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 it does not allow a discrimination between the sqr and sox pathways. 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. 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. just like 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. starting compound (reactant) intermediate or end compounds(products) organism average 34s fractionation (product/reactant) details oxidant reference sulfide sulfate t. neopolitanus, t. intermedius and t. ferrooxidans (chemolithotrophs) -2 to -5.5‰ (no temperature provided) aerobicph 5 to 6 carbon dioxide toran (1986) polythionates (sno62-)elemental sulfursulfate t. concretivorus (chemolithotroph) 0.6 to 19‰ (30 °c)-2.5 to 1.2‰ (30 °c)-18 to -10.5‰ (30 °c) aerobic carbon dioxide kaplan & rittenberg (1964) sulfate a. ferrooxidans (chemolithotroph) −1.5‰−4‰ (no temperature provided) aerobic anaerobic carbon dioxide thurston et al. (2010) sulfate t. denitrificans (chemolithotroph)sulfurimonas denitrificans (chemolithotroph) −4.3 to −1.3‰ (30 °c) −2.9 to −1.6‰ (28 °c) anaerobic, coupled to dnr, sqr pathwayanaerobic, coupled to dnr, sox pathway carbon dioxide poser et al. (2014) sulfate thiomicrospira sp. strain cvo (chemolithotroph) 1‰ (no temperature provided) anaerobic, coupled to dnr, no intermediates in complete oxidation of sulfide to sulfate (potentially only uses sox pathway) carbon dioxide hubert et al. (2009) elemental sulfur chlorobium thiosulphatophilum(green sulfur bacteria) 5‰ (no temperature provided) anaerobic carbon dioxide kelly et al. (1979) thiosulfate oscillatoria sp. (cyanobacteria) calothrix sp. (cyanobacteria) 0‰ (30 °c) anaerobic, anoxygenic photosynthesis carbon dioxide habicht et al.(1988) elemental sulfur sulfate chromatium vinosum (purple sulfur bacteria) 0‰ (30-35 °c) 2‰ (30-35 °c) anaerobic, anoxygenic photosynthesis fry et al. (1985) elemental sulfur sulfate ectothiorhodospira shaposhnikovii (purple sulfur bacteria) ±5‰ (no temperature provided) anaerobic, anoxygenic photosynthesis ivanov et al. (1976) polythionates (sno62-)elemental sulfursulfate chromatium sp. (purple sulfur bacteria) 4.9 to 11.2‰ (30 °c)-10 to -3.6‰ (30 °c)-2.9 to -0.9‰ (30 °c) anaerobic kaplan & rittenberg (1964) thiosulfate sulfate t. intermedius (chemolithotroph) -4.7‰ (no temperature provided) aerobic kelly et al. (1979) sulfate t. versutus (chemolithotroph) 0‰ (28 °c) aerobic fry et al. (1986) elemental sulfur + sulfate chromatium vinosum (purple sulfur bacteria) 0‰ (30-35 °c) anaerobic fry et al. (1985) sulfate desulfovibrio sulfodismutans (chemolithotroph) d. thiozymogenes (chemolithotroph; "cable bacteria") for both bacteria: 0‰ (30 °c; compared to the sulfonate functional group); 2 to 4‰ (30 °c; compared to the sulfane functional group) anaerobic, disproportionation habicht et al.(1988) elemental sulfur sulfate desulfocapsa thiozymogenes (chemolithotroph; "cable bacteria") enrichment culture 17.4‰ (28 °c) 16.6‰ (28 °c) anaerobic, disproportionation, in the presence of iron scavengers böttcher et al. (2001) desulfocapsa sulfoexigens desulfocapsa thiozymogenes (chemolithotrophs; "cable bacteria") desulfobulbus propionicus (chemoorganotroph) marine enrichments and sediments 16.4‰ (30 °c) 17.4‰ (30 °c) 33.9‰ (35 °c) 17.1 to 20.6‰ (28 °c) anaerobic, disproportionation canfield et al. (1998) desulfocapsa thiozymogenes (chemolithotroph; "cable bacteria") enrichment culture −0.6 to 2.0‰ (28 °c) −0.2 to 1.1‰ (28 °c) anaerobic, disproportionation, attenuated isotope effect due to reoxidation by manganese oxides böttcher & thamdrup (2001) sulfite sulfate desulfovibrio sulfodismutans (chemolithotroph) d. thiozymogenes (chemolithotroph; "cable bacteria") 9 to 12‰ (30 °c) 7 to 9‰ (30 °c) anaerobic, disproportionation habicht et al.(1988)