Sulfur and oxygen isotope fractionation during bacterial sulfur disproportionation under anaerobic haloalkaline conditions

ABSTRACTSulfur and oxygen isotope fractionation of elemental sulfur disproportionation at anaerobic haloalkaline conditions were evaluated for the first time. Isotope enrichment factors of the strains <i>Desulfurivibrio alkaliphilus</i> and <i>Dethiobacter alkaliphilus</i> growing at pH 9 or 10 were −0.9 ‰ to −1 ‰ for sulfide (³⁴ϵ), +3.6 ‰ to +4.7 ‰ for sulfate (³⁴ϵ), and for oxygen in sulfate (¹⁸ϵ) in the range of +3.5 ‰ to +7.7 ‰. These values are significantly smaller compared to previously published values of sulfur disproportionators at neutral pH. We propose that this discrepancy is caused by masking effects due to preferential formation of polysulfides at high pH leading to accelerated internal sulfur turnover rates, but cannot rule out distinct isotope effects due to specific enzymatic disproportionation reactions under haloalkaline conditions. The results imply that the microbial sulfur cycle in haloalkaline environments is characterized by specific stable sulfur and oxygen isotope patterns.