Stable Sulfur and Oxygen Isotope Fractionation of Anoxic Sulfide Oxidation by Two Different Enzymatic Pathways

The microbial oxidation of sulfide is a key reaction of the microbial sulfur cycle, recycling sulfur in its most reduced valence state back to more oxidized forms usable as electron acceptors. Under anoxic conditions, nitrate is a preferential electron acceptor for this process. Two enzymatic pathways have been proposed for sulfide oxidation under nitrate reducing conditions, the sulfide:quinone oxidoreductase (SQR) pathway and the Sox (sulfur oxidation) system. In experiments with the model strains Thiobacillus denitrificans and Sulfurimonas denitrificans, both pathways resulted in a similar small sulfur and oxygen isotope fractionation of -2.4 to -3.6 parts per thousand for S-34 and -2.4 to -3.4 parts per thousand for O-18. A similar pattern was detected during the oxidation of sulfide in a column percolated with sulfidic, nitrate amended groundwater. In experiments with O-18-labeled water, a strong oxygen isotope fractionation was observed for T. denitrificans and S. denitrificans, indicating a preferential incorporation of O-18-depleted oxygen released as water by nitrate reduction to nitrogen. The study indicates that nitrate-dependent sulfide oxidation might be monitored in the environment by analysis of O-18-depleted sulfate.