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 <i>Thiobacillus denitrificans</i> and <i>Sulfurimonas denitrificans</i>, both pathways resulted in a similar small sulfur and oxygen isotope fractionation of −2.4 to −3.6‰ for ³⁴S and −2.4 to −3.4‰ for ¹⁸O. A similar pattern was detected during the oxidation of sulfide in a column percolated with sulfidic, nitrate amended groundwater. In experiments with ¹⁸O-labeled water, a strong oxygen isotope fractionation was observed for <i>T. denitrificans</i> and <i>S. denitrificans</i>, indicating a preferential incorporation of ¹⁸O-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 ¹⁸O-depleted sulfate