Large sulfur isotope fractionation during abiotic consumption of hydrogen sulfide after cessation of bacterial growth in batch culture experiments of sulfate reducing bacteria

Microbial sulfate reduction is a key process in the oceanic sulfur cycle and imparts a large sulfur isotope fractionation. The mechanism of sulfur isotope fractionation during microbial sulfate reduction has been studied from the geochemical and biochemical aspects since the 1950s. Recently, however, the large sulfur isotope fractionation, exceeding 47‰, has been observed in some pure culture experiments of sulfate reducing bacteria and the tentative understanding of microbial sulfur isotope fractionation has not well explained the mechanism. Here we quantified growth phase dependent sulfur isotope fractionation of a type sulfate reducer Desulfovibrio desulfuricans (DSM642) and revealed that the magnitude of isotope fractionation increased from -13.4 ± 3.6‰ in early exponential phase to -65.9 ± 21.0‰ in later exponential phase having clear negative correlation with cell specific sulfate reduction rate. Our results show the importance of cells growth phase and states that control cell specific sulfate reduction rate and sulfur isotope fractionation. Although microbial sulfate reduction likely continued to stationary phase of cells, microbial sulfur isotope fractionation could not be quantified because of the decreasing total dissolved sulfide (ΣH2S: H2S, HS−, and S2−) concentration after stationary phase. A non-microbial ΣH2S-consuming reaction is interpreted to have occurred during the stationary phase and this had large sulfur isotope fractionations, -10.5 ± 1.1‰ and -45.6 ± 12.4‰. Besides, the non-microbial ΣH2S-consuming reaction decreased Δ33S’ value of the ΣH2S. While the end-product of the non-microbial ΣH2S-consuming reaction in the stationary phase remains unidentified, we observed precipitates of some sulfide minerals and organic sulfur in the stationary phase media. Regardless of the end-product, the reaction increases the sulfur isotopic composition of dissolved ΣH2S, which can account for the high sulfur isotopic compositions of ΣH2S compared to pyrite and organic sulfur observed in some modern marine sediments.