Mass-Independent Isotope Variations

Sulfur isotopic compositions have been demonstrated as excellent biomarkers of microbial life and unique proxies of ancient atmospheric and oceanic chemistry. This study investigates multiple sulfur isotope fractionation during microbial sulfate reduction, focusing on three sulfate-reducing bacteria: Desulfovibrio gigas, Thermodesulfovibrio yellowstonii, and Desulfobacter hydrogenophilius. Batch culture experiments were performed under controlled conditions to achieve optimal growth regarding electron donors, salinity, and temperature. The isotopic composition was measured using isotope-ratio mass spectrometry, and the isotopic fractionation was calculated based on a Rayleigh fractionation model. Results indicate an excellent agreement of fractionation factors with previous studies for Thermodesulfovibrio yellowstonii, ranging from 7.6 to 21.7 per mil. The λ values obtained varied between 0.5092 and 0.5122, aligning with model predictions and supporting previous results. However, no systematic correlation was found regarding the variation of organic substrate and temperature across different strains. This research provides insights into the isotopic dynamics of microbial sulfate reduction and its implications for understanding past environmental conditions.