Rates of microbial elemental sulfur oxidation and 18O and 34S isotopic fractionation under varied nutrient and temperature regimes

Globally escalating excess supplies of elemental S (S0) are often stored in large outdoor S0 blocks that are exposed over time to the environment. Here S0 oxidation experiments were conducted under ambient conditions to investigate (i) the rate of S0 oxidation at a range of ambient temperatures (6–32 °C) in the presence and absence of Acidithiobacillus thiooxidans   and (ii) the O- and S-isotope fractionation during oxidation to identify sources of O incorporated into the resulting SO42-. Experiments were conducted using four different δ18OH2O values in a nutrient and nutrient free solution. Results showed that >95% of total SO42- generated could be attributed to autotrophic microbial activity. Experiments conducted in the nutrient solution showed oxidation rates increased with temperature (Q10 ≈ 1.7–1.9); while experiments conducted in a nutrient-free solution showed no increase in oxidation rate with temperatures between 12 and 32 °C. The contribution of water-derived O to SO42- by S0 oxidation ranged from 84% to 97% for all treatments. The final δ18O(SO4) value indicated nearly all SO42- oxygen originates from water, and the ε18OSO4–H2O was estimated to be between −0.9‰ and −6.2‰ with a mean of −3.6 ± 2.7‰ for the nutrient tests and +1.1‰ to −3.4‰ with a mean of −1.5 ± 2.4‰ for nutrient free tests. A relationship between the proportions of water O incorporated from O2 or water into the SO42- and temperature was not observed. The δ34S data showed that the oxidation of S0 produces SO42- with a δ34S value that is nearly indistinguishable from the parent S0. The findings suggest that SO42- accumulation in S0 block effluent would be minimized by limiting H2O influx and preventing microbial inhabitation of the block.

► Measured S oxidation rates in a nutrient solution were 9.9 × 10−4 (6°C) to 6.2 × 10−3 (32°C) μg S0 cm-2 d-1 (Q10 ≈ 1.6-1.9). ► Measured S oxidation rates in a DI water solution were (3.8-4.9 × 10−4 μg S0 cm−2 d−1) and did not increase with temperature. ► Based on in situ hybridization analyses, > 95% of total SO42- generated is attributed to autotrophic microbial activity. ► Essentially all oxygen incorporated into generated SO42- during oxidation originates from water.