nai

Project Progress

We have cultured two strains of bacteria, Acidithiobacillus ferrooxidans and Leptospirrilum ferrooxidans at room temperature and have adapted cultures to grow as low as 4°C. Both strains have been adapted to grow in sulfate-free medium so that biogenic sulfate may be uniquely characterized. To identify the source of oxygen used in the production of sulfate, either an atmospheric source or oxygen from water, we used two types of media, one with “normal” water and the other spiked with excess oxygen-18. The isotopic spike produces water that has relatively less oxygen-17 relative to O-18, which normally correlate, but can be distinguished by comparing the results to mass-dependent relations. Bacteria appear to use atmospheric oxygen to initiate oxidation of pyrite sulfur but subsequently an inorganic process becomes dominant that uses oxygen in water. The switch in oxygen sources and utilization is accompanied by a rapid increase in oxidation kinetics and transition to entirely biologic oxidation of iron by bacteria. Previously, ¹⁸O/¹⁶O analyses using non-spiked isotope compositions have shown qualitative involvement of atmospheric oxygen but these earlier studies could not explain all aspects of the isotopic data, nor could they quantify contributions from different oxygen reservoirs. Analysis of all three oxygen isotopes in microbially produced sulfate has provided the first quantitative constraints on the extent and duration of microbial utilization of atmospheric oxygen to produce sulfate. Future work will explore a large range in environmental conditions to determine other fractionation processes and constrain changes in the oxygen utilization pathways.