CARBON SOURCE CONTROLS ON ISOTOPIC COMPOSITION OF SULFATE BY SULFATE-REDUCING BACTERIA AND IMPLICATIONS FOR EVOLUTION OF THE EARLY ATMOSPHERE

The composition of Earth's atmosphere, especially molecular oxygen content, played a key role in the evolution of life. To elucidate the geological history and evolution of atmospheric oxygen, the fundamental assumption has been made that sulfate concentrations increased in the upper ocean when oxygen became a prominent constituent of atmosphere. It is also likely that Sulfate-Reducing Bacteria (SRB) evolved with the appearance of their respiratory substrate. Therefore, the isotopic composition of sulfur bearing minerals, such as marine sulfate and sedimentary sulfide, will reflect the atmospheric changes. The answer to when Earth's atmosphere became oxygen rich should be obtained by determining when the ocean became sulfate rich and SRB became active. The ä18O values of sulfate preserved in the ancient rocks can be used as proxy to elucidate activity of SRB. Our research is focused on understanding the environmental factors that control the fractionation of oxygen and sulfur isotopes by different sulfate-reducing bacteria. To investigate influence of the utilized carbon source on the magnitude of oxygen and sulfur isotope fractionation, D. Desulfuricans G20, D. autotrophicum and D. Salexigens were grown in batch cultures on various carbon sources with an initial sulfate/sulfite concentration of 28 mM. Lactate, butyrate, formate, succinate, fumarate and CO2 were used individually as the sole carbon source. In the experiments, the oxygen of the remaining sulfate shows fairly constant ä18O values regardless of the carbon source and the strain. An oxygen isotope fractionation of ~17 ‰ between sulfate and water (ÄSO4-H2O) was measured at 37 oC for all the experiments. Similar experimental results from all three strains indicate that ä18O values of sulfate during reduction is largely controlled by ä18O values of water via exchange between ä18O of water and the remaining sulfate. These experimental results suggest that ä18O values of sulfate preserved in the ancient rocks may largely reflect isotopic exchange with ambient water, which is facilitated by bacterial reactions and can in turn help us to trace SRB in modern and ancient environments.