Geobiological controls on the fractionation of sulfur isotopes at deep -sea hydrothermal vents
Seafloor hydrothermal systems have existed throughout most of Earth history. Ancient hydrothermal deposits may provide clues to understand the evolution of life on Earth. Many thermophilic chemolithoautotrophic organisms found in modern deep-sea hydrothermal-vent ecosystems are sulfur- and sulfate-reducing microorganisms, which are deeply branching in the universal ‘ Tree of Life’. These ancient lineages may provide clues for recognizing and interpreting biosignatures in ancient hydrothermal deposits. The fractionation of sulfur isotopes during dissimilatory sulfate reduction produces sulfide that is depleted in 34S. This biogenic sulfide is often preserved as metal sulfides (e.g. pyrite), and has been widely used as a biosignature for dissimilatory sulfate reduction. My dissertation research is focused on understanding the environmental controls on the formation of stable-isotope biosignatures formed from the thermophilic chemolithotrophic sulfate-reducing bacterium Thermodesulfatator indicus, isolated from a deep-sea hydrothermal vent. Fractionation during sulfate reduction was explored under a wide range of temperatures, and under high and low H 2 concentrations. For temperature-dependent fractionation experiments, the range of fractionations (1.5–10‰) was typical for growth with hydrogen as the electron donor. Experiments measuring the effect of H 2 concentration on fractionation were conducted using a bioreactor, developed for this study, designed for controlled growth experiments under a wide range of H2 flux conditions. H2-limited experiments revealed (high) fractionations of 24–37‰. Our results are consistent with a model where fractionation is controlled by the competition of forward and reverse enzymatic reaction rates during sulfate reduction and by sulfate transport into the cell.
In addition, a culture-independent molecular phylogenetic survey was carried out to determine the microbial diversity of a mineralized crust coating a sulfide spire from the Central Indian Ridge. Results from these analyses confirm the dominance of sulfur-reducing ϵ-Proteobacteria bacteria. Sulfate-reducing bacteria were also detected using a functional gene approach. Scanning electron micrographs of the mineralized crust reveal abundant filamentous rods encased in mineral precipitates, suggesting that these microorganisms may influence mineralization and vent architecture.