The evolution of life on Earth has been tightly coupled to Earth’s geochemical evolution. Understanding Earth’s earliest ecosystems and their influence on element cycling is an important step to understand the evolution of biologically important element cycles, as well as the impact this evolution had on the development of more complex life on Earth, and the possibility of recognizing signatures of life in the rock record of early Earth as well as on other planetary bodies. Because seafloor hydrothermal systems are likely to have prevailed throughout Earth’s history the extant organisms in these systems and their genomes may serve as living records of change over geological time.

The deeply branching lineages of the ‘universal tree of life’ are all occupied by thermophiles. Many of these deep-branching thermophiles are sulfur-metabolizing species that have been isolated from deep-sea hydrothermal vents, and include organisms that grow from the reduction of elemental sulfur and sulfate coupled to the oxidation of H2. These metabolic pathways are likely very ancient. Recent evidence from the sulfur isotope record in ancient pyrites provides support for the early emergence of sulfur metabolizing organisms.

Constraints on the timing of the first appearance of sulfate reduction, and the isolation of deeply-branching sulfate-reducing bacteria from deep-sea hydrothermal vents, lend support to the importance of biogeochemical sulfur cycling in ancient deep-sea hydrothermal environments. Modern hydrothermal systems therefore may represent analogs of ancient autotrophic ecosystems, and can provide a basis for interpreting biosignatures from ancient hydrothermal systems.

Dissimilatory sulfate-reducing bacteria generate sulfide that is depleted in 34S. The biogenic sulfide is often preserved in the geologic record as metal sulfides (e.g. pyrite), and has been widely used as a biosignature for dissimilatory sulfate reduction. However, despite extensive research on sulfur-isotope fractionation by sulfate-reducing microorganisms, very little is actually known about how the deeply branching sulfate-reducing bacteria from deep-sea hydrothermal systems fractionate sulfur.

Our research is focused on understanding the environmental factors that control the fractionation of sulfur isotopes by thermophilic chemolithotrophic sulfate-reducing bacteria isolated from deep-sea hydrothermal vents. Additionally, by combining FISH and secondary ion mass spectrometry (SIMS), we will identify S-isotope signatures of sulfide minerals associated with sulfur metabolizing organisms from deep-sea hydrothermal vents. By integrating these techniques, we may improve our interpretation of the biogenic component of the sulfur isotope record in modern and ancient deep-sea hydrothermal sulfide deposits.

Aims of Project

The long-term objective of our research is to better understand and interpret the biogenic component of the sulfur isotope record in modern and ancient deep-sea hydrothermal sulfide deposits on Earth and on other planetary bodies. This research is addressed specifically with the following goals:

1. Measure the fractionation of sulfur isotopes by deeply rooted thermophilic sulfate-reducing chemolithotrophs from deep-sea hydrothermal vents under environmental conditions.

2. Analyze biogenic signatures in sulfide minerals associated with biofilms of hydrothermal vents.

The results from these experments may provide additional clues about the structure and diversity of early Earth ecosystems centered on ancient hydrothermal vents.