Collaborative Research: Kinetics and stable isotopic fractionation for abiotic and microbial transformations of elemental sulfur at seafloor hydrothermal environments

Carnegie Institution Of Washington, Washington DC

Investigators

Abstract

Elemental sulfur (So) is a key intermediate species for biochemical sulfur cycling in chimney deposits, the shallow subsurface in diffuse flow areas, and hydrothermal plumes at mid-ocean ridges. Although the floc found in the water column after a volcanic eruption is predominantly So and many microbial isolates from vents are found to utilize So, no significant reservoir of So has been found in chimney deposits, suggesting high turnover rates of elemental sulfur in the subsurface. The kinetic rate constants for abiotic So oxidation and reduction are unknown, even though they are critical for constraining the metastability, and thus bioavailability, of So in the presence of H2(aq) and O2(aq) at temperatures and pH conditions relevant to hydrothermal mixing environments. Nor have the rate constants describing microbial transformations of So been studied at environmentally relevant conditions. As a result, there is a discrepancy between predictions based on energetics and the observed physiology of microorganisms that have been isolated and grown in laboratory conditions. Accurate and realistic biotic and abiotic kinetic rate constants are essential for modeling biogeochemical transformations involving intermediate sulfur species and active microbial consortia at the variable pH and redox conditions found in subsurface underlying basalt- and ultramafic-hosted hydrothermal systems. In this project, researchers at Washington University of St. Louis (WU) and at the Carnegie Institute of Washington (CIW) will experimentally evaluate the kinetics of elemental sulfur abiotic oxidation/reduction at a range of T, pH and H2(aq)-O2(aq) concentrations relevant to the shallow subsurface of basalt and ultramafic hydrothermal systems: 250 bars, 40-120 deg C, pH 4 - 9, 0.1-20 mM H2(aq) and 0-0.25 mM O2(aq). Simultaneously, they will characterize the catabolic reaction rates of thre key bacterial species that mediate elemental sulfur redox transformations to determine if fractionation increases as environmental conditions begin to inhibit growth, as has been seen for SO4 reducing bacteria. By characterizing the geochemical and isotopic effect of So-oxidizing/reducing thermophilic autotrophs from vent systems, they expect to be able to evaluate the impact of complex subsurface microbial ecological systems on associated plume environments contributing to the global ocean sulfur cycle. Broader impacts: The proposed project brings together experimental, microbial, and sulfur isotope expertise, establishes a new collaboration between WU and the CIW, and reintroduces a female scientist back into full-time research by facilitating a leading role in this project. Two distinct undergraduate research projects crossing biology, geochemistry, and computer science will be supported. Furthermore, the research team will develop an online interactive simulation for secondary students that will allow them to use the real scientific data produced by this study in student-driven investigations. This educational outreach activity will also establish a working relationship with a secondary school teacher with experience adapting scientific data and methods for online learning.