Organic Sulfur Compounds as Metabolic Energy Sources for Thermophilic Microbial Communities in Deep-Sea Hydrothermal Environments (Barker, Rogers, Schulte)

R.L. Barker¹*, K.L. Rogers¹, & M.D. Schulte¹

Corresponding author: rlb42c@mizzou.edu
¹University of Missouri, Geological Sciences Dept., Columbia, MO, 65211

Abstract:
Deep-sea hydrothermal environments at mid-ocean ridges produce abundant amounts of chemical energy that have the potential to sustain life. Biotic and abiotic cycling of organic sulfur compounds has been well documented in low temperature anaerobic environments [Visscher et al., 2003] and the thermodynamic properties of organic sulfur compounds at temperatures relevant to deep-sea hydrothermal systems have been recently predicted [Schulte and Rogers, 2004]. These thermodynamic data for organic sulfur compounds, together with constraints imposed by deep-sea systems, were incorporated into geochemical models that suggest that abiotic synthesis of organic sulfur compounds is feasible in modern deep-sea hydrothermal vent environments [Schulte and Rogers, 2004]. These models allow further investigation of the potential for organic sulfur metabolisms in environments such as Lau Basin and the East Pacific Rise where there are great quantities of carbon and sulfur. We evaluated the potential for metabolism of methanethiol, ethanethiol, 1-propanethiol, 1-butanethiol, dimethylsulfide, diethylsulfide, and dipropylsulfide at deep-sea hydrothermal environments. We considered various metabolic strategies including 1) the complete oxidation of the organic sulfur compound coupled with sulfate reduction to H2S; 2) the dismutation of these compounds by methanogens to CO2 and CH4; 3) complete organic sulfur compound reduction with H2; and 4) aerobic organic sulfur compound oxidation. Using fluid compositions based on mixing end-member vent fluid with seawater [McCollom and Shock, 1997], we calculated the overall free energy for potential metabolic reactions between 25°C and 125°C. Overall, disproportionation reactions yield energy only at low temperatures. Aerobic respiration has the highest energy yields followed by reduction with hydrogen. Finally sulfate reduction coupled to organic sulfur compound oxidation can provide substantially less energy to microbial communities in deep-sea vents.

Contributions to Integration and Synthesis:
Our geochemical models and analyses of potential metabolic energy yields will be used to guide enrichments of organic sulfur-metabolizing thermophilic heterotrophs. Currently we are using these data to enrich for novel thermophiles from the EPR and Lau Basin ISS. Additionally, our models can be extended to investigate the viability of these metabolisms in the various geochemical environments that are obtained in other deep-sea vent systems. These efforts will facilitate the isolation of novel species with unique metabolic strategies from other deep-sea vent environments.

References:
McCollom, T. M. and E. L. Shock (1997) Geochemical constraints on chemolithoautotrophic metabolism by microorganisms in seafloor hydrothermal systems, Geochim. Cosmochim. Acta 61, 4375-4391.
Schulte, M. D. and K. L. Rogers (2004) Thiols in hydrothermal solution: Standard partial molal properties and their role in the organic geochemistry of hydrothermal environments, Geochim. Cosmochim. Acta 68, 1087-1097.
Visscher P. T., L. K. Baumgartner, D. H. Buckley, D. R. Rogers, M. E. Hogan, C. D. Raleigh, K. A. Turk and D. J. Des Marais (2003) Dimethyl sulphide and methanethiol formation in microbial mats: potential pathways for biogenic signatures, Env. Microb. 5(4) 296-308.