Using geochemical models to find novel thermophiles in deep-sea vent environments: the case for organic-sulfur based metabolisms

K.L. Rogers¹*, A.F. Bennett¹, R.L. Barker¹ & M.D. Schulte¹

Corresponding author: rogerskl@missouri.edu
¹University of Missouri, Department of Geological Sciences, Columbia, MO, 65203

Abstract:
Carbon and sulfur are ubiquitous components of deep-sea vent fluids, however, their combined interactions, as well as their role in supporting local microbial communities is poorly understood. To better understand the role of organic sulfur compounds in vent environments we use a variety of geochemical techniques to evaluate the potential for both abiotic synthesis and microbial metabolism of organic sulfur compounds in these systems. Recently, we have predicted the thermodynamic properties of aqueous alkyl thiols and sulfides at elevated temperature and pressure (Schulte & Rogers, 2004; Schulte 2009). Using these data and constraints imposed by deep-sea systems, geochemical models suggest that abiotic synthesis of organic sulfur compounds is feasible in modern hydrothermal vent environments (Schulte and Rogers, 2004), with organic sulfide concentrations in excess of 10-3 molal (Schulte, 2009). Furthermore, reaction path modeling of vent fluid mixing with seawater suggests that organic sulfur compounds could account for a majority of the carbon distributed in vent fluids. Additionally, the potential for organic sulfur compounds to serve as metabolic substrates for deep-sea vent microbial communities can be evaluated by calculating the overall Gibbs free energy of potential metabolic reactions. Results of these calculations suggest that both methanogenesis and sulfate reduction of methanethiol and dimethylsulfide, as well as longer chain thiols and sulfides, are exergonic under hydrothermal conditions. These results have spurred recent enrichment and isolation efforts to culture novel vent thermophiles that use organic sulfur compounds in their metabolism. We are currently pursuing a number of positive enrichment cultures that were obtained by inoculating geochemically-designed media with sulfide slurries from the Lau Basin hydrothermal vent fields.

Keywords:
organic sulfur, metabolism, thermophiles, geochemical models

Contributions to Integration and Synthesis:
Geochemical modeling of ridge systems is often disconnected from the search for novel organisms that inhabit these environments and directly influence the geochemistry of these systems. However, understanding the connections between vent fluid geochemistry and resident microbial communities is essential for understanding element cycling in these systems. Our research shows how application of geochemical models can facilitate the search for and isolation of novel organisms that were previously unknown in ridge environments. By expanding our knowledge of the metabolic diversity of vent microbial communities, we will be able to more accurately model both geochemical and ecosystem dynamics in these environments. We hope that this approach can serve as a model for isolating novel species from ridge environments where traditional cultivation techniques have not captured the full diversity of the microbial community.

References:
Schulte M. (2009) Organic sulfides in hydrothermal solution: Standard partial molal properties and role in the organic geochemistry of hydrothermal environments. Aquatic Geochem., submitted.
Schulte M. D and Rogers K. L. (2004) Thiols in hydrothermal solution: Standard partial molal properties and their role in the organic geochemistry of hydrothermal environments. GCA 68, 1087-1097.