Marine sediments harbor microbial communities that mediate globally relevant biogeochemical cycles. Sedimented hydrothermal vents are unique ecosystems as both high organic load from overlying sediments and high temperature fluids influence the microbial communities associated with these ecosystems. Methane and other short-chain alkanes (C₂ - C₄ hydrocarbons) can serve as key sources of metabolic energy for survival in such environments. Recent integrative studies focused on the anaerobic oxidation of methane (AOM) have elucidated much about the identity (Hinrichs et al., 1999), functional potential (Kruger et al., 2003; Hallam et al., 2004), and ecophysiology (Girguis et al., 2003, 2005) of the marine microbial consortia that mediate this widespread process. Furthermore, C₂ - C₄ hydrocarbons are a major component of the utilizable carbon pool in deep sea sediments worldwide, including hydrothermal vent systems. Recent studies have also demonstrated that sulfate-reducing microbes isolated from the deep sea can survive solely on propane (C₃) and butane (C₄) (Kniemeyer et al., 2007).

However, the influence of environmental and geochemical gradients on these microbial communities remains largely uncharacterized. Middle Valley is a sediment-filled rift valley that represents one of the most geochemically distinct environments at Juan de Fuca Ridge. Geochemical characterization of these sediments has revealed high levels of Fe, Cr, and As in these hydrothermally altered sediments with surface layers exhibiting high amounts of Mn, Cu, and Zn as a result of diagenetic processes. A recent study indicated that there is a greater flexibility of different electron acceptors that can be utilized for AOM than previously assumed, such as manganese and iron oxides (Beal et al., 2009). We are utilizing an integrative biogeochemical approach to investigate how the availability of methane and C₂ - C₄ hydrocarbons affects the metabolic activity of microbial populations in Middle Valley. We are combining quantitative molecular and geochemical techniques including laboratory experiments, in situ studies, and thermodynamic modeling to determine the degree to which the anaerobic oxidation of methane and C₂ - C₄ hydrocarbons supports these communities.

Contributions to Integration and Synthesis: Preliminary results from the lab of Dr. Peter Girguis at Harvard show that methane and C₂ - C₄ hydrocarbons are metabolized to varying degrees in sediments from Middle Valley. Recent data from our group also demonstrates the first evidence to date constraining AOM at temperatures >50°C and indicating that temperature limits drive the community composition of putative methanotrophs mediating AOM (M.M. Adams and S.D. Wankel, in preparation). However, a better understanding of metabolic processes in such anaerobic, high temperature environments is necessary to determine the extent to which these communities use hydrocarbons for energy generation. Working with the larger community to develop such integrative approaches and synthesize a more complete picture of hydrocarbon metabolism in sedimented hydrothermal vents will shed light on how vent biogeochemistry is linked to the microbial communities associated with sediments in Middle Valley.