Acetate and hydrogen are common products of microbial fermentation and chemical pyrolysis of organic matter. They are also common energy sources for microbial respiration reactions. Thus, these molecules are expected to be key intermediates in subsurface microbial activities. Methane, a primary product of acetoclastic methanogenesis or carbonate reduction, coexists in the pore water or as gas hydrate in close proximity to methanogens.
Globally, much of the methane found in seafloor hydrates has a light stable isotope signature for carbon that suggests it was produced by microbial (i.e., methanogenic) activity (Kvenvolden, 1995). Methanogens, members of the domain Archaea, are important contributors to the oxidation of organic matter. Despite the difficulty in detecting methanogens in sediments that contain hydrates (Reed et al., 2002; Inagaki et al., 2003), it is expected that their presence is key to the formation of large methane hydrate deposits that possess biogenic methane.
Methanogens are found in numerous anoxic environments on Earth. They depend upon aerobic and anaerobic microbial communities to oxidize more complex forms of organic materials into the small organic molecules that they can use (Fenchel and Blackburn, 1979). Many methanogens also use hydrogen as a source of energy. Hydrogen can be generated from biological or abiological processes (Morita, 2000) and may serve as a fundamental means of survival for many organisms in the subsurface. In subseafloor environments, acetate and hydrogen are two of the most likely sources of energy for methanogens (Whiticar et al., 1986). Acetoclastic methanogens have been suggested as key contributors to the methane that exists within hydrates on Blake Ridge, one of the most intensively studied hydrate sites in the world (Wellsbury et al., 1997). At Blake Ridge, Wellsbury et al. (1997) detected high concentrations of acetate that corresponded to high rates of acetoclastic methanogenesis as measured in the laboratory.
The concentration of energy-yielding compounds in microbial habitats is a critical element for estimating the rate of microbial activity in these environments. Acetate concentrations in anoxic marine sediments are typically low and maintained at those low levels because of microbial activity. In shallow Blake Ridge sediments, acetate concentrations in pore waters remained close to ~7 µM through the uppermost 80 m of sediments (Wellsbury et al., 1997). However, as depth increased in this location, higher values were detected (up to 15,000 µM).
Acetate concentrations have been measured on a few occasions in deep marine boreholes such as Ocean Drilling Program (ODP) Leg 164 on Blake Ridge, (Egeberg and Barth, 1998), Leg 177 in the Southern Ocean (Wellsbury et al., 2001), Leg 201 on the Peruvian continental margin (Shipboard Scientific Party, 2002), and the present study. Only the Southern Ocean study area did not have gas hydrate deposits associated with some of the drilling locations. Total bacterial abundances have been measured in sediments for each of these four legs, and, more specifically, during Leg 204 methanogenic archaea were counted by Boyd (2005).
Hydrogen concentrations in marine sediments vary according to the terminal electron-accepting process (TEAP) that dominates in a particular location in the sediments. For example, in Cape Lookout bight sediments, hydrogen concentrations ranged from a low of 0.031 nM, where nitrate reduction was the dominant TEAP, to a high of 133 nM, where acetogenesis was the dominant TEAP (Hoehler et al., 1998).
Hydrogen concentrations have been previously measured during Leg 201, where they reached a maximum concentration of 102 nM at Site 1231 (Shipboard Scientific Party, 2002). At Site 1230, where gas hydrates were recovered from multiple depths ranging from 123 to 200 meters below seafloor (mbsf), hydrogen concentrations reached a maximum of 1.45 nM; however, the sample density was too low to derive a correlation of hydrogen concentrations with intervals bearing gas hydrate.