Utilization of hydrogen, acetate, and “noncompetitive”; substrates by methanogenic bacteria in marine sediments

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Abstract
Methanogenesis occurs at low rates in many marine sediments containing sulfate, in spite of an unfavorable thermodynamic and kinetic environment. One hypothesis explaining the coexistence of sulfate reduction and methanogenesis in marine sediments is that methanogens use substrates for which competition is minimal or nonexistent. Methylamines and methanol in particular have been suggested as “noncompetitive”; substrates. Data from studies of the physiology of recently isolated marine methanogens and studies using radiotracers at in situ concentrations support the notion that methylamines are important methane precursors in surface sediments. In contrast, sulfate‐reducing bacteria appear to outcompete methanogens for methanol at low concentrations (μ?), while the reverse is true at higher concentrations (e.g., > 1 mM). At present, sources of methyl‐amines are the subject of some speculation; several have been suggested, including trimethylamine oxide, choline, and glycine betaine. Of these, glycine betaine appears especially significant because of its high concentration in much of the marine biota. Studies of glycine betaine fermentation in anoxic intertidal sediments indicate that the production of stoichio‐metric amounts of acetate and trimethylamine may support both sulfate reduction and methanogenesis simultaneously. Although methanogenesis is quantitatively unimportant in carbon mineralization in most marine sediments, the use of methylamines suggests that methanogens may play an important role in the mineralization of an important class of organic nitrogen‐containing compounds.