Activity and Distribution of Thermophilic Prokaryotes in Hydrothermal Fluid, Sulfidic Structures, and Sheaths of Alvinellids (East Pacific Rise, 13°N)

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Abstract
Processes of inorganic carbon assimilation, methanogenesis, sulfate reduction, and acetate oxidation to CO 2 occurring in samples from the East Pacific Rise at 13°N were traced, using radioisotopically labeled substrates, at temperatures ranging from 65 to 100°C. Molecular hydrogen stimulated lithotrophic methanogenesis and sulfate reduction but inhibited inorganic carbon assimilation. Active mineralization of acetate was observed in an organic-rich Alvinella -associated system at 80°C. Members of the Thermococcales were the most numerous hyperthermophilic archaea in these samples, their density achieving 10 8 cells per cm 3 , while the numbers of cultured hydrogen-utilizing thermophilic lithotrophs were several orders of magnitude lower.