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Thermophilic methanogens are common autotrophs at hydrothermal vents, but their growth constraints and dependence on H 2  syntrophy  in situ  are poorly understood. Between 2012 and 2015, methanogens and H 2 -producing heterotrophs were detected by growth at 80°C and 55°C at most diffuse (7–40°C) hydrothermal vent sites at Axial Seamount. Microcosm incubations of diffuse hydrothermal fluids at 80°C and 55°C demonstrated that growth of thermophilic and hyperthermophilic methanogens is primarily limited by H 2  availability. Amendment of microcosms with NH 4  +  generally had no effect on CH 4  production. However, annual variations in abundance and CH 4  production were observed in relation to the eruption cycle of the seamount. Microcosm incubations of hydrothermal fluids at 80°C and 55°C supplemented with tryptone and no added H 2  showed CH 4  production indicating the capacity  in situ  for methanogenic H 2  syntrophy. 16S rRNA genes were found in 80°C microcosms from H 2 -producing archaea and H 2 -consuming methanogens, but not for any bacteria. In 55°C microcosms, sequences were found from H 2 -producing bacteria and H 2 -consuming methanogens and sulfate-reducing bacteria. A co-culture of representative organisms showed that  Thermococcus paralvinellae  supported the syntrophic growth of  Methanocaldococcus bathoardescens  at 82°C and  Methanothermococcus  sp. strain BW11 at 60°C. The results demonstrate that modeling of subseafloor methanogenesis should focus primarily on H 2  availability and temperature, and that thermophilic H 2  syntrophy can support methanogenesis within natural microbial assemblages and may be an important energy source for thermophilic autotrophs in marine geothermal environments.

Hydrogen Limitation and Syntrophic Growth among Natural Assemblages of Thermophilic Methanogens at Deep-sea Hydrothermal Vents