Several recent discoveries suggest that novel thermophilic microorganisms that couple autotrophic CO2 fixation with the reduction of nitrate to ammonia may play an important role in both primary production and nitrogen cycling in marine geothermal environments. We obtained several novel extremely thermophilic organisms in pure culture originating from active deep-sea hydrothermal vents on both the East Pacific Rise and the Mid-Atlantic Ridge. These bacteria are capable of chemolithoautotrophic growth by coupling H2-oxidation to NO3- reduction. The ecological significance of such a community at deep-sea hydrothermal vents is twofold: 1) these organisms contribute to the primary productivity by fixing CO2, and 2) their nitrate respiratory metabolism (namely, the reduction of NO3- to NH4+) implicate that nitrogen is conserved within the vent system and is recycled into the vent nitrogen cycling.