Isolation and characterization of a thermophilic, chemolithotrophic nitrate-reducing bacterium from deep-sea hydrothermal vents

Abstract

The nitrogen cycle at deep-sea hydrothermal vents has yet to be thoroughly studied. Novel thermophilic microorganisms that couple autotrophic CO₂ fixation with the reduction of nitrate to ammonia have been suggested in recent studies to be important for primary production and nitrogen cycling in marine geothermal environments (Blochl, et al. 1997, Alain, et al. 2002b, Huber, et al. 2002, Miroshnichenko, et al. 2004, Vetriani, et al. 2004). In the summer of 2001, hydrothermal samples were collected from the Rainbow deep-sea vent field on the Mid-Atlantic Ridge (MAR), and several novel thermophilic organisms were obtained in pure culture. Among these novel isolates, cells of strain TB-1 were found to be Gram-negative rods with optimal growth occurring at 55°C, slightly acidic pH, and a salt concentration lower than that of seawater. This bacterium was capable of chemolithoautotrophic growth by coupling H₂-oxidation to NO₃^- reduction, which was reduced to ammonium. Under these conditions the generation time of TB-1 was about 50 minutes.

Isolate TB-1 is phylogenetically related to the epsilon-proteobacteria (genus Caminibacter). The ecological significance of such deep-sea hydrothermal vent bacteria 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⁺) implicates that nitrogen is conserved and recycled within the vent system.