A novel microbial habitat in the mid-ocean ridge subseafloor

The subseafloor at the mid-ocean ridge is predicted to be an excellent microbial habitat, because there is abundant space, fluid flow, and geochemical energy in the porous, hydrothermally influenced oceanic crust. These characteristics also make it a good analog for potential subsurface extraterrestrial habitats. Subseafloor environments created by the mixing of hot hydrothermal fluids and seawater are predicted to be particularly energy-rich, and hyperthermophilic microorganisms that broadly reflect such predictions are ejected from these systems in low-temperature (≈15°C), basalt-hosted diffuse effluents. Seven hyperthermophilic heterotrophs isolated from low-temperature diffuse fluids exiting the basaltic crust in and near two hydrothermal vent fields on the Endeavour Segment, Juan de Fuca Ridge, were compared phylogenetically and physiologically to six similarly enriched hyperthermophiles from samples associated with seafloor metal sulfide structures. The 13 organisms fell into four distinct groups: one group of two organisms corresponding to the genus Pyrococcus and three groups corresponding to the genus Thermococcus. Of these three groups, one was composed solely of sulfide-derived organisms, and the other two related groups were composed of subseafloor organisms. There was no evidence of restricted exchange of organisms between sulfide and subseafloor habitats, and therefore this phylogenetic distinction indicates a selective force operating between the two habitats. Hypotheses regarding the habitat differences were generated through comparison of the physiology of the two groups of hyperthermophiles; some potential differences between these habitats include fluid flow stability, metal ion concentrations, and sources of complex organic matter.

Dominance of one bacterial phylotype at a Mid-Atlantic Ridge hydrothermal vent site.

Microbial community structure in natural environments has remained largely unexplored yet is generally considered to be complex. It is shown here that in a Mid-Atlantic Ridge hydrothermal vent habitat, where food webs depend on prokaryotic primary production, the surface microbial community consists largely of only one bacterial phylogenetic type (phylotype) as indicated by the dominance of a single 16S rRNA sequence. The main part of its population occurs as an ectosymbiont on the dominant animals, the shrimp Rimicaris exoculata, where it grows as a monoculture within the carapace and on the extremities. However, the same bacteria are also the major microbial component of the free-living substrate community. Phylogenetically, this type forms a distinct branch within the epsilon-Proteobacteria. This is different from all previously studied chemoautotrophic endo- and ectosymbioses from hydrothermal vents and other sulfidic habitats in which all the bacterial members cluster within the gamma-Proteobacteria.