A novel microbial habitat in the mid-ocean ridge subseafloor

The subseafloor at the mid-ocean ridge is predicted to be an excellent micr obial habitat, because there is abundant space, fluid flow, and geochemical energy in the porous, hydrothermally influenced oceanic crust. These chara cteristics also make it a good analog for potential subsurface extraterrest rial habitats. Subseafloor environments created by the mixing of hot hydrot hermal fluids and seawater are predicted to be particularly energy-rich, an d hyperthermophilic microorganisms that broadly reflect such predictions ar e ejected from these systems in low-temperature (approximate to 15 degreesC ), basalt-hosted diffuse effluents. Seven hyperthermophilic heterotrophs is olated from low-temperature diffuse fluids exiting the basaltic crust in an d 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 sulf ide structures. The 13 organisms fell into four distinct groups: one group of two organisms corresponding to the genus Pyrococcus and three groups cor responding to the genus Thermococcus. Of these three groups, one was compos ed solely of sulfide-derived organisms, and the other two related groups we re composed of subseafloor organisms. There was no evidence of restricted e xchange of organisms between sulfide and subseafloor habitats, and therefor e this phylogenetic distinction indicates a selective force operating betwe en the two habitats. Hypotheses regarding the habitat differences were gene rated through comparison of the physiology of the two groups of hyperthermo philes; some potential differences between these habitats include fluid flo w stability, metal ion concentrations, and sources of complex organic matte r.