| Mid-ocean ridge subseafloor environments are dynamic and rich in chemical energy sources; they are predicted to be excellent microbial habitats and to contain novel microorganisms. Hyperthermophilic microorganisms (optimal growth above 80°C) were detected in low-temperature (12--14°C), basalt-hosted upflow zones from the Endeavour Segment, Juan de Fuca Ridge, but were not detectable in ambient seawater. Enhanced levels of these organisms in seafloor effluents with temperatures well below their growth range indicate the presence of a warm, stable habitat within the oceanic crust and confirm that the cultured hyperthermophiles are subseafloor microorganisms. These subseafloor hyperthermophiles are phylogenetically distinct from similar organisms cultured from nearby sulfide structures, implying significant differences between the sulfide and subseafloor habitats.; Hyperthermophiles were also present in two event plumes associated with the 1996 eruption on the North Gorda Ridge but not in any of the potentially entrained fluids (midwater or near-bottom seawater). The presence of hyperthermophiles indicates that some portion of the event plume tapped a subseafloor fluid reservoir. These event plume samples yielded novel isolate GR1, which is phylogenetically distant from known members of the order Thermococcales and displays a suite of traits that are predicted for subseafloor microorganisms. The traits include biofilm formation, growth over a wide thermal range, and oxidation of small organic compounds including acetate using Fe(III) as the electron acceptor.; Hot (60--130°C) sediments at a site of secondary hydrothermal recharge in Middle Valley, Juan de Fuca Ridge, contained low abundances (10 5/cm3) of microorganisms as observed via direct microscopic counts and phospholipid fatty acid analysis. Samples had similar suites of fatty acids at all temperatures, no samples contained detectable archaeal lipids, and hyperthermophiles were not detectable by culturing; from this analysis it appears likely that microorganisms in these hot sediments were not active. However, at a sealed borehole kilometers distant from these hydrothermally active sites, a novel hyperthermophile related to isolate GR1 was cultured from a subseafloor fluid sample. The discovery of these two novel phylotypes in two different samples from the subseafloor is strong evidence for the presence of novel organisms in the subseafloor. |
