New Mineral and Microbial Evidence for High Temperature Hydrothermal Habitats Beneath an Abyssal Hill on the East Pacific Rise Flank at 9°27'N

During Alvin dives in May 2002, orange-brown filamentous flocculations were sampled from an axis-facing abyssal hill fault scarp located on 0.5 Ma seafloor west of the EPR at 9°27'N (MM site). We report here on hydrothermal sulfide minerals and hyperthermophilic microbes identified in the orange-brown flocculations collected from the fault scarp. Powder X-ray diffractometry and Environmental Scanning Electron Microscopy (ESEM) were used to characterize mineral phases in fault scarp materials. Chalcopyrite, pyrrhotite, and pyrite constitute a significant portion of the mineral assemblage. Presence of chalcopyrite suggests that high temperature (>250°C) hydrothermal fluids vented from the fault scarp, and presence of pyrrhotite indicates a low oxidation state of these fluids. Molecular phylogenetic surveys are consistent with these findings. Archaeal 16s rRNA clone libraries show that most archaeal phylotypes cluster within hyperthermophilic, chemoautotrophic groups including Thermoproteales, Desulfurococcales, Thermococcales, Korarchaeota, and Methanopyrales. Bacterial 16s rRNA sequences grouped mostly within the alpha, delta, epsilon, and gamma subdivisions of Proteobacteria. The dominant bacterial sequence group from the fault scarp belonged to the epsilon Proteobacteria, a common component of hydrothermal bacterial communities on the ridge crest. Sequences closely related to the thermophilic bacterial species, Oceanithermus desulfurans, were also identified. The physiology and common metabolites of these archaeal and bacterial groups indicate that hydrothermal fluids along the fault scarp likely exceeded 80°C and contained a variety of reduced species such as H2S, H2, CO2, and CH4. Because hydrothermal fluids emerging along ridge flank fault scarps must tap crustal fluid reservoirs, the MM site appears to be an accessible window into the potentially vast subsurface biosphere on the EPR ridge flank.