Microbial diversity of fault scarps and marine sediment on the ridge flank abyssal hill terrain of the East Pacific Rise: Implications and new techniques for the geomicrobiology of subseafloor biospheres

Hydrothermal fluids circulating in basaltic oceanic crust and the microbial communities contained within them are significant components of the chemistry, biology, and mineral geochemistry of the deep ocean. Beneath the flanks of the global mid-ocean ridge system these hydrothermal reservoirs constitute a vast and virtually unexplored high temperature marine habitat. Geomicrobiological research on these subseafloor microbial habitats has been limited by the paucity of known seafloor hydrothermal sites on ridge flanks and by the absence of non-destructive molecular techniques that can be used to study microbe-mineral interactions within these systems.; This dissertation presents molecular and mineral characterizations of biologic materials collected from a hydrothermal venting site (MM site) located on young (<0.5My) ridge flank oceanic crust of the East Pacific Rise. Sulfide minerals identified with x-ray diffractometery and scanning electron microscopy (SEM) indicate that hydrothermal fluids at MM site were high temperature (>250°C) and oxygen-depleted. Archaeal community surveys (using 16S rRNA clone libraries) are consistent with these results revealing a number of sequences representing hyperthermophilic (>80°C), anaerobic organisms within the orders Thermoproteales, Desulfurococcales, Methanopyrales and Korarchaeota . Most bacterial and eukaryotic sequences recovered from MM site belong to the epsilon-Proteobacteria and to a hydrothermal vent aplacophoran (genus Helicoradomenia) respectively. While these sequences also appear hydrothermal in origin, they likely reflect biologic communities adapted to the lower temperature (<80°C) mixing environments surrounding venting sites on the seafloor.; Lastly, an improved technique for rRNA-based in situ hybridization is developed here to study microbe-mineral interactions in geologic materials. This method uses nanoparticles of gold covalently attached to oligonucleotide probes. When hybridized to intracellular rRNA targets, these probes can be detected inside cells using SEM and energy dispersive x-ray spectroscopy. Existing nanogold hybridization techniques were optimized to increase the binding efficiency and signal detection of gold probes inside bacterial cells. These modifications were tested on three different microbe-mineral systems: archaeal cultures grown with pyrite and sulfur, bacterial cultures grown on sand grains, and bacterial cells adhered to hydrothermal sulfide assemblages. Spatial variety statistics were needed to analyze SEM images and to distinguish between rRNA-targeted gold probe hybridization and mineral-based gold precipitation.