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Project Progress

Deep-sea hydrothermal vents are potential analogs to subsurface, extraterrestrial microbial habitats. We have been investigating diversity of microbial populations in the organic-rich, hydrothermally heated sediments of Guaymas Basin vent sites in the Gulf of California using a combination of molecular sequencing and characterizations of lipids. The vent fluids have an unusually high carbonate content and support rich bacterial and archaeal diversity, including hyperthermophilic methanogens (Methanococcus, Methanopyrus), sulfate reducers (Archaeoglobus) and massive bacterial mats of sulfur-oxidizing, filamentous Beggiatoa spp. We sequenced 100 cloned 16S-rRNA genes from the upper 2 cm of Guaymas Basin sediment core A (Alvin Dive 3202, April 1998). The core was covered with a mesophilic, sulfur-oxidizing Beggiatoa mat, whereas the sedimentary surface layers contain a homogeneous assemblage of methanogens that are related to archaea from cold marine methane seeps at the Eel River Basin, California. These archaea are candidates for anaerobic methane oxidation, based on their association with diagnostic C-13-depleted archaeal lipids, including high levels of archaeol and 3-hydroxyarchaeol. The Guaymas sediments apparently harbor a novel archaeal community driven by anaerobic methane oxidation. This discovery suggests a sedimentary microbial methane cycle of anaerobic methanotrophs and methanogens that functions without oxygen input, a new microbial model system for anoxic extraterrestrial habitats.

The bacterial community is considerably more diverse but is dominated by Epsilon proteobacterial exosymbionts and Delta proteobacterial propionate-oxidizing, sulfate-reducing Desulfobulbus and Desulforhopalus. The delta-proteobacteria clones are candidates for hydrogenotrophic, sulfate-reducing anaerobes within anaerobic methane-oxidizing consortia, which would scavenge the methane and educt hydrogen and thus make anaerobic methane oxidation thermodynamically possible. This community also includes the green, non-sulfur bacteria, members of the OP11 and OP8 subdivisions originally described in a hot Yellowstone spring, and a collection of gram-positive bacteria, the Planctomycetales, the Nitrospira phylum, and the candidate subdivision OP1. Except for the possibly aerobic epsilon Proteobacteria, the clones indicate a predominantly anaerobic, mesophilic bacterial community in the Guaymas sediment surface. During the next year we will extend the analysis of core A by at least 1 cm and study the upper 6 cm layers of core C, a sediment core with a less steep temperature profile than core A. Lipid analyses will follow as practical.