| When ultramafic rocks interact with liquid water, an exothermic geochemical reaction (termed 'serpentinization') causes the release of heat, hydrogen, and organic compounds. At the Lost City hydrothermal field, densely populated microbial communities are supported by the products of serpentinization. Because serpentinization can occur on any planetary body with ultramafic rocks and liquid water, it has tremendous astrobiological importance. This thesis dissertation represents the most comprehensive analysis of microbial communities in a serpentinization-driven ecosystem.;A general taxonomic survey of archaea and bacteria in Lost City carbonate chimney minerals and fluids identified a predominance of organisms expected to mediate methane and sulfur cycling. In particular, the hottest, most actively venting chimneys were nearly completely dominated by a single archaeal phylotype belonging to order Methanosarcinales. A more sensitive analysis of archaeal and bacterial DNA sequences in chimney samples of known ages detected many phylotypes that can remain rare for >100 years before 'blooming' and becoming dominant when environmental conditions allow. Sequencing of highly variable intergenic regions from carbonate chimneys indicated the presence of multiple abundant genotypes within the one dominant Methanosarcinales phylotype. These chimney samples also showed remarkable genomic, morphological, and metabolic diversity, suggesting that the Methanosarcinales phylotype has differentiated into multiple subpopulations, each adapted to a specialized niche within the biofilm community. Metagenomic sequencing of a carbonate chimney also revealed a surprising abundance and diversity of genes encoding transposases, enzymes that can mediate the lateral transfer of DNA. The metagenomic dataset also provided insights into the ecology of bacteria in Lost City chimneys, as it included many bacterial genes involved in sulfur oxidation, sulfate reduction, and carbon acquisition. The final chapter of the dissertation proposes that the biofilm communities of Lost City carbonate chimneys offer a valuable opportunity to study many of the same processes (eg. lateral gene transfer, metabolic networks, and utilization of serpentinization products) that are expected to have been involved in the origin and early evolution of metabolisms and cells. |
