GEOBIOCHEMISTRY IN HYDROTHERMAL ECOSYSTEMS

The merger of geochemical models with advances in molecular biology enable new investigations of the interplay between geochemical and biochemical processes. Hydrothermal ecosystems, which are overwhelmingly microbial, offer opportunities for testing emerging hypotheses from this merger. Thermodynamic analysis of hot spring geochemistry reveals a wide variety of oxidation-reduction disequilibria that are potential sources of chemical energy that chemosynthetic microbes could use. These assessments rely on accurate calculations of fluid speciation, which, in turn, can be used to assess the availability of nutrients, metals and toxins. Molecular data from environmental samples can reveal the presence of genes involved in accessing the geochemical energy sources, those linked to the uptake and cycling of nutrients and metals, and others that may be involved in the transformation or elimination of toxins. Merging these results reveals cases where geochemically provided conditions and potential biochemical pathways converge.

Thermodynamic analysis of geochemical data from hot spring ecosystems in Yellowstone National Park (YNP) allows ranking of chemical energy supplies across a wide range of temperature (40° - 93°C; boiling at the elevation of YNP), pH (1.9-9.2), and several order-of-magnitude variations in nutrient and metal concentrations. These results show that oxidation of carbon monoxide, hydrogen sulfide, methane and hydrogen all yield abundant energy across the composition spectrum, while the energy yields from oxidation of aqueous ferrous iron to magnetite, hematite or goethite increase dramatically as pH increases (Shock and others, 2010). Environmental genomic and other molecular data are far less abundant than geochemical data for YNP hot springs, although the inventory is growing rapidly. Where data are abundant, as in the case for the Bison Pool Environmental Genome Project, they reveal the presence of genes for enzymes directly involved in hydrogen oxidation, carbon dioxide reduction, ammonia oxidation, nitrogen fixation and numerous other processes. Tying these observations to the results of geochemical analyses and thermodynamic models enables rapid assessment of which microbial processes can occur where and why. Gene expression coupled with kinetic studies of oxidation-reduction processes will then pave the way for determining how geochemical challenges drive biochemical solutions, which greatly influence microbial diversity.

Shock, E.L., Holland, M.E., Meyer-Dombard, D.R., Amend, J.P. Osburn, G.R. and Fischer, T., 2010, Quantifying inorganic sources of geochemical energy in hydrothermal ecosystems, Yellowstone National Park, USA., Geochim. Cosmochim. Acta (in press).

YNP = Yellowstone National Park