MICROBIOLOGICAL CLUES TO CARBON FLOW THROUGH THE SERPENTINITE SUBSURFACE BIOSPHERE

Serpentinizing ultramafic rocks are conduits for the exchange of carbon and energy between the deep Earth and the surface environment. The growth and activity of microbial communities hosted within serpentinites may play important roles in controlling the composition and mobility of carbon-bearing compounds. However due to a complex interplay of carbon sources and sinks and a paucity of data on the microorganisms associated with serpentinites, the significance of microbial processes has proven difficult to constrain. Concerted studies have allowed us to begin to define the identity and functional potential of microbial communities associated with serpentinizing ophiolites, their energy sources, and their impacts upon carbon speciation. Taxonomic data indicate that Betaproteobacteria predominate at shallower oxic/anoxic interfaces, whereas anaerobic taxa within the order Clostridiales are abundant in deeper, more anoxic regions of the serpentinites. Metagenomic and functional genomic analyses support and expand upon these observations, documenting evidence of both hydrogen oxidation and hydrogen production through fermentation of organic matter, and in some cases for sulfur red-ox transformations. Additionally, these studies provide evidence for aerobic carbon monoxide assimilation and carbon fixation via the Calvin cycle and the acetyl CoA pathway. Surprisingly, to date only intermittent evidence of archaeal methanogens has been found within the habitats sampled. These data provide important targets for quantification of subsurface biogeochemical processes and their impact upon the characteristics of circulating fluids and their host rocks.