PNNL

Abstract

Terrestrial subsurface environments are inhabited almost exclusively by microorganisms and are in essence 'aphotic' ecosystems. Photosynthesis plays only an indirect role in subsurface microbial ecology, providing reduced organic compounds that can be metabolized by aerobic or anaerobic heterotrophic bacteria. Organic compounds are introduced into the subsurface, in general, via burial of detrital organic matter or as solutes that are transported to the subsurface in the form of dissolved organic carbon (DOC) in waters that percolate downward and recharge aquifers. Microbial generation of energy in deep subsurface environments results from biochemical reactions involving the oxidation of reduced compounds and the subsequent transfer of electrons to an adjacent oxidized compound. It is these metabolic processes that have a great impact on microbial ecological interactions in the subsurface and subsequent impacts of microbial metabolism on groundwater geochemistry and geological processes such as diagenesis (1). This article will provide an overview of the sources of energy that drive microbial metabolism in the subsurface and the physical constraints on the presence and function of subsurface microorganisms. The distributions and general characteristics of microorganisms in the subsurface will be examined and critical issues with regards to sampling the subsurface and enumerating associated microorganisms will be discussed. Finally, the extent of the subsurface biosphere on earth will be explored along with how this concept has focused the search for life elsewhere in the solar system to the subsurface of other planetary bodies.