Geochemical Investigations of Subsurface Microbial Ecosystems
James P. MCKINLEY*
Pacific Northwest National Laboratory, Richland, Washington, 99352, U.S.A.
Subsurface microbial ecosystems consist of interdependent microbial and geochemical components. Geochemical measurements provide a sensitive tool for the detection of microbial communities functioning at low activity levels under inhospitable conditions. These measurements target two aspects of the microbial ecosystem: 1) chemical limitations on microbial function and 2) the chemical signatures of community activity. Because generalized geochemical variations may be effected by microbial guilds functioning in small, restricted environments within nominally inhospitable systems, the scale of chemical measurements may be important to understanding community dynamics. Several firsthand examples follow, illustrating the utility of geochemical measurements to reveal and understand microbial processes at depth; they are arranged also toward ordered measurements at fine scale, to illustrate the rich spatial heterogeneity. of microbial activity.
A broad geochemical study of a subsurface lithoautotrophic microbial ecosystem (SLiME) showed 1) the dominance of either sulfate reducers (SRB) or methanogens (MB), depending on the availability of dissolved sulfate, and 2) the utilization of dissolved inorganic carbon to produce methane, as shown by the microbial alteration of δ13CDIC by MB (1, 2). This study was based on laboratory manipulations of groundwater-resident microbial communities and on geographically widespread variations in microbially-coupled groundwater chemistry. A finer-scale investigation of coupled fermentative, sulfate-reducing, and iron-reducing bacteria in anaerobic sediments, within an otherwise aerobic aquifer, indicated interdependence between the microbial guilds (3). Anaerobically and aseptically sampled sediments were cultured, and selected chemical properties and extracted porewater compositions were determined at a 1 m scale. The mobility and supply of electron acceptors acted to limit overall microbial activity, and resulted in the long-term persistence of sediment organic carbon and the apparent microbe-mediated precipitation of secondary calcite. Finally, meter-scale studies of sediment microbiology and chemistry were combined with cm-scale measurements of groundwater chemistry in a system of alternating sandstones and shales. These investigations showed cross-stratigraphic limitations on microbial activity, along with the microbial effects on groundwater chemistry, and the existence of MB under conditions overwhelmingly favoring competing SRB (4). SRB activity and sulfide production were maximized at shale sandstone boundaries, where
