SPRINGS AS WINDOWS TO THE SUBSURFACE BIOSHERE

The inaccessibility of subsurface environments is a significant obstacle to understanding microbial processes that contribute to sediment, rock, and pore fluid diagenesis over geologic time. Drilling, coring, and mining technologies, which provide much of our access to subsurface environments, are always accompanied by disturbances. These disturbances, in turn, can alter or obscure in situ microbial processes and ecology. Springs, which are naturally occurring discharges of subsurface pore fluids, also provide access to subsurface environments. The extent to which the natural flora of spring waters reflects the microbial ecology of subsurface environments, however, is not presently known. A study of the microbial ecology of spring waters in Idaho, using molecular methods, illustrates this issue. One set of springs taps hot, deeply buried rhyolitic rocks. The flora of these hot springs are characterized by very low biomass (~10³ cells/ml) and are dominated by Archaea that are most closely related to methanogens. Bacteria are present, but are a small percentage (<10%) of the biomass. The other set of springs taps talus sediments recharged by cold snowmelt. Contrary to expectation, the flora of the cold springs exhibit even lower biomass (~10² cells/ml)and also has a relatively high percentage of Archaea. One interpretation of these results is that the flora of spring waters are representative of biofilms attached to aquifer rocks and sediments in the subsurface, and that the Archaea have a competitive advantage over the Bacteria. Another interpretation is that Archaea lack the highly developed attachment mechanisms characteristic of Bacteria, and thus only appear to predominate in these ecosystems. In either case, while springs may provide naturally occurring windows to subsurface ecosystems, the extent to which they alter the view must be carefully considered.