THE GROUND-WATER CHEMISTRY AND MICROBIAL ECOLOGY OF HOT AND COLD SPRINGS, SNAKE RIVER PLAIN, IDAHO

The Snake River Plain is underlain by a significant source of geothermal energy that produces thousands of hot springs and artesian wells in Idaho. The mountains surrounding the Snake River Plain are also sources of snowmelt that produce an abundance of cold springs as well. Water produced from the Lidy Hot Springs(~ 60 C) is anoxic, has very low concentrations of dissolved organic carbon (DOC) and contains measurable concentrations of dissolved iron and sulfide. In contrast, Blue Springs, which is located less than a mile away, produces cold water (~8 C) that contains high concentrations of dissolved oxygen (~6 mg/L), lacks dissolved iron and sulfide, and also has low concentrations of DOC. These contrasting water compositions suggest that the microbial ecology of the hot and cold aquifers would be similarly different. To examine this initial hypothesis, these spring waters were studied using quantitative PCR methods designed to detect specific physiological classes of bacteria. The Lidy Hot Springs taps hot, deeply buried rhyolitic rocks and the water is characterized by very low biomass (10-1000 cells/ml). The bacterial component of this microbial population lacks members of the genus Geobacter that are known to reduce ferric oxyhydroxides. This water also contain a relatively high percentage (~20%) of bacteria containing the dissimilatory sulfite reductase (DSR) gene which reflects the ability to carry out sulfate reduction. The Blue Springs tap a mountain talus aquifer, and the water contains even lower biomass (1-100 cells/ml). In contrast to expectations, the bacterial population that is present contains a high percentage (~80%) of bacteria containing the DSR gene. These results show that both sets of spring waters are highly oligotrophic, and that the aquifers producing the waters support very low numbers of microorganisms. The presence of DSR-bearing microorganisms is consistent with the chemisty of the hot springs. However, the lack of evidence for sulfate reduction in the aerobic Blue Springs water, combined with the surprisingly high portion of bacteria containing the DSR gene, suggests that ground-water chemistry may not always reflect the physiological capabilities of the microorganisms present.