CARBON SOURCE UTILIZATION PROFILES FOR MICROBIAL COMMUNITIES FROM HYDROLOGICALLY DISTINCT ZONES IN A BASALT AQUIFER

The Eastern Snake River Plain aquifer has hydrologically distinct zone s in basalt flow units and interbedded sediments. The zones that diffe r markedly in physical features (e.g., porosity and permeability) have similar groundwater chemistries. The primary objective of this study was to determine whether intervals within the aquifer that contrast on the basis of permeability have distinct communities of unattached mic roorganisms based on functional attributes. Aquifer sampling was condu cted using a submersible pump to obtain whole-well (w) samples, and a straddle-packer pump (SPP) to obtain samples from specific aquifer int ervals that were vertically distributed in the open borehole. The SPP intervals ranged from 4.6 to 6.1 m in length and were located from 142 to 198 m below land surface. A community-level physiological profile (CLPP) was used to determine functional characteristics of the microbi al community in the groundwater samples based on the community respons e to 95 sole organic carbon sources. Surface soil samples at the site were analyzed in a similar manner for comparison. The total bacterial population in the groundwater samples was determined using acridine or ange direct counts. Principal components analysis (PCA) of the CLPP da taset distinguished between surface soil and aquifer microbial communi ties. Soils scored low in the respiration of polymers, esters, and ami nes and high in bromosuccinate, when compared to aquifer samples. The W samples were distinct from SPP samples. The 180- to 198-m interval, with the lowest hydraulic conductivity of all intervals, yielded sampl es that grouped together by PCA and cluster analysis. Direct counts va ried between 10(4) and 10(5) cells ml(-1), and showed no relationship to the depth of the sample or to the hydraulic conductivity of the sam ple interval. Differences between microbial communities based on respi red carbon compounds were discerned in separate, hydrologically distin ct intervals within the borehole, although these differences were slig ht. Differences among aquifer intervals were less apparent than differ ences between surface soils and groundwater, and may be related to var iations in hydrologic properties over the intervals sampled. The resul ts suggest that free-living microbial communities in basalt aquifers, as characterized by CLPP are relatively unaffected by wide ranges in h ydraulic conductivity when other abiotic factors are essentially equal .