Molecular analysis of microbial community structures in pristine and contaminated aquifers: Field and laboratory microcosm experiments

This study used phylogenetic probes in hybridization analysis Ito (i) deter mine in situ microbial community structures in regions of a shallow sand aq uifer that were oxygen depleted and fuel contaminated (FC) or aerobic and n oncontaminated (NC) and (ii) examine alterations in microbial community str uctures resulting from exposure to toluene and/or electron acceptor supplem entation (nitrate). The latter objective was addressed by using the NC and FC aquifer materials for anaerobic microcosm studies in which phylogenetic probe analysis was complemented by microbial activity assays. Domain probe analysis of the aquifer samples showed that the communities were predominan tly Bacteria; Eucarya and Archaea were not detectable. At the phylum and su bclass levels, the FC and NC aquifer material had similar relative abundanc e distributions of 43 to 65% beta- and gamma-Proteobacteria (B+G), 31 to 35 % alpha-Proteobacteria (ALF), 15 to 18% sulfate-reducing bacteria, and 5 to 10% high G+C gram positive bacteria. Compared to that of the NC region, th e community structure of the FC material differed mainly in an increased ab undance of B+G relative to that of ALF. The microcosm communities were like those of the field samples in that they were predominantly Bacteria (83 to 101%) and lacked detectable Archaea but differed in that a small fraction (2 to 8%) of Eucarya was detected regardless of the treatment applied. The latter result was hypothesized to reflect enrichment of anaerobic protozoa. Addition of nitrate and/or toluene stimulated microbial activity in the mi crocosms, but only supplementation of toluene alone significantly altered c ommunity structures. For the NC material, the dominant subclass shifted fro m B+G to ALF, while in the FC microcosms 55 to 65% of the Bacteria communit y was no longer identifiable by the phylum or subclass probes used. The lat ter result suggested that toluene exposure fostered the proliferation of ph ylotype(s) that were otherwise minor constituents of the FC aquifer communi ty. These studies demonstrated that alterations in aquifer microbial commun ities resulting from specific anthropogenic perturbances can be inferred fr om microcosm studies integrating chemical and phylogenetic probe analysis a nd in the case of hydrocarbon contamination may facilitate the identificati on of organisms important for in situ biodegradation processes. Further wor k integrating and coordinating microcosm and field experiments is needed to explore how differences in scale, substrate complexity, and other hydrogeo logical conditions may affect patterns observed in these systems.