Mineralization of PAHs in coal–tar impacted aquifer sediments and associated microbial community structure investigated with FISH

The microbial community structure and mineralization of polycyclic aromatic hydrocarbons (PAHs) in a coal–tar contaminated aquifer were investigated spatially using fluorescence in situ hybridization (FISH) and in laboratory-scale incubations of the aquifer sediments. DAPI-detected microbial populations in the contaminated sediments were three orders of magnitude greater than nearby uncontaminated sediments, suggesting growth on coal–tar constituents in situ. Actinobacteria, β- and γ-Proteobacteria, and Flavobacteria dominated the in situ aerobic (>1 mg l−1 dissolved oxygen) microbial community, whereas sulfate-reducing bacteria comprised 37% of the microbial community in the sulfidogenic region of the aquifer. Rapid mineralization of naphthalene and phenanthrene were observed in aerobic laboratory microcosms and resulted in significant enrichment of β- and γ-Proteobacteria potentially explaining their elevated presence in situ. Firmicutes, Flavobacteria, α-Proteobacteria, and Actinobacteria were also enriched in the mineralization assays, but to a lesser degree. Nitrate- and sulfate-limited mineralization of naphthalene in laboratory microcosms occurred to a small degree in aquifer sediments from locations where groundwater chemistry indicated nitrate- and sulfate-reduction, respectively. Some iron-limited mineralization of naphthalene and phenanthrene was also observed in sediments originating near groundwater measurements of elevated ferrous iron. The results of this study suggest that FISH may be a useful tool for providing a much needed link between laboratory microcosms and groundwater measurements made in situ necessary to better demonstrate the potential for natural attenuation at complex PAH contaminated sites.