The potential for anaerobic benzene oxidation in the Fe(III)reduction zone of petroleum-contaminated aquifers was evaluated. Sediments were incubated under strict anaerobic conditions without any amendments in order to simulate in situ conditions. [ 14 C]Benzene was not oxidized to 14 CO 2 at most sites examined, which is consistent with previous studies that have found that benzene tends to persist in unamended, anaerobic aquifer materials and/or long periods of time are required in order to adapt the microbial population for benzene degradation. However, at one site located in Bemidji, MN, [ 14 C]benzene was oxidized to 14 CO 2 in unamended sediments without an apparent lag, suggesting that benzene was anaerobically degraded in situ. Benzene was not significantly oxidized in sediments from nearby Fe(III)-reducing sites nor in sediments collected from an uncontaminated background site in the same aquifer. Culturing and 16S rRNA-based molecular studies of the Bemidji aquifer demonstrated that while all sites contained similar numbers of Fe(III)-reducing microorganisms closely related to Geothrix fermentens, the site at which anaerobic benzene degradation was observed was greatly enriched with microorganisms in the family Geobacteraceae. This study provides the first data consistent with in situ anaerobic oxidation of benzene to carbon dioxide in the Fe(III)reducing zone of a petroleum-contaminated aquifer and suggests that comparative studies on the size of the Geobacteraceae community in petroleum-contaminated aquifers might aid in the location of zones in which benzene degradation coupled to Fe(III) reduction is taking place.
To evaluate which microorganisms might be responsible for microbial reduction of humic substances in sedimentary environments, humic-reducing bacteria were isolated from a variety of sediment types. These included lake sediments, pristine and contaminated wetland sediments, and marine sediments. In each of the sediment types, all of the humic reducers recovered with acetate as the electron donor and the humic substance analog, 2,6-anthraquinone disulfonate (AQDS), as the electron acceptor were members of the family Geobacteraceae. This was true whether the AQDS-reducing bacteria were enriched prior to isolation on solid media or were recovered from the highest positive dilutions of sediments in liquid media. All of the isolates tested not only conserved energy to support growth from acetate oxidation coupled to AQDS reduction but also could oxidize acetate with highly purified soil humic acids as the sole electron acceptor. All of the isolates tested were also able to grow with Fe(III) serving as the sole electron acceptor. This is consistent with previous studies that have suggested that the capacity for Fe(III) reduction is a common feature of all members of the Geobacteraceae. These studies demonstrate that the potential for microbial humic substance reduction can be found in a wide variety of sediment types and suggest that Geobacteraceae species might be important humic-reducing organisms in sediments.
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