Benzene has often been observed to be resistant to microbial degradation under anoxic conditions. A number of recent studies, however, have demonstrated that anaerobic benzene utilization can occur. This study extends the previous reports of anaerobic benzene degradation to sediments that varied with respect to contamination input, predominant redox condition, and salinity. In spite of differences in methodology, microbial degradation of benzene was noted in slurries constructed with sediments from various geographical locations and range from aquifer sands to fine-grained estuarine muds, under methanogenic, sulfate-reducing, and iron-reducing conditions. In aquifer sediments under methanogenic conditions, benzene loss was concomitant with methane production, and microbial utilization of [14C]benzene yielded 14CO2 and 14CH4. In slurries with estuarine and aquifer sediments under sulfate-reducing conditions, the loss of sulfate in amounts consistent with the stoichiometric degradation of benzene or the conversion of [14C]benzene to 14CO2 indicates that benzene was mineralized. Benzene loss also occurred in the presence of Fe(III) in sediments from freshwater environments. Microbial benzene utilization, however, was not observed under denitrifying conditions. These results indicate that the potential for the anaerobic degradation of benzene, which was once thought to be resistant to non-oxygenase attack, exists in a variety of aquatic sediments from widely distributed locations.
A sulfate-reducing bacterial enrichment that anaerobically metabolized benzene was obtained from a petroleumcontaminated aquifer. During biodegradation, we observed the transient accumulation of phenol and benzoate as putative benzene intermediates. As these compounds are intermediates in many anaerobic metabolic pathways, we investigated their relation to anaerobic benzene decay with 13 C-labeled starting material. We were able to confirm the presence of [ 13 C]phenol and [ 13 C]benzoate as intermediates of anaerobic [ 13 C-UL]benzene decay. Mass spectral evidence indicated that the carboxyl group of benzoate also originated from 13 C-labeled benzene. Benzoate was also found as a putative benzene intermediate when inoculum from the same site was incubated under methanogenic conditions or when organisms enriched from a different petroleum-contaminated location were incubated with chelated Fe(III) as an electron acceptor. These findings are the first to confirm the importance of benzoate during anaerobic benzene metabolism and suggest that concerns over the accumulation of potentially recalcitrant intermediates in anaerobic environments contaminated with this substrate are unwarranted.
The ability of anaerobic microorganisms to degrade a wide variety of crude oil components was investigated using chronically hydrocarbon-contaminated marine sediments as the source of inoculum. When sulfate reduction was the predominant electron-accepting process, gas chromatographic analysis revealed almost complete n-alkane removal (C 15 -C 34 ) from a weathered oil within 201 d of incubation. No alteration of the oil was detected in sterile control incubations or when nitrate served as an alternate electron acceptor. The amount of sulfate reduced in the oilamended nonsterile incubations was more than enough to account for the complete mineralization of the n-alkane fraction of the oil; no loss of this anion was observed in sterile control incubations. The mineralization of the alkanes was confirmed using 14 C-14,15-octacosane (C 28 H 58 ), with 97% of the radioactivity recovered as 14 CO 2 . These findings extend the range of hydrocarbons known to be amenable to anaerobic biodegradation. Moreover, the rapid and extensive alteration in the n-alkanes can no longer be considered a defining characteristic of aerobic oil biodegradation processes alone.
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