In this study we analysed the relationship between bacterial community structures and geochemistry of groundwater in a sandstone aquifer (SIReN site) impacted mainly by BTEX hydrocarbons (benzene, toluene, ethylbenzene and xylenes), of which benzene is most abundant. The long-term presence of benzene reduced bacterial diversity: in groundwaters contaminated with more than 1.8 x 10(4) microg l(-1) of benzene, bacterial diversity was half of that in clean groundwaters. Terminal restriction fragment length polymorphism (T-RFLP) analysis of 16S rDNA revealed that the community structures were very similar in uncontaminated groundwaters, whereas communities subjected to long-term benzene contamination were different, not only from uncontaminated groundwater communities, but also from each other. Canonical correspondence analysis of the community profiles and the geochemical data showed that this divergence in community structure was not primarily caused by the direct toxic or stressful effects of benzene, but by the environmental changes brought about by benzene metabolism, in particular a decrease in redox potential.
Aims: To investigate the factors affecting benzene biodegradation and microbial community composition in a contaminated aquifer. Methods and Results: We identified the microbial community in groundwater samples from a benzene‐contaminated aquifer situated below a petrochemical plant. Eleven out of twelve groundwater samples with in situ dissolved oxygen concentrations between 0 and 2·57 mg l−1 showed benzene degradation in aerobic microcosm experiments, whereas no degradation in anaerobic microcosms was observed. The lack of aerobic degradation in the remaining microcosm could be attributed to a pH of 12·1. Three groundwaters, examined by 16S rRNA gene clone libraries, with low in situ oxygen concentrations and high benzene levels, each had a different dominant aerobic (or denitrifying) population, either Pseudomonas, Polaromonas or Acidovorax species. These groundwaters also had syntrophic organisms, and aceticlastic methanogens were detected in two samples. The alkaline groundwater was dominated by organisms closely related to Hydrogenophaga. Conclusions: Results show that pH 12·1 is inimical to benzene biodegradation, and that oxygen concentrations below 0·03 mg l−1 can support aerobic benzene‐degrading communities. Significance and Impact of the Study: These findings will help to guide the treatment of contaminated groundwaters, and raise questions about the extent to which aerobes and anaerobes may interact to effect benzene degradation.
Aims: To isolate benzene‐degrading strains from neutral and alkaline groundwaters contaminated by benzene, toluene, ethylbenzene, xylenes (BTEX) from the SIReN aquifer, UK, and to test their effective pH range and ability to degrade TEX. Methods and Results: The 14 isolates studied had an optimum pH for growth of 8, and could degrade benzene to below detection level (1 μg l−1). Five Rhodococcus erythropolis strains were able to metabolize benzene up to pH 9, two distinct R. erythropolis strains to pH 10, and one Arthrobacter strain to pH 8·5. These Actinobacteria also degraded benzene at least down to pH 5·5. Six other isolates, a Hydrogenophaga and five Pseudomonas strains, had a narrower pH tolerance for benzene degradation (pH 6 to 8·5), and could metabolize toluene; in addition, the Hydrogenophaga and two Pseudomonas strains utilized o‐, m‐ or p‐xylenes. None of these strains degraded ethylbenzene. Conclusions: Phylogenetically distinct isolates, able to degrade BTX compounds, were obtained, and some degraded benzene at high pH. Significance and Impact of the Study: High pH has previously been found to inhibit in situ degradation of benzene, a widespread, carcinogenic groundwater contaminant. These benzene‐degrading organisms therefore have potential applications in the remediation or natural attenuation of alkaline waters.
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