Mixed aerobic and anaerobic microbial communities in benzene-contaminated groundwater.

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.

Isolation of alkali-tolerant benzene-degrading bacteria from a contaminated aquifer.

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 microg 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.

Isolation of haloarchaea that grow at low salinities.

Summary Archaea, the third domain of life, were long thought to be limited to environmental extremes. However, the discovery of archaeal 16S rRNA gene sequences in water, sediment and soil samples has called into question the notion of Archaea as obligate extremophiles. Until now, none of these novel Archaea has been brought into culture, a critical step for discovering their ecological roles. We have cultivated three novel halophilic Archaea (haloarchaea) genotypes from sediments in which the pore-water salinity was close to that of sea water. All previously reported haloarchaeal isolates are obligate extreme halophiles requiring at least 9% (w/v) NaCl for growth and are typically the dominant heterotrophic organisms in salt and soda lakes, salt deposits and salterns. Two of these three newly isolated genotypes have lower requirements for salt than previously cultured haloarchaea and are capable of slow growth at sea-water salinity (2.5% w/v NaCl). Our data reveal the existence of Archaea that can grow in non-extreme conditions and of a diverse community of haloarchaea existing in coastal salt marsh sediments. Our findings suggest that the ecological range of these physiologically versatile prokaryotes is much wider than previously supposed.

Proposal of a new halobacterial genus Natrinema gen. nov., with two species Natrinema pellirubrum nom. nov. and Natrinema pallidum nom. nov.

A phylogenetic analysis of 69 halobacterial 16S rRNA gene sequences has been carried out, integrating data from new isolates, previously described halobacteria and cloned sequences from uncultivated halobacteria. Halobacterium halobium NCIMB 777, Halobacterium trapanicum NCIMB 784 and Halobacterium salinarium NCIMB 786, together with several other strains (strains T5.7, L11 and Halobacterium trapanicum NCIMB 767) constitute a distinct lineage with at least 98.2% sequence similarity. These strains have been incorrectly assigned to the genus Halobacterium. Therefore, based on a variety of taxonomic criteria, it is proposed that Halobacterium salinarium NCIMB 786 is renamed as Natrinema pellirubrum nom. nov., the type species of the new genus Natrinema gen. nov., and that Halobacterium halobium NCIMB 777 and Halobacterium trapanicum NCIMB 784 are renamed as a single species, Natrinema pallidum nom. nov. It was notable that halobacteria closely related to the proposed new genus have been isolated from relatively low-salt environments.

Halobacteria: the evidence for longevity.

Subterranean salt deposits are the remains of ancient hypersaline waters that presumably supported dense populations of halophilic microorganisms including representatives of the haloarchaea (halobacteria). Ancient subterranean salt deposits (evaporites) are common throughout the world, and the majority sampled to date appear to support diverse populations of halobacteria. The inaccessibility of deep subsurface deposits, and the special requirements of these organisms for survival, make contamination by halobacteria from surface sites unlikely. It is conceivable that these subterranean halobacteria are autochthonous, presumably relict populations derived from ancient hypersaline seas that have been revived from a state of dormancy. One would predict that halobacteria that have been insulated and isolated inside ancient evaporites would be different from comparable bacteria from surface environments, and that it might be possible to use a molecular chronometer to establish if the evolutionary position of the subsurface isolates correlated with the geological age of the evaporite. Extensive comparisons have been made between the 16S rRNA genes of surface and subsurface halobacteria without showing any conclusive differences between the two groups. A further phylogenetic comparison exploits an unusual feature of one particular group of halobacteria that possess at least two heterogeneous copies of the 16S rRNA gene, the sequences of which may have been converging or diverging over geological time. However, results to date have yet to show any gene sequence differences between surface and evaporite-derived halobacteria that might arguably be an indication of long-term dormancy.