Bioremediation of high molecular weight polycyclic aromatic hydrocarbons: a review of the microbial degradation of benzo[a]pyrene.

Bacteria have evolved a diverse potential to transform and even mineralize numerous organic compounds of both natural and xenobiotic origin. This article describes the occurrence of N-heteroaromatic compounds and presents a review of the bacterial degradation of pyridine and its derivatives, indole, isoquinoline, and quinoline and its derivatives. The bacterial metabolism of these compounds under different redox conditions – by aerobic, nitrate-reducing, sulfate-reducing and methanogenic bacteria – is discussed. However, in natural habitats, various environmental factors, such as sorption phenomena, also influence bacterial conversion processes. Thus, both laboratory and field studies are necessary to aid our understanding of biodegradation in natural ecosystems and assist the development of strategies for bioremediation of polluted sites. Occurring predominantly near (former) wood-treatment facilities, creosote is a frequent contaminant of soil, subsoil, groundwater, and aquifer sediments. In situ as well as withdrawal-and-treatment techniques have been designed to remediate such sites, which are polluted with complex mixtures of aromatic and heterocyclic compounds.

Bacterial degradation of pyridine, indole, quinoline, and their derivatives under different redox conditions

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous pollutants, some of which are on the US Environmental Protection Agency priority pollutant list. Consequently, timely clean-up of contaminated sites is important. The lower-mol-wt PAHs are amenable to bioremediation; however, higher-mol-wt PAHs seem to be recalcitrant to microbial degradation. The rates of biodegradation of PAHs are highly variable and are dependent not only on PAH structure, but also on the physicochemical parameters of the site as well as the number and types of microorganisms present. PAHs sorb to organic matter in soils and sediments, and the rate of their desorption strongly influences the rate at which microorganisms can degrade the pollutants. Much of the current PAH research focuses on techniques to enhance the bioavailability and, therefore, the degradation rates of PAHs at polluted sites. Degradation products of PAHs are, however, not necessarily less toxic than the parent compounds. Therefore, toxicity assays need to be incorporated into the procedures used to monitor the effectiveness of PAH bioremediation. In addition, this article highlights areas of PAH research that require further investigation.

Environmental aspects of PAH biodegradation

Pentachlorophenol (PCP) was the most prevalent wood preservative for many years worldwide. Its widespread use had led to contamination of various environments. Traditional methods of PCP clean-up include storage in land-fill sites, incineration and abiotic degradation processes such as photodecomposition. Some aerobic and anaerobic microorganisms can degrade PCP under a variety of conditions. Axenic bacterial cultures, Flavobacterium sp., Rhodococcus sp., Arthrobacter sp., Pseudomonas sp., Sphingomonas sp., and Mycobacterium sp., and fungal cultures, Phanerochaete sp. and Trametes sp. exhibit varying rates and extent of PCP degradation. This paper provides some general information on properties of PCP and reviews the influence of nutrient amendment, temperature and pH on PCP degradation by various aerobic and anaerobic microorganisms. Where information is available, proposed degradation pathways, intermediates and enzymes are reviewed.

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Microbial degradation of pentachlorophenol

The diversity of bacteria isolated from creosote- contaminated soils in the United States, Norway, and Germany was determined by comparing their ability to degrade polycyclic aromatic hydrocarbons (PAHs), their phospholipid ester-linked fatty acid (GC-FAME) profiles, sole carbon source utilization patterns (Biolog assays), and 16S rRNA sequences Bacteria were initially obtained by enrichment with phenanthrene and fluoranthene Many were capable of degrading a broad range of the PAHs found in creosote Phenanthrene- or fluoranthene-degraders were abundant in most of the soils tested Several of the fluoranthene-degrading isolates clustered with Sphingomonas (formerly Pseudomonas) paucimobilis strain EPA505 in the GC-FAME and Biolog analyses and three of the isolates examined by 16S rRNA sequence comparisons showed a close relationship with Sphingomonas In addition, the Sphingomonas strains showed the most extensive degradation of 4- & 5-ring PAHs in creosote Burkholderia cepacia strains isolated on phenanthrene from PAH-contaminated soils had limited ability to attack higher molecular weight PAHs either individually or in creosote Thus, degradation capabilities appeared to be associated with members of certain taxa, independent of the origin of the soils from which the bacteria were isolated

Phylogenetic and physiological comparisons of PAH-degrading bacteria from geographically diverse soils.

A two-stage, continuous-flow, sequential inoculation bioreactor strategy for the bioremediation of ground water contaminated with creosote and pentachlorophenol (PCP) was evaluated at the bench- and pilot-scale levels. Performance of continually stirred tank reactors (CSTR) using specially-selected microorganisms was assessed according to chemical analyses of system influent, effluent and bioreactor residues, performing a mass balance evaluation, and comparative biological toxicity and teratogenicity measurements. When specially-selected bacteria capable of utilizing (mineralizing) high-molecular-weight polycyclic aromatic hydrocarbons (HMW PAHs) as primary growth substrates were used in pilot-sale bioreactors (120 gal), the concentration of creosote constituents was reduced from ca. 1,000 ppm in the ground water feed (flow rate = 30 GPD) to 99%). Notably, the cumulative concentration of 8 HMS PAHs (containing 4 or more fused rings) was reduced from 368 ppm in the ground water fed to 5.2 ppm in the system effluent. Moreover, the toxicity and teratogenicity of the bioreactor effluent was significantly reduced. Biodegradation of PCP was limited (ca. 18%) due in large part to poor inoculation and a high degree of abiotic loss (bioaccumulation and adsorption). In general, field data correlated well with those obtained from bench-scale studies.

Derek Ross

Papers

Strategy using bioreactors and specially selected microorganisms for bioremediation of groundwater contaminated with creosote and pentachlorophenol