Biodegradation of polycyclic aromatic hydrocarbons

Abbreviations
HCCA--2-hydroxychromene-2-carboxylate; PAH--polycyclic aromatic hydrocarbon; PCR-polymerase chain reaction; tHBPA--trans-O-hydroxybenzylidenepyruvate.
Introduction
The use of bioremediation to degrade hazardous chemicals is a practical alternative to traditional remediation treatment technologies. The effectiveness of bioremediation in the detoxification of potentially hazardous organic chemicals that contaminate soil is directly related to the chemical structure of the pollutants. Polycyclic aromatic hydrocarbons (PAHs) are an important class of xenobiotics that persist in soils and sediments (Fig. 1). PAHs are lipophilic chemicals that are formed as by-products of fossil fuel combustion and residues of coal processing. They are ubiquitous as contaminants from petroleum and coal utilization and are of environmental concern because of their toxic, mutagenic and carcinogenic properties [1]. Because of their persistence in the environment and their genotoxicity, m u c h research effort has b e e n aimed at the remediation of PAH-contaminated wastes [2]. PAHs are removed from contaminated sites principally by microbial degradation. However, other possible fates of PAHs are volatilization, photooxidation, chemical oxidation, bioaccumulation, adsorption and adhesion to the soil matrix (Fig. 2) [3]. PAHs with four and five fused b e n z e n e rings are more resistant to biodegradation than PAHs with two or three rings. Their carcinogenicity and lipophilicity m e a n that the PAHs are h u m a n health and ecological risks (Fig. 1).
the laboratory, these c o m p o u n d s still persist in the environment. A factor that limits their degradation is the p o o r bioavailability caused by their low water solubility, a low dissolution rate, and strong adsorption to the soil matrix [7"-9"]. Effective PAH bioremoval from soils is also strongly influenced b y a wide variety of environmental and microbial factors, including soil type, moisture content, concentration of the PAH, redox conditions, sediment toxicity, temperature, pH, electron acceptors, per cent organic matter, seasonal factors, the presence of PAH-degrading microorganisms, inorganic nutrient availability, depth, diffusion, and physicochemical properties of the PAH [5,10]. Even if these factors are optimized, PAH degradation may be slow unless the bioavailability of the PAH for microbial metabolism is enhanced. This review will focus on recent progress in the field of microbial degradation of PAHs. Four areas of research that have advanced over the past few years are: first, the isolation of microorganisms that degrade highly condensed PAHs; second, innovative methods to detect PAH-degrading microorganisms and determine their biodegradative potential; third, studies of the genetics and regulation of PAH degradation; and finally, n e w methods for enhancing the bioremediation of PAHs in soils and sediments.
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331
332
Environmental biotechnology PAH SolubiUty mg 11 Carcinogenicity
Naphthalene
31.7
Non-carcinogen
Acenaphthene
3.9
Non-carcinogen
Anthracene
0.07
Non-carcinogen
Phenanthrene
1.3
Non-carcinogen
amount of PAH biodegradation activity in soil [11]. Until recently, information on the bacterial degradation of PAHs containing more than three fused b e n z e n e rings as sole sources of carbon and energy was scant. Recently, Pseudomonas sp. [12,13,14"', 15"], Alcaligenes sp. [13,14"], Rhodococcussp. [16], Beijerinckia sp. [17], Mycobacterium sp. [18,19..,20.,21], Staphylococcus sp. [22] and Arthrobacter sp. [23,24] have b e e n isolated that can completely degrade or co-metabolize higher molecular weight PAHs, such as fluoranthene, pyrene, fluorene and benz[a]anthracene. Proposed pathways for the biodegradation of these PAHs have also b e e n elucidated. Nevertheless, further research into the initial oxidative steps, enzymatic activities and metabolic intermediates of higher molecular weight PAHs, such as chrysene and benzo[a]pyrene, is necessary.
Biodegradation of polycyclic aromatic hydrocarbons
Carl E. Cerniglia
National Center for Toxicological Research, Jefferson, USA
Polycyclic aromatic hydrocarbons, the products of the incomplete combustion of fossil fuels, are ubiquitous in nature. Some of these chemicals are of environmental concern because of their genotoxic and carcinogenic potential and their persistence in the environment. Over the last few decades, many investigators have focused on the biodegradation of these pollutants. Bioremediation technologies have been developed to clean up contaminated soils. A better understanding of the metabolism, enzyme mechanisms, and genetics of polycyclic aromatic hydrocarbon degrading microorganisms is critical for the optimization of these bioremediation processes. Current Opinion in Biotechnology 1993, 4:331-338
Degradation by bacteria Microbiห้องสมุดไป่ตู้l metabolism of PAHs
Microbial metabolism of PAHs has b e e n extensively studied and the reader is referred to recent reviews [4,5,6"']. Although the degradation of m a n y PAHs by microorganisms has b e e n repeatedly demonstrated in Gram-positive and Gram-negative bacteria with the ability to metabolize naphthalene, phenanthrene and anthracene can be readily isolated from soils (Table 1). Generally, soils with a high total PAIl content contain more PAH utilizers (105-1010 bacteria per gram of