Microbial transformation is a major environmental process affecting the fate of PAHs in both terrestrial and aquatic ecosystems. The microbial degradation of PAHs, having two or three ring, is well documented, but in the last decade, a number of bacteria, that metabolize larger PAH molecules, have also been isolated. Biological technologies are now being explored for their potential in the remediation of contaminated sites. However, their successful application demands a broader understanding of the biochemical pathways by which PAHs are degraded, both individually and in mixtures. An extensive literature describing the degradation of individual PAHs by microorganisms which are able to utilize them as sole sources of carbon and energy, does exist. These studies have yielded fundamental information about the biodegradability of individual compounds. The rates of biodegradability of PAHs are highly variable and are dependent not only on PAHs structure, but also on the physic-chemical 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 research is focused on techniques to enhance the bioavailability and consequently the degradation rates 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. Aerobic bacteria have been extensively studied for use in remediation processes and both enzymologic and genetic studies are being carried out for the purpose of effective biodegradation. PAHs are degraded by microorganisms either in metabolism or co-metabolism.Co-metabolism is very important for degradation of mixtures of PAHs and high molecular weight PAHs. In contrast, several two-, three- and four-ring PAHs have been known to be growth substrates for bacteria.
A few microorganisms have been shown to utilize four ring PAHs for their growth in the absence of co-factors or surfactants However, a Mycobaterium sp., isolated from PAH contaminated freshwater sediments, was found to be capable of mineralizing phenanthrene, pyrene were detected with the nahAc gene probe, indicating that enaymes involed in PAH metabolism were not related to the well characterized naphthalene dioxy genase gene.
The catabolism of PAHs, possessing three or less fused aromatic rings, has been well studied, while the metabolism of higher PAHs containing four or more rings has not been investigated extensively. The processes involving biodegradation are proportional to the ring size of PAH molecules. The lower molecular weight PAHs are degraded more rapidly than the higher weight PAHs. Till the late 1980s, there were no reports of axenic microbial cultures utilizing PAHs containing four or more fused rings as the sole source of carbon and energy. Since then, a number or pure cultures have been reported which are capable of degrading higher PAHs such as fluoranthene, pyrene, chrysene and benz[a]anthracene. The biochemical pathways involved in the catabolism of these of these PAHs have been well identified. However, microorganisms capable of degrading PAHs containing five benzene rings have been difficult to obtain. The very low solubility of more complex PHAs strongly reduces their bioavailability, due to which they do not serve as amenable substrates for microbial metabolism.