Introduction

Hydrocarbons are ubiquitous in the environment, but they are most abundant where coal and petroleum fuels are stored, processed, or burned (TuhaCkova et al. 2001; Warshawsky 1999). Because many hydrocarbons are toxic, their biological effects have been studied extensively. Occupational exposure to polycyclic aromatic hydrocarbons (PAHs) in the aluminum, coke, and steel industries has been linked to lung and bladder cancer (Mastrangelo et al. 1996) and exposure to hydrocarbon fuels may produce neurotoxic effects (Ritchie et al. 2001). Mammalian metabolites of benzo[a]pyrene, the most important carcinogen in natural PAH mixtures, form adducts with macromolecules and interfere with cellular signaling pathways (Miller and Ramos 2001). Some PAHs are either weakly estrogenic or antiestrogenic (Santodonato 1997). The widespread ability of yeasts and filamentous fungi to transform hydrocarbons suggests that they may be involved in the recycling of naturally occurring hydrocarbons in the environment as well as in the biodeterioration of liquid fuels (Lindley 1992). Their versatility in degrading hydrocarbons is due to the broad substrate specificity of their enzymes (Cerniglia and Sutherland 2001). Selected fungi are now being exploited for the bioremediation of soils contaminated with hydrocarbon wastes (Atlas and Cerniglia 1995; Cerniglia and Sutherland 2001) and other species are used for the biotransformation of hydrocarbons to higher-value compounds (Trudgill 1994).

The recycling of hydrocarbons by fungi is probably common, though usually unnoticed, in the environment. For instance, certain yeasts can utilize the methane produced in lakes (Wolf and Hanson 1979; 1980). Fungi that are pathogenic to insects can degrade the hydrocarbons found in the epicuticular waxes of their hosts (Crespo et al. 2000;

Napolitano and Juárez 1997). Many filamentous fungi and yeasts grow abundantly in soils contaminated by petroleum residues (April et al. 1998; 2000; Ekundayo and Obuekwe 2000). Fungi also degrade hydrocarbons in streams and lakes (Griffin and Cooney 1979; Romero et al. 2001) and in oil-polluted seawater (Cerniglia 1997; Zinjarde et al. 1998). Filamentous fungi, especially Hormoconis resinae [the anamorph of Amorphotheca resinae], may contaminate aviation fuels when water is present (Parbery 1971). Yeasts are responsible for biodeterioration of liquid fuels at the interface with water and mycelial fungi may physically block the fuel lines of ships and aircraft (Lindley 1992). The ability of fungi to metabolize hydrocarbons is now being exploited to clean up the environment. Several fungi are used to remove PAHs from wastewater (Giraud et al. 2001; Liao et al. 1997) and yeasts are used to decompose petroleum residues in estuaries (Nwachukwu 2000). Fungi have been used to inoculate contaminated soils to degrade hydrocarbons (May et al. 1997; Novotny et al. 1999; Rama et al. 2000). Cultures that grow on toluene, ethylbenzene, and styrene can be used in biofilters to remove these compounds from industrial waste gases (Cox et al. 1996; García-Pena et al. 2001; Prenafeta-Boldu et al. 2001).

Many purified hydrocarbons can be transformed stereo-specifically by fungi to produce higher-value products. The monoterpenes a- and b-pinene (Prema and Bhattacharyya 1962), b-myrcene (Yamazaki et al. 1988), and limonene (Noma et al. 1992; de Oliveira and Strapasson 2000) as well as several sesquiterpenes (Abraham et al. 1992; Miyazawa et al. 1995; 1997; 1998) are transformed to chiral metabolites. (+ )-Limonene can be transformed to perillyl alcohol, an anticancer drug (de Oliveira and Strapasson 2000). Ethyl-benzene and propylbenzene are transformed stereospecifi-cally to (+ )-1-phenylethanol and (+ )-1-phenylpropanol, which may be used as chiral building blocks for chemical synthesis (Uzura et al. 2001). Some species of fungi are more abundant in sites with hydrocarbon deposition (Tuhackova et al. 2001). The fungal genera that have been studied most extensively for use in the biotransformation of hydrocarbons are the white-rot basidiomycetes Phanerochaete, Pleurotus, and Trametes, the zygomycete Cunninghamella, the hypho-mycetes Aspergillus and Penicillium, and the yeast Candida. Several of these fungi have been used in the experimental bioremediation of soils contaminated with oil (Atlas and Cerniglia 1995; Colombo et al. 1996). Phanerochaete chrysosporium causes a white rot of wood by degrading lignin (Hammel 1995). It oxidizes PAHs to trans-dihydro-diols, phenols, quinones, glucoside conjugates, aromatic acids, and eventually CO2 (Bumpus 1989; Cerniglia and Sutherland 2001; Sutherland et al. 1991). Several enzymes are responsible, including lignin peroxidase, manganese per-oxidase, and cytochrome P450 (Bogan et al. 1996; Cerniglia and Sutherland 2001; Hammel et al. 1991; 1992). P. chrysosporium also cometabolizes single-ring aromatic compounds, such as benzene and toluene, to CO2 (Yadav and Reddy 1993). P. laevis, another species which oxidizes PAHs to polar metabolites (Bogan and Lamar 1996), produces manganese peroxidase and a small amount of laccase but not lignin peroxidase.

Pleurotus ostreatus, the cultivated oyster mushroom, is another white-rot fungus with the ability to mineralize PAHs to CO2 (Bezalel et al. 1996a; Wolter et al. 1997). Other metabolites include trans-dihydrodiols, quinones, and aromatic acids (Bezalel et al. 1996b). Evidence supports the involvement of a cytochrome P450 and an epoxide hydrolase in the early steps of PAH metabolism; manganese peroxidase and laccase may be involved in the later steps (Bezalel et al. 1996b; Schützendübel et al. 1999). P. pulmonarius, a related species, produces a cytochrome P450 that hydroxylates benzo[a]pyrene (Masaphy et al. 1999). Trametes versicolor, another basidiomycete that causes a white rot of wood, also degrades PAHs in soil (Rama et al. 2000). It produces a manganese peroxidase that oxidizes fluorene and phenan-threne (Collins and Dobson 1996). T. versicolor also produces laccases, which can oxidize anthracene and benzo[a]pyrene in the presence of chemical mediators (Collins et al. 1996; Johannes and Majcherczyk 2000). A second species, T. hirsuta, produces a laccase that has been shown to oxidize phenanthrene in the presence of linoleic acid and the mediator 1-hydroxybenzotriazole (Böhmer et al. 1998). Cunninghamella elegans, a zygomycete, oxidizes biphenyl and a large variety of PAHs to metabolites with higher water solubility (Cerniglia 1992; 1993; 1997; Dodge et al. 1979; Cerniglia et al. 1992; Sutherland 1992; Pothuluri and Cerniglia 1994; Cerniglia and Sutherland 2001). C. elegans first oxidizes a PAH to an arene oxide by a cytochrome P450 monooxygen-ase reaction (Cerniglia 1992; 1997). It then quickly hydrates the arene oxide, forming a trans-dihydrodiol, by an epoxide hydrolase reaction (Sutherland et al. 1995). Alternatively, the arene oxide may rearrange nonenzymatically to form a phenol. The trans-dihydrodiols and phenols may be conjugated later with sulfate or glucoside (Casillas et al. 1996; Pothuluri et al. 1990; 1996). Another species, C. blakesleeana, oxidizes cyclohexylcyclohexane (Davies et al. 1986).

Aspergillus niger and A. fumigatus both metabolize terpenes and PAHs. A. niger converts the terpene ß-myrcene to dihydroxylated derivatives (Yamazaki et al. 1988). Another strain of this species oxidizes the PAHs phenanthrene and pyrene to phenols and methyl ethers (Sack et al. 1997a), and there is even a report of the ability of A. niger to cleave the rings of naphthalene, anthracene, and phenanthrene (Yogambal and Karegoudar 1997). A. cellulosae [A. fumigatus] transforms (+)- and (— )-limonene by hydroxylation, double-bond reduction, and ketone formation (Noma et al. 1992). A. fumigatus also produces a cytochrome P450 that hydroxylates benzo[a]pyrene (Venkateswarlu et al. 1996). Penicillium glabrum oxidizes pyrene to mono- and dihydroxylated derivatives, quinones, methyl ethers, and a sulfate (Wunder et al. 1997). P. digitatum hydrates one of the double bonds in (+ )-limonene to form (+ )-a-terpineol (Demyttenaere et al. 2001; Tan et al. 1998) and a different Penicillium sp. strain transforms a-pinene to verbenone (Agrawal and Joseph 2000). P. janthinellum oxidizes high-molecular-weight PAHs to phenols and quinones (Boonchan et al. 2000; Launen et al. 2000). Candida maltosa, a yeast that grows on n-alkanes, and some other Candida spp. produce cytochrome P450 monooxygenases that hydroxylate n-hexadecane and n-octadecane (Scheller et al. 1996). Alkane-utilizing Candida sp. strains from oil-polluted soils may produce biosurfactants as well as oxidative enzymes (Ekundayo and Obuekwe 2000). A strain of C. utilis that utilizes n-alkanes has been used for the bioremediation of mangrove sites (Nwachukwu 2000). Among the hydrocarbons that can be transformed by fungi are alkanes (Lindley 1992; Morgan and Watkinson 1994), terpenes (Trudgill 1994), monocyclic aromatic compounds (Prenafeta-Boldii et al. 2001; Yadav and Reddy 1993), and PAHs (Cerniglia 1992; 1993; 1997; Cerniglia and Sutherland 2001; Cerniglia et al. 1992; Juhasz and Naidu 2000; Hammel 1995; Müncnerova and Augustin 1994; Pothuluri and Cerniglia 1994; Sutherland 1992; Sutherland et al. 1995). This chapter will emphasize the recent research on hydrocarbon transformation by fungi.

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