Biodegradation of Fenthion by Phanerochaete chrysosporium

Fenthion was metabolised by the white-rot fungus Phanerochaete chrysosporium in liquid culture when applied to the limit of its solubility in aqueous media (7.2 μM). Using GCNPD, no evidence for either adsorption to or accumulation in intra-cellular compartments was obtained and after 5 days culture, less than 1% of fenthion remained in the extracellular medium. Fenthion did not inhibit spore germination at the concentrations supplied, but appeared to inhibit the rate of spread of the fungus on solid media. Fenthion was oxidised by lignin peroxidase (LiP), but enzyme turnover was dependent on the concentration of H2O2 supplied, with inhibition accompanying the formation of the inactive intermediate Compound III at an H2O2 concentration equal to the km of LiP for H2O2 (c. 0.4 mM). Available evidence suggests that fenthion was metabolised via the fungal ligninolytic system and it represented the reducing substrate for LiP Compound I to return to the native state. INTRODUCTION Fenthion is an organophosphorus pesticide (Figure 1) used to control bird pests in agricultural systems. In particular, it is used against the Red-billed Quelea, Quelea quelea, a major pest of cereal crops in semi-arid areas of sub-Saharan Africa. For instance, during the 1997 98 season more than 5000 l of a commercial formulation (Queletox R) were sprayed at 7 l ha to protect wheat, sorghum, millet and sunflower crops in South Africa (Geertsema, 1998). After application, fenthion and toxic degradation products, particularly the sulphone and sulphoxide derivatives for example, can be persistent, depending on environmental conditions (Minelli et al., 1996; Rotunno et al., 1997; Lacorte et al., 1997). In addition, off-target drift for 3 km was detected in samples taken at a height of 6 m 20 h after application and at 9 m after 64 h (van der Walt et al., 1998; van der Walt, 2000 or see page 91). Also, birds that have collected a lethal dose of fenthion may be able to disperse and contaminate other sites up to 30 km distant, resulting in secondary contamination. Both primary and secondary contamination have been detected in non-target birds such as raptors (van der Walt et al., 1998) and recent reports suggest that fenthion persists