Biodegradation

Abstract

An essential component of the biogenic halogen cycle is the degradation of organohalogens into their constitutive elements when an organism dies. Following the style of the earlier review (1), coverage here will be brief since biodegradation of organohalogens, both natural and anthropogenic, is an enormous topic and excellent reviews are available (2119, 2432–2442).

Several organisms are capable of degrading methyl halides, including Methylobacterium chloromethanicum (2443, 2444), Hyphomicrobium chloromethanicum (2444), Aminobacter spp. (2444), and others (2445), including marine bacteria (2446, 2447). The biodegradation of 1,2-dichloroethane has received particular interest, and the haloalkane dehalogenase from Xanthobacter autotrophicus has been extensively investigated (2448–2452). The key step in this degradation is an S_N2 displacement of chloride that is supported by chlorine isotope effect studies (2453). Other bacterial enzymes also biodegrade 1,2-dichloroethane and related haloalkanes and haloacetic acids (278, 2454–2458). The dry cleaning agents, trichloroethylene (TCE) and perchloroethylene (PERC), which also have a natural source, are degraded by bacterial enzymes (2459–2462), as are dichloroethylenes (2462, 2464) and vinyl chloride (2464). The white-rot fungus Trametes versicolor also mineralizes TCE (2465). Transgenic plants degrade TCE, 1,2-dibromoethane, and other organohalogens (2466).