Bacterial transformation of nitroaromatic compounds and its implicatins in detoxification

Abstract

Nitroaromatic compounds form a major group of environmental pollutants because of their high toxicity, widespread use, and recalcitrance to microbial degradation. The studies incorporated in this thesis aimed at understanding the microbial transformation of this class of compounds, assessment of detoxification potential of such transformations, and possible applications of microorganisms in the treatment of contaminated effluents and wastes. An organism capable of degrading 4-nitrophenol was isolated and identified to be a new species of bacteria. Bacterial transformation of seven nitroaromatic compounds, e.g., 4- and 3-nitrophenols, 2,4-dinitrophenol, 3-nitroaniline, 3,5-dinitrobenzoic acid, picric acid, and TNT, were studied using the isolated Bacillus sp. along with two other bacterial strains, namely, Pseudomonas sp. and a Corynebacterium sp. The metabolites were identified from their chromatographic and spectral properties, and transformation pathways were elucidated. A new enzyme which can hydroxylate 4-nitrophenol to 4-nitrocatechol was isolated from the Pseudomonas sp. It requires molecular oxygen, NADH, and FAD for its activity. Preliminary characterization of this enzyme has been carried out. From a study of the comparative phytotoxicities of three nitrophenols and their microbial metabolites, it could be inferred that microbial transformations are essentially detoxification reactions. The efficacy of the immobilized bacterial cells in decontamination of 4-nitrophenol and methylparathion-containing effluents was examined. Such systems proved to be quite promising for use over prolonged periods.