| Methyl parathion (MP) is widely used as an insecticide. p-Nitrophenol (PNP) and p-Aminophenol (PAP) are well-known metabolites of MP biodegradation. The toxicity and fate of these chemicals in the environment need to be understood to prevent health risks and other ecological concerns. In the first part of this study, these three chemicals were examined for their effect on a methanogenic consortium using an Anaerobic Toxicity Assay (ATA). Results showed that MP was the most toxic to methanogenesis, followed by PNP and PAP. Second part was to examine the biotransformation of MP, PNP and PAP under methanogenic, sulfidogenic, denitrifying and ferrogenic conditions using Raritan River sediments as the inoculum. MP loss was observed under all four reducing conditions. Methanogenic, sulfidogenic and denitrifying enrichment cultures mineralized MP coupled with bicarbonate, sulfate and nitrate as electron acceptors, respectively. In contrast, only low concentrations of PNP and PAP were biotransformed under all four anoxic conditions. Based on the detected metabolites of MP biotransformation, MP was biotransformed to PNP under four anoxic conditions; PAP was found as the major intermediate in PNP biotransformation studies, which confirmed that MP biotransformation occurs via a nitro-reduction pathway. The third step was confirmed by PAP biotransformation enrichment cultures. p-Hydroxybenzoate (PHB) was detected as the major metabolite under all four reducing conditions. Therefore, the proposed pathway of MP under the four anoxic conditions tested followed a pathway where MP is converted in sequence to PNP, PAP, and then to PHB. The third part of this study was to isolate the novel denitrifier from the denitrifying consortium. The bacterium was designated MP-SN1 and identified as Pseudomonas stutzeri according to the Biolog System, fatty acid methyl ester (FAME) analysis, and 16s rRNA gene phylogeny. P. stutzeri MP-SN1 grew in the presence of either oxygen or nitrate as the terminal electron acceptor. This strain degraded MP, PAP, and benzoate aerobically faster than anaerobically. Degradation of MP by MP-SN1 was compared to P. stutzeri strain KC DSM 7136, ATCC 55595), P. denitrificans, and P. aeruginosa. Our results showed that MP is more toxic but less recalcitrant than its metabolites under four anoxic conditions. MP-SN1 could be interesting to further study in terms of enzymology and molecular approaches. |
