Bioremediation Of Chlorinated Toxic Chemicals Using Plant: Microbial Interactions
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ProgramMaster of Science
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The impact of chlorinated organo-chemicals on human health and the environment has received wide attention because of their long geochemical half-lives and ecotoxic nature. Due to the presence of highly electro-negative Cl atoms, the compounds become electron deficient in nature and chemically inactive in reaction. As a result, such compounds gain water insolubility and are sequestered in the soil. In general, biodegradation of chlorinated organo-chemicals is a two step process: (i) dechlorination, mainly operated by anaerobic bacteria which increases the chemical reactivity of chlorinated organo-chemicals and enhances the bioavailability for aerobic organisms followed by (ii) oxidative degradation and mineralization. Due to low abundance of the anaerobic organisms on the earth's crust, and large amount of chlorinated toxic chemicals such as PCBs, dechlorination will be very slow. For example, in areas like the Hudson River where large amounts of PCB were dumped, abundance of anaerobes might be very limited for dechlorination. This creates a barrier for oxidative degradation. Recent findings of a denitrifying bacterium, Thauera aromatica strain 3CB-1, capable of dehalogenation using nitrate as the terminal electron acceptor, stimulated us to seek the role of nitrate reductase in dechlorination. In the present investigation, we have observed that a single plant or microbe by itself cannot degrade to mineralization PCBs, DDT and PVC. Thus, several intermediate metabolites remained, and they may be more eco-toxic especially in PCBs biodegradation. Furthermore, nitrate reductase (NR) along with constant supplies of reduced coenzymes such as NADH or NADPH enhances the dechlorination of the compounds and makes them more bio-available for oxidative degradation and further mineralization. These findings have been confirmed by chlorine release assay and infrared analysis using pure nitrate reductase. Light and glucose stimulate this dechlorination reaction in vivo by providing a constant supply of reduced coenzymes such as NADPH or NADH by inducing the non-cyclic photophosphorylation and Kreb's cycle, respectively. This experimental evidence suggests that dechlorination is possible without anaerobic organisms, but in the presence of nitrate reductase and constant supplies of reduced coenzymes such as NADH/NADPH.