Environmental Health - Toxic Substances
Role of Acetogens in Anaerobic Oxidation of Vinyl Chloride
A recent microcosm study conducted with stream bed sediments demonstrated rapid degradation of [1,2-14C] Vinyl Chloride (VC) and simultaneous production of 14CO2 and 14CH4. The observed inhibition of 14CH4 production in the presence of BES indicated that methanogens were responsible for VC degradation to CH4. The results of acetate mineralization studies indicated that these sediments contained active acetotrophic methanogens. However, autotrophic methanogenesis was not significant in this study. Based on these observations, it was concluded that methanogens indigenous to the stream bed sediments were able to degrade VC to CO2 and CH4 via acetotrophic methanogenesis. A subsequent investigation demonstrated that microbial degradation of VC to CH4 and CO2 involved an initial oxidative acetogenic step followed by acetotrophic methanogenesis.
These studies have important implications for the current understanding of the microbial ecology of anaerobic VC biodegradation. The fact that acetate is an intermediate of anaerobic VC mineralization indicates that acetogens are responsible for the initial anaerobic degradation of VC to a nontoxic product. This conclusion is consistent with the consensus that acetogens are a metabolically diverse group of microorganisms capable of degrading a wide variety of substrates including chlorinated compounds. Moreover, this conclusion suggests that the subsequent processes of acetotrophic methanogenesis and anaerobic respiration contribute only indirectly to VC degradation by consuming the acetate produced by acetogens and thereby promoting further acetogenesis.
Based on the cumulative evidence from these studies, a conceptual model for anaerobic microbial degradation of VC to non-chlorinated products can be proposed. According to this model, VC can be reduced to ethene via the ubiquitous but characteristically inefficient process of anaerobic cometabolic reductive dechlorination or via the relatively efficient process of chlororespiration. Alternatively, VC can be degraded via oxidative acetogenesis as shown in this study. The acetate formed during oxidative acetogenesis of VC can then be mineralized to CO2 and CH4 via acetotrophic methanogenesis or to CO2 via microbial humic acids reduction or microbial reduction of various inorganic electron acceptors. In addition, it has been suggested that, in the presence of an appropriate terminal electron acceptor, individual microorganisms may be able to directly catalyze the anaerobic oxidation of VC to CO2. In light of the current results, however, further investigation is needed to determine if anaerobic oxidation of VC to CO2 is possible within individual microorganisms or if the process requires the cooperation of multiple metabolic groups as observed here.