Chlorophenol dechlorination and subsequent degradation in denitrifying microcosms fed low concentrations of nitrate

The potential of using nitrate as a terminal electron acceptor to stimulate anaerobic degradation of mixtures of monochlorophenols (MCPs) or dichlorophenols (DCPs) was evaluated. Contaminated and non-contaminated soils were added to water saturated anaerobic microcosms supplemented with 1 mM or 5 mM nitrate. Denitrification and dechlorination activity were present in three diverse soil types and were maintained upon refeeding both nitrate and the appropriate chlorophenol. However, dechlorination activity could only be serially transferred in enrichments with an added electron donor such as acetate. Dehalogenation activity in enrichments from four of the primary microcosms showed at least five different dechlorination reactions, each mediated by different microbial communities. Three of these are distinct ortho-dechlorinating paths; two are meta-dechlorinating and one is the para-dechlorination of 3,4-DCP. Simultaneous dechlorination and denitrification was observed and both activities could be maintained in microcosms but only in the presence of low nitrate concentrations. Dechlorination and denitrification were mediated by two separate microbial communities; one that dechlorinates without use of nitrate and one that denitrifies while oxidizing the dechlorinated aromatic ring. There was no evidence that dechlorination is mediated by the denitrifying community, however the maintenance of a denitrification potential using low (< 1 mM) nitrate concentrations may be useful for completing the food chain by stimulating the mineralization of phenol and benzoate.