Initial transformation step of dicamba by a sulfate-reducing consortium enriched from sediment of the Pearl River of China

Dicamba is a readily detectable chemical in run-off and groundwater of agricultural areas. Biotransformation of dicamba was investigated in laboratory anaerobic microcosms incubated at two temperatures (15 and 25 degrees C) under sulfate-reducing condition using sediment from the Pearl River of Southern China as a source of microorganisms. Initial results of sulfate-reducing microcosms showed that dicamba was transformed by 86% in 120 days of incubation at 25 degrees C compared with the sterile controls. In comparison, only < 60% of the initial dicamba concentrations disappeared at 15 C. Subsequent enrichment cultures were established from the initial microcosms and accelerated transformation of dicamba was observed in the enrichment transfer cultures by 30 days of incubation for 95% and 78% removal at 25 and 15 degrees C, respectively. Through enrichment transfer technique, the lag phase and the time required for dicamba biotransformation were all shortened from the initial microcosms, indicating the enrichment of microorganisms capable of transforming dicamba. Addition of 0.1% yeast extract enhanced the dicamba biotransformation by the dicamba-transforming consortium. Transformation of dicamba was further investigated by incubating C-14-ring labeled dicamba with the dicamba-transforming consortium. Results confirmed that dicamba was transformed by the consortium but no substantial amount of (CO2)-C-14 was recovered, suggesting that the initial degradation was an demethoxylation reaction removing the o-methyl group forming 2-hydroxy-3,5-dichlorobenzoic acid. Our results suggest that dicamba is quickly transformed through demethoxylation, but further degradation of the transformation intermediate is much slower in the environment. (C) 2008 Elsevier Ltd. All rights reserved.