Bacterial community adaptation to chlorinated pollutant challenge: implications for ready biodegradation testing.

This study aimed to investigate the RBT outcome as a function of the bacterial inoculum during adaptation to degrade the HAs dichloroacetic acid (DCA), trichloroacetic acid (TCA) and 2-monochloropropionic acid (2MCPA). The HAs investigated were ranked, in order of greatest recalcitrance; TCA > 2MCPA > DCA.  DCA degradation was associated with enrichment of a Ralstonia like phylotype and dehII expression.  TCA degradation in replicate RBTs was consistently associated with a Bradyrhizobium like phylotype and dehI gene expression, regardless of the inoculum concentration used, suggesting that TCA imposes a highly selective pressure on the community. TCA degradation was affected by the inoculum concentration, as the rate of degradation was slower at the lowest inoculum concentration used, implying that TCA degradation was accomplished by cometabolism. 2MCPA was associated with various Alpha-, Beta- and Gammaproteobacteria, and the presence and expression of both dehI and dehII genes. Biodegradation of 2MCPA was strongly influenced by inoculum concentration. At lower inoculum concentrations, 2MCPA was commonly associated with a biphasic dechlorination curve, which has not been reported previously.  Biphasic dechlorination curves were attributed to the enrichment of an initial degrading organism, always associated with dehII expression, which was succeeded following 50% dechlorination by a second organism, in association with dehI expression, which were supposed to act sequentially on the L- and D-2MCPA isomers, respectively. The delayed enrichment of a dehI containing organism was attributed to their low environmental abundance, relative to dehII containing organisms. This study highlights the utility of cultivation-independent methods to link more precisely community structure and function during adaptation to degrade xenobiotic compounds