Effects of endogenous substrates on adaptation of anaerobic microbial communities to 3-chlorobenzoate

Lengthy adaptation periods in laboratory studies evaluating the potential for contaminant biodegradation in natural or engineered environments may indicate that the native microbial communities are not metabolizing the contaminants in situ. In this study, we characterized the adaptation period preceding the biodegradation of 3-chlorobenzoate in anaerobic communities derived from lake sediment and wastewater sludge digesters. The importance of alternative mechanisms of adaptation of the anaerobic communities to 3-chlorobenzoate was evaluated by monitoring the concentrations of metabolic substrates and products as well as the levels of total small subunit (SSU) rRNA and SSU rRNA from populations thought to be important in 3-chlorobenzoate mineralization. The anaerobic environments from which the 3-chlorobenzoate- degrading communities were derived contained different levels of endogenous substrates. Increasing methane levels in the digester and sediment communities and decreasing chemical oxygen demand concentrations in the sediment community during the adaptation periods revealed that endogenous substrates were preferentially utilized relative to 3-chlorobenzoate. Methane and chemical oxygen demand concentrations leveled off concomitantly with the onset of 3-chlorobenzoate biodegradation, suggesting that depletion of the preferentially degraded endogenous substrates stimulated 3-chlorobenzoate metabolism. Consistent with these observations, adaptation to 3-chlorobenzoate occurred more rapidly in digester samples that were depleted of endogenous substrates compared to samples that contained high levels of these biodegradable compounds. Other potential adaptation mechanisms, e.g., genetic change or selective population enrichment, appeared to be less important based on the reproducibility and relative lengths of the adaptation events, trends in the SSU rRNA levels, and/or amplification of SSU rRNA genes from key populations.