Effects of auxiliary carbon sources and electron acceptors on methanogenic degradation of chlorinated phenols

We studied the degradation of chlorinated phenols under methanogenic conditions by establishing enrichment cultures on 4‐chlorophenol and 2,4‐dichlorophenol with or without a supplementary substrate. p‐Cresol was chosen as a nonchlorinated aromatic compound structurally similar to the chlorophenols, and propionate was chosen as a readily utilizable carbon source. 2,4‐Dichlorophenol was dechlorinated to 4‐chlorophenol, which was degraded without further detection of metabolites. The rates of chlorophenol metabolism were enhanced by subsequent refeeding of the respective chlorophenols and supplementary substrates. The addition of p‐cresol or propionate as an auxiliary carbon source enhanced the rate of 4‐chlorophenol degradation. Methanogenic cultures capable of ortho dechlorination were repeatedly subcultured by dilution into fresh media and refeeding of 2,6‐dichlorophenol and either p‐cresol or propionate as auxiliary substrates. 2,6‐Dichlorophenol was sequentially dechlorinated to 2‐chlorophenol and phenol and ultimately mineralized to methane and carbon dioxide. Cultures adapted to 2,4‐ or 2,6‐dichlorophenol also readily dechlorinated other dichlorophenols containing an ortho chlorine. The alternative electron acceptors nitrate, sulfite, and thiosulfate completely inhibited dechlorination of 2,6‐dichlorophenol, whereas sulfate slowed the dechlorination rate.