Indirect evidences for PCB congener degradation by Geobacteraceae in anaerobic sediment-free microcosms

Although PCBs production was brought to a halt thirty years ago, recalcitrance to degradation makes them a major environmental pollutant. Previous studies confirmed that organohalide-respiring bacteria (OHRB) were capable of utilizing chlorinated congeners as sole electron acceptor using reductive dehalogenases. OHRB belonging to the Phylum Chloroflexi and Firmicutes are considered as the main degradation drivers. In this study, we aimed at exploring degradation contributions by other taxa using sediment-free microcosms (SFMs) and Delor PCB mixture. Results showed high rates of congener degradation (up to 93.5%) in the long term (692 days). Bacterial communities were dominated by Chloroflexi, Proteobacteria, and Firmicutes, among community structures composed of twelve major phyla only. In a first batch of SFMs, Dehalococcoides mccartyi strains CG4 and CBDB1 were considered as the main congener degraders. Addition of 2-bromoethanesulfonate (BES) in a second batch of SFMs had an adverse effect on the abundance of Dehalococcoides sp. Two sequences could be detected in two BES-treated SFMs, contributing to a mere 0.04% of the communities. BES-treated SFMs showed different community structures, especially in the contributions of organisms involved in syntrophic activities. Indirect evidences provided by both statistical and phylogenetic analysis validated the significant implication of a new cluster of actors, affiliated with the Family Geobacteraceae (Phylum δ-Proteobacteria) in the degradation of low chlorinated congeners. Members of this Family are known already for the degradation capacity of chlorinated solvent. This study gives primary evidences for the possible implication of members the Geobacteraceae in PCB congener degradation.