Indirect Evidence Link PCB Dehalogenation with Geobacteraceae in Anaerobic Sediment-Free Microcosms

Although polychlorinated biphenyls (PCBs) production was brought to a halt 30 years ago, recalcitrance to degradation makes them a major environmental pollutant at a global scale. Previous studies confirmed that organohalide-respiring bacteria (OHRB) were capable of utilizing chlorinated congeners as electron acceptor. OHRB belonging to the Phyla Chloroflexi and Firmicutes are nowadays considered as the main PCB-dechlorinating organisms. In this study, we aimed at exploring the involvement of other taxa in PCB dechlorination using sediment-free microcosms (SFMs) and the Delor PCB mixture. High rates of congener dehalogenation (up to 96%) were attained in long-term incubations of up to 692 days. Bacterial communities were dominated by Chloroflexi, Proteobacteria, and Firmicutes, among strictly simplified community structures composed of 12 major phyla only. In a first batch of SFMs, Dehalococcoides mccartyi closely affiliated with strains CG4 and CBDB1 was considered as the main actor associated with congener dehalogenation. Addition of 2-bromoethanesulfonate (BES), a known inhibitor ofmethanogenic activity in a second batch of SFMs had an adverse effect on the abundance of Dehalococcoides sp. Only two sequences affiliated to this Genus could be detected in two (out of six) BES-treated SFMs, contributing to a mere 0.04% of the communities. BES-treated SFMs showed very different community structures, especially in the contributions of organisms involved in fermentation and syntrophic activities. Indirect evidence provided by both statistical and phylogenetic analysis validated the implication of a new cluster of actors, distantly affiliated with the Family Geobacteraceae (Phylum d-Proteobacteria), in the dehalogenation of low chlorinated PCB congeners. Members of this Family are known already for their dehalogenation capacity of chlorinated solvents. As a result, the present study widens the knowledge for the phylogenetic reservoir of indigenous PCB dechlorinating taxa

WHO IS INTERESTED IN THE REDUCTIVE DECHLORINATION OF POLYCHLORINATED BIPHENYLS (PCBs)?

PCBs are persistent organic pollutants which have entered the environment through both use and disposal. The environmental dissemination of PCBs is complex and nearly global in scale, although their production was banned in the 80’s. Nowadays, specific bacterial species were shown to degrade these compounds by attacking the bond between the aromatic core and the flanking chlorine atoms. This anaerobic dechlorination process could be a possible way for environmental bioremediation. Nevertheless, at present time, little is known about the diversity of the bacterial guild and enzymes involved in the process. In this context, anaerobic microcosms were inoculated with PCB-contaminated sediments collected from a water channel leaving a former chemical factory (Strazske, Slovak Republic). Total RNA and DNA were extracted after six months of continuous growth. The first goal was to identify the members of the dechlorinating bacterial guild, as well as identifying the enzymes which were involved in the reductive dehalogenation of the PCBs. A second goal was to identify environmental variables that influence the biological removal of PCBs in the contaminated sites. The anaerobic conditions imposed to the microcosms resulted in the enrichment of putative PCB dechlorinators (Dehalococcoides sp. as well as members of the Chloroflexi class). T-RFLP analysis of the initial bacterial communities and bacterial consortia in microcosms showed a clear shift in their respective compositions. Two microcosms samples are currently being analyzed in details using a cloning-sequencing strategy, leading to a sequence analysis and the construction of phylogenetic trees. The analysis of the PCB congeners present in the microcosms is currently under way. In parallel, a comprehensive list of all dehalogenase sequences found in databases was produced. Based on these sequences, new sets of degenerated primers were designed for the detection of putative genes involved in biodegradation of highly chlorinated PCBs

Divergent PCB organohalide-respiring consortia enriched from the efflux channel of a former Delor manufacturer in Eastern Europe

Polychlorinated biphenyl (PCB) organohalide-respiring communities from the efflux channel of a former Delor manufacturer in Eastern Slovakia were assessed using metagenomic, statistical and cultivation-adapted approaches. Multivariate analysis of environmental factors together with terminal restriction fragment length polymorphisms of the bacterial communities in the primary sediments revealed both temporal and spatial heterogeneity in the distribution of microbial populations, which reflects the dynamic pattern of contamination and altered conditions for biodegradation activity along the channel. Anaerobic microcosms were developed from eight sediments sampled along the channel, where high concentrations of PCBs – from 6.6 to 136 mg/kg dry weight, were measured. PCB dehalorespiring activity, congruent with changes in the microbial composition in all microcosms, was detected. After 10 months of cultivation, the divergently evolved consortia achieved up to 35.9 percent reduction of the total PCB concentration. Phylogenetic-analysis of the active Chloroflexi-related organohalide respiring bacteria by partial sequencing of 16S rRNA genes in cDNA from microcosms with the highest PCB dechlorination activity revealed diverse and unique complexity of the populations. The predominant organohalide respirers were either affiliated with Dehalococcoides sp. and Dehalococcoides-like group (DLG) organisms or were composed of currently unknown distant clades of DLG bacteria. The present study should encourage researchers to explore the full potential of the indigenous PCB dechlorinating populations to develop effective bioremediation approaches that can perform the complete mineralization of PCBs in polluted environments