Dechlorination Of Polychlorinated Biphenyls Research Articles

Global prevalence of organohalide-respiring bacteria dechlorinating polychlorinated biphenyls in sewage sludge

BackgroundMassive amounts of sewage sludge are generated during biological sewage treatment and are commonly subjected to anaerobic digestion, land application, and landfill disposal. Concurrently, persistent organic pollutants (POPs) are frequently found in sludge treatment and disposal systems, posing significant risks to both human health and wildlife. Metabolically versatile microorganisms originating from sewage sludge are inevitably introduced to sludge treatment and disposal systems, potentially affecting the fate of POPs. However, there is currently a dearth of comprehensive assessments regarding the capability of sewage sludge microbiota from geographically disparate regions to attenuate POPs and the underpinning microbiomes.ResultsHere we report the global prevalence of organohalide-respiring bacteria (OHRB) known for their capacity to attenuate POPs in sewage sludge, with an occurrence frequency of ~50% in the investigated samples (605 of 1186). Subsequent laboratory tests revealed microbial reductive dechlorination of polychlorinated biphenyls (PCBs), one of the most notorious categories of POPs, in 80 out of 84 sludge microcosms via various pathways. Most chlorines were removed from the para- and meta-positions of PCBs; nevertheless, ortho-dechlorination of PCBs also occurred widely, although to lower extents. Abundances of several well-characterized OHRB genera (Dehalococcoides, Dehalogenimonas, and Dehalobacter) and uncultivated Dehalococcoidia lineages increased during incubation and were positively correlated with PCB dechlorination, suggesting their involvement in dechlorinating PCBs. The previously identified PCB reductive dehalogenase (RDase) genes pcbA4 and pcbA5 tended to coexist in most sludge microcosms, but the low ratios of these RDase genes to OHRB abundance also indicated the existence of currently undescribed RDases in sewage sludge. Microbial community analyses revealed a positive correlation between biodiversity and PCB dechlorination activity although there was an apparent threshold of community co-occurrence network complexity beyond which dechlorination activity decreased.ConclusionsOur findings that sludge microbiota exhibited nearly ubiquitous dechlorination of PCBs indicate widespread and nonnegligible impacts of sludge microbiota on the fate of POPs in sludge treatment and disposal systems. The existence of diverse OHRB also suggests sewage sludge as an alternative source to obtain POP-attenuating consortia and calls for further exploration of OHRB populations in sewage sludge.7vWBgxzBnCPA6QWxMbshMgVideo

Modeled Pathways and Fluxes of PCB Dechlorination by Redox Potentials.

Dechlorination is one of the main processes for the natural degradation of polychlorinated biphenyls (PCBs) in an anaerobic environment. However, PCB dechlorination pathways and products vary with PCB congeners, types of functional dechlorinating bacteria, and environmental conditions. The present study develops a novel model for determining dechlorination pathways and fluxes by tracking redox potential variability, transforming the complex dechlorination process into a stepwise sequence. The redox potential is calculated via the Gibbs free energy of formation, PCB concentrations in reactants and products, and environmental conditions. Thus, the continuous change in the PCB congener composition can be tracked during dechlorination processes. The new model is assessed against four measurements from several published studies on PCB dechlorination. The simulation errors in all four measurements are calculated between 2.67 and 35.1% under minimum (n = 0) and maximum (n = 34) numbers of co-eluters, respectively. The dechlorination fluxes for para-dechlorination pathways dominate PCB dechlorination in all measurements. Furthermore, the model also considers multiple-step dechlorination pathways containing intermediate PCB congeners absent in both the reactants and the products. The present study indicates that redox potential might be an appropriate indicator for predicting PCB dechlorination pathways and fluxes even without prior knowledge of the functional dechlorinating bacteria.

Under explored roles of microbial ligninolytic enzymes in aerobic polychlorinated biphenyl transformation.

Polychlorinated biphenyls (PCBs) are persistent organic pollutants in the environment that are responsible for many adverse health effects. Bioremediation appears to be a healthy and cost-effective alternative for remediating PCB-contaminated environments. While some microbial species have been observed to be capable of transforming PCBs, only two different microbial pathways (rdh and bph pathways) have been described to be involved in PCB transformations. Ligninolytic enzymes have been observed or are under suspicion in some microbial PCB transformations. However, the role of these promising PCB-transforming enzymes, which are produced by fungi and some aerobic bacteria, is still unclear. The present review describes their role by identifying microbial PCB-transforming species and their reported ligninolytic enzymes whether proven or suspected to be involved in PCB transformations. There are several lines of evidence that ligninolytic enzymes are responsible for PCB transformations such as (1) the ability of purified laccases from Myceliophthora thermophila, Pycnoporus cinnabarinus, Trametes versicolor, Cladosporium sp, and Coprinus cumatus to transform hydroxy-PCBs; (2) the increased production of laccases and peroxidases by many fungi in the presence of PCBs; and (3) the enhanced PCB transformation by Pseudomonas stutzeri and Sinorhizobium meliloti NM after the addition of ligninolytic enzyme enhancers. However, if the involvement of ligninolytic enzymes in PCB transformation is clearly demonstrated in some fungal species, it does not seem to be implicated in all microbial species suggesting other still unknown metabolic pathways involved in PCB transformation and different from the bph and rdh pathways. Therefore, PCB transformation may involve several metabolic pathways, some involving ligninolytic enzymes, bph or rdh genes, and some still unknown, depending on the microbial species. In addition, current knowledge does not fully clarify the role of ligninolytic enzymes in PCB oxidation and dechlorination. Therefore, further studies focusing on purified ligninolytic enzymes are needed to clearly elucidate their role in PCB transformation.

Metabolic Synergy of Dehalococcoides Populations Leading to Greater Reductive Dechlorination of Polychlorinated Biphenyls.

Polychlorinated biphenyls (PCBs) are dioxin-like pollutants that cause persistent harm to life. Organohalide-respiring bacteria (OHRB) can detoxify PCBs via reductive dechlorination, but individual OHRB are potent in dechlorinating only specific PCB congeners, restricting the extent of PCB dechlorination. Moreover, the low biomass of OHRB frequently leads to the slow natural attenuation of PCBs at contaminated sites. Here we constructed defined microbial consortia comprising various combinations of PCB-dechlorinating Dehalococcoides strains (CG1, CG4, and CG5) to successfully enhance PCB dechlorination. Specifically, the defined consortia consisting of strains CG1 and CG4 removed 0.28-0.44 and 0.23-0.25 more chlorine per PCB from Aroclor1260 and Aroclor1254, respectively, compared to individual strains, which was attributed to the emergence of new PCB dechlorination pathways in defined consortia. Notably, different Dehalococcoides populations exhibited similar growth when cocultivated, but temporal differences in the expression of PCB reductive dehalogenase genes indicated their metabolic synergy. Bioaugmentation with individual strains (CG1, CG4, and CG5) or defined consortia led to greater PCB dechlorination in wetland sediments, and augmentation with the consortium comprising strains CG1 and CG4 resulted in the greatest PCB dechlorination. These findings collectively suggest that simultaneous application of multiple Dehalococcoides strains, which catalyze complementary dechlorination pathways, is an effective strategy to accelerate PCB dechlorination.

Genome-Resolved Metagenomics and Metatranscriptomics Reveal Insights into the Ecology and Metabolism of Anaerobic Microbial Communities in PCB-Contaminated Sediments.

Growth of organohalide-respiring bacteria such as Dehalococcoides mccartyi on halogenated organics (e.g., polychlorinated biphenyls (PCBs)) at contaminated sites or in enrichment culture requires interaction and support from other microbial community members. To evaluate naturally occurring interactions between Dehalococcoides and key supporting microorganisms (e.g., production of H2, acetate, and corrinoids) in PCB-contaminated sediments, metagenomic and metatranscriptomic sequencing was conducted on DNA and RNA extracted from sediment microcosms, showing evidence of both Dehalococcoides growth and PCB dechlorination. Using a genome-resolved approach, 160 metagenome-assembled genomes (MAGs), including three Dehalococcoides MAGs, were recovered. A novel reductive dehalogenase gene, distantly related to the chlorophenol dehalogenase gene cprA (pairwise amino acid identity: 23.75%), was significantly expressed. Using MAG gene expression data, 112 MAGs were assigned functional roles (e.g., corrinoid producers, acetate/H2 producers, etc.). A network coexpression analysis of all 160 MAGs revealed correlations between 39 MAGs and the Dehalococcoides MAGs. The network analysis also showed that MAGs assigned with functional roles that support Dehalococcoides growth (e.g., corrinoid assembly, and production of intermediates required for corrinoid synthesis) displayed significant coexpression correlations with Dehalococcoides MAGs. This work demonstrates the power of genome-resolved metagenomic and metatranscriptomic analyses, which unify taxonomy and function, in investigating the ecology of dehalogenating microbial communities.

Substrate-dependent strategies to mitigate sulfate inhibition on microbial reductive dechlorination of polychlorinated biphenyls

Sulfate widely co-exists with polychlorinated biphenyls (PCBs) at various concentrations in the subsurface environment. Previous studies have suggested that sulfate often hampers microbial degradation of aliphatic chlorinated solvents such as chloroethenes. However, the impact of sulfate on microbial reductive dechlorination of aromatic PCBs and the underlying mechanisms have received limited attention. Likewise, strategies to mitigate such inhibition remain scarce. Here we found that the mechanisms and mitigation strategies of sulfate inhibition on PCB dechlorination were substrate-dependent. Under electron donor-limiting conditions, even a low concentration of sulfate (2 mM) resulted in a decreased PCB dechlorination rate by 88.7% in a co-culture comprising Dehalococcoides mccartyi CG1 and the sulfate-reducing bacterium Desulfovibrio desulfuricans F1, an inhibition which was attributed to the competition for electron donor between sulfate reduction and PCB dechlorination. As expected, re-amendment of 5 mM lactate effectively re-initiated PCB dechlorination. However, in the presence of a higher concentration of sulfate (5 mM), the PCB dechlorination rate in the co-culture was 77.7% lower than in the control, even with excessive electron donor supply. This inhibition was linked to high concentration of sulfide (∼5 mM) produced from sulfate reduction, as suggested by high availability of electron donor, recovery of dechlorination activity after removal of sulfide, and negligible influence of sulfate on PCB dechlorination in the axenic culture of D. mccartyi CG1. Indeed, sulfide (>5 mM) was found to directly suppress expression of PCB-dechlorinating reductive dehalogenase gene. The highest transcriptional level of pcbA1 was 2.9 ± 0.3 transcripts·cell−1 in the presence of ∼5 mM sulfide, which was increased to 37.4 ± 5.0 transcripts·cell−1 when sulfide was removed. Under this scenario, introduction of ferrous salts (5 mM) efficiently alleviated sulfide inhibition on PCB dechlorination. Interestingly, the augmentation of methanogens in the co-culture was also effective in mitigating sulfide inhibition on PCB dechlorination, offering a new approach to protect Dehalococcoides under sulfide stress. Collectively, these findings deepen our understanding of the influence of sulfate on microbial reductive dechlorination of PCBs and contribute to developing appropriate strategies based on geochemical conditions to alleviate sulfate inhibition during bioremediation of PCB-contaminated sites.

Short-chain fatty acids facilitated long-term dechlorination of PCBs in Taihu Lake sediment microcosms: Evidence from PCB congener and microbial community analyses

Microbial reductive dechlorination hosts great promise as an in situ bioremediation strategy for polychlorinated biphenyls (PCBs) contamination. However, the slow dechlorination in sediments limits natural attenuation. Short-chain fatty acids, as preferred carbon sources and electron donors for dechlorinating microorganisms, might stimulate PCB dechlorination. Herein, two sets of short-chain fatty acids, sole acetate and a fatty acid mixture (acetate, propionate, and butyrate), were amended periodically into Taihu Lake (China) sediment microcosms containing nine PCB congeners (PCB5, 12, 64, 71, 105, 114, 149, 153, and 170) after 24 weeks of incubation. Short-chain fatty acids facilitated the long-term PCB dechlorination and the promoting effect of the fatty acid mixture compared favorably with that of sole acetate. By the end of 108 weeks, the total PCB mass concentrations in acetate amended and fatty acid mixture amended microcosms significantly declined by 7.6% and 10.3% compared with non-amended microcosms (P < 0.05), respectively. Short-chain fatty acids selectively favored the removal of flanked meta and single-flanked para chlorines. Notably, a rare ortho dechlorination pathway, PCB25 (24-3-CB) to PCB13 (3-4-CB), was enhanced. Supplementary fatty acids significantly increased reductive dehalogenases (RDase) gene pcbA5 instead of improving the growth of Dehalococcoides. These findings highlight the merits of low cost short-chain fatty acids on in situ biostimulation in treating PCBs contamination.

Unlocking the potential of resuscitation-promoting factor for enhancing anaerobic microbial dechlorination of polychlorinated biphenyls

Microbial dechlorination of polychlorinated biphenyls (PCBs) is limited by the slow growth rate and low activity of dechlorinators. Resuscitation promoting factor (Rpf) of Micrococcus luteus, has been demonstrated to accelerate the enrichment of highly active PCB-dechlorinating cultures. However, it remains unclear whether the addition of Rpf can further improve the dechlorination performance of anaerobic dechlorination cultures. In this study, the effect of Rpf on the performance of TG4, an enriched PCB-dechlorinating culture obtained by Rpf amendment, for reductive dechlorination of four typical PCB congeners (PCBs 101, 118, 138, 180) was evaluated. The results indicated that Rpf significantly enhanced the dechlorination of the four PCB congeners, with residual mole percentages of PCBs 101, 118, 138 and 180 in Rpf-amended cultures being 16.2–29.31 %, 13.3–20.1 %, 11.9–14.4 % and 9.4–17.3 % lower than those in the corresponding cultures without Rpf amendment after 18 days of incubation. Different models were identified as appropriate for elucidating the dechlorination kinetics of distinct PCB congeners, and it was observed that the dechlorination rate constant is significantly influenced by the PCB concentration. The supplementing Rpf did not obviously change dechlorination metabolites, and the removal of chlorines occurred mainly at para- and meta- positions. Analysis of microbial community and functional gene abundance suggested that Rpf-amended cultures exhibited a significant enrichment of Dehalococcoides, Dehalogenimonas and Desulfitobacterium, as well as non-dechlorinators belonging to Desulfobacterota and Bacteroidetes. These findings highlight the potential of Rpf as an effective additive for enhancing PCB dechlorination, providing new insights into the survival of functional microorganisms involved in anaerobic reductive dechlorination.

Photo-aging promotes the inhibitory effect of polystyrene microplastics on microbial reductive dechlorination of a polychlorinated biphenyl mixture (Aroclor 1260)

Polychlorinated biphenyls (PCBs) and microplastics (MPs) commonly co-exist in various environments. MPs inevitably start aging once they enter environment. In this study, the effect of photo-aged polystyrene MPs on microbial PCB dechlorination was investigated. After a UV aging treatment, the proportion of oxygen-containing groups in MPs increased. Photo-aging promoted the inhibitory effect of MPs on microbial reductive dechlorination of PCBs, mainly attributed to the inhibition of meta-chlorine removal. The inhibitory effects on hydrogenase and adenosine triphosphatase activity by MPs increased with increasing aging degree, which may be attributed to electron transfer chain inhibition. PERMANOVA showed significant differences in microbial community structure between culturing systems with and without MPs (p < 0.05). Co-occurrence network showed a simpler structure and higher proportion of negative correlation in the presence of MPs, especially for biofilms, resulting in increased potential for competition among bacteria. MP addition altered microbial community diversity, structure, interactions, and assembly processes, which was more deterministic in biofilms than in suspension cultures, especially regarding the bins of Dehalococcoides. This study sheds light on the microbial reductive dechlorination metabolisms and mechanisms where PCBs and MPs co-exist and provides theoretical guidance for in situ application of PCB bioremediation technology.

Salinity determines performance, functional populations, and microbial ecology in consortia attenuating organohalide pollutants.

Organohalide pollutants are prevalent in coastal regions due to extensive intervention by anthropogenic activities, threatening public health and ecosystems. Gradients in salinity are a natural feature of coasts, but their impacts on the environmental fate of organohalides and the underlying microbial communities remain poorly understood. Here we report the effects of salinity on microbial reductive dechlorination of tetrachloroethene (PCE) and polychlorinated biphenyls (PCBs) in consortia derived from distinct environments (freshwater and marine sediments). Marine-derived microcosms exhibited higher halotolerance during PCE and PCB dechlorination, and a halotolerant dechlorinating culture was enriched from these microcosms. The organohalide-respiring bacteria (OHRB) responsible for PCE and PCB dechlorination in marine microcosms shifted from Dehalococcoides to Dehalobium when salinity increased. Broadly, lower microbial diversity, simpler co-occurrence networks, and more deterministic microbial community assemblages were observed under higher salinity. Separately, we observed that inhibition of dechlorination by high salinity could be attributed to suppressed viability of Dehalococcoides rather than reduced provision of substrates by syntrophic microorganisms. Additionally, the high activity of PCE dechlorinating reductive dehalogenases (RDases) in in vitro tests under high salinity suggests that high salinity likely disrupted cellular components other than RDases in Dehalococcoides. Genomic analyses indicated that the capability of Dehalobium to perform dehalogenation under high salinity was likely owing to the presence of genes associated with halotolerance in its genomes. Collectively, these mechanistic and ecological insights contribute to understanding the fate and bioremediation of organohalide pollutants in environments with changing salinity.

Mechanistic insight into the Dehalococcoides-mediated reductive dechlorination of polychlorinated biphenyls.

Microbial reductive dechlorination provides a green and highly desirable approach to address the pollution raised by the substantial legacies of polychlorinated biphenyls (PCBs) in soil, sediment, and underground water. It has been shown that the reaction event is catalyzed by supernucleophilic cob(I)alamin housed in reductive dehalogenases (RDases). However, the mechanism still remains elusive. Herein, we unravel the mechanism via quantum chemical calculations, considering a general model of RDase and the dechlorination regioselectivity of two representative PCB congeners, 234-236-CB and 2345-236-CB. The B12-catalzyed reductive dechlorination of PCBs starts with the formation of a reactant complex, followed by a proton-coupled two-electron transfer (PC-TET) and a subsequent single-electron transfer (SET). The PC-TET yields a cob(III)alamin-featured intermediate, which is quickly reduced by the latter SET fueled by significant energetic benefits (∼100 kcal mol-1). It rationalizes the exclusive detection and characterization of cob(I/II)alamins in RDase-mediated dehalogenation experiments. The determined mechanism successfully reproduces the experimental dechlorination regioselectivity and reactivity, as observed with Dehalococcoides mccartyi strain CG1.

Metagenomic study of humic acid promoting the dechlorination of polychlorinated biphenyls.

Polychlorinated biphenyls (PCBs) are persistent organic pollutants that degrade slowly in the environment. Humic acid (HA), the main component of soil organic matter, or more specifically, the quinone moieties in HA, is generally regarded as an "electron shuttle" between pollutants and microorganisms, which could promote microbial remediation of contamination. In this study, we examined the dechlorination of PCB153 by adding HA and anthraquinone-2,6-disulfonate (AQDS, a model compound of quinones) to systems containing PCB dechlorinators, analyzed the composition and functional gene network of the microbial community by metagenomics, and explored the role of HA by modifying or substituting carbon sources or electron donors. However, this study found that HA accelerated microbial dechlorination of PCBS, while AQDS did not. Moreover, HA without quinone activity still promoted dechlorination, but not without carbon source or electron donor. Metagenomic analysis showed that HA did not promote the growth of PCB dechlorinator (Dehalococcoides), but the transmembrane electron carriers in the HA group were higher than those in the AQDS group and the control group, so HA may have promoted the electron transport process. This study is helpful for microbial remediation of PCB contamination, and provides new insights into the role that HA plays in the biogeochemical cycle.

Enhancement of relatively non-toxic dechlorination of PCBs from thermal desorption off-gas over manganese-doped iron-based bimetallic catalyst

In view of the problems existing in catalytic degradation, such as low degradation efficiency, poor dechlorination efficiency and large amount of exhaust gas production, three iron-based bimetallic catalysts were developed for the dechlorination of polychlorinated biphenyls (PCBs) from thermal desorption off-gas, among which manganese-doped iron-based bimetallic catalyst showed the highest degradation efficiency (93.6%) and dechlorination efficiency (92.2%). Specifically, the effect of dechlorination under the air conditions was studied. The GC–MS analysis of intermediate products suggested the presence of three synergistic degradation pathways, namely, hydrodechlorination (HDC), preliminary oxidation, and deep oxidation over manganese-doped iron-based bimetallic catalyst. Through the hydrodechlorination, the chlorine atoms of PCBs were successively removed to obtain the final product biphenyl. During the oxidation stage including preliminary oxidation and deep oxidation, PCBs and intermediates were oxidized by reactive oxygen species (•OH and O2–•). This study provides an efficient and safe dechlorination technology of PCBs in the presence of oxygen with little exhaust gas generated.

Development and microbial characterization of Bio-RD-PAOP for effective remediation of polychlorinated biphenyls

Polychlorinated biphenyls (PCBs) as typical halogenated persistent organic pollutants are widely distributed in natural environments, and can be enriched and magnified in organisms via food webs. It is consequently urgent and necessary to develop techniques to completely remove these persistent organohalides. In this study, we developed a process (Bio-RD-PAOP) by integrating microbial reductive dechlorination (Bio-RD) with subsequent persulfate activation and oxidation process (PAOP) for effective remediation of PCBs. Results showed the synergistic combination of advantages of Bio-RD and PAOP in dechlorination of higher-chlorinated PCBs and of PAOP in degradation/mineralization of lower-chlorinated PCBs, respectively. For the PAOP, both experimental evidences and theoretical calculations suggested that degradation rate and efficiency decreased with the increased PCB chlorine numbers. Relative to the Bio-RD and PAOP, Bio-RD-PAOP had significantly higher PCB removal efficiencies, of which values were PCB congener-specific. For example, removal efficiency of Bio-RD-PAOP in removing PCB88 is 2.50 and 1.86 times of that of Bio-RD and PAOP, respectively. In contrast, the efficiency is 1.66 and 3.35 times of Bio-RD and PAOP, respectively, for PCB180 removal. The PAOP-derived oxidizing species (mainly sulfate free radical) significantly decreased microbial abundance, particularly of the organohalide-respiring Dehalococcoides. Notably, co-existence of other microorganisms alleviated the inhibitive effect of oxidizing species on the Dehalococcoides, possibly due to formation of microbial flocs or biofilm. This study provided a promising strategy for extensive remediation of organohalide-contaminated sites, as well as new insight into impact of PAOP-derived oxidizing species on the organohalide-respiring community.

Effect of microplastics on microbial dechlorination of a polychlorinated biphenyl mixture (Aroclor 1260)

Microplastics (MPs) and polychlorinated biphenyls (PCBs) generally coexist in the environment, posing risks to public health and the environment. This study investigated the effect of different MPs on the microbial anaerobic reductive dechlorination of Aroclor 1260, a commercial PCB mixture. MP exposure inhibited microbial reductive dechlorination of PCBs, with inhibition rates of 39.43%, 23.97%, and 17.53% by polyethylene (PE), polypropylene (PP), and polystyrene (PS), respectively. The dechlorination rate decreased from 1.63 μM Cl− d−1 to 0.99–1.34 μM Cl− d−1 after MP amendment. Chlorine removal in the meta-position of PCBs was primarily inhibited by MPs, with no changes in the final PCB dechlorination metabolites. The microbial community compositions in MP biofilms were not significantly different (P > 0.05) from those in suspension culture, although possessing greater Dehalococcoides abundance (0.52–0.81% in MP biofilms; 0.03–0.12% in suspension culture). The co-occurrence network analysis revealed that the presence of MPs attenuated microbial synergistic interactions in the dechlorinating culture systems, which may contribute to the inhibitory effect on microbial PCB dechlorination. These findings are important for comprehensively understanding microbial dechlorination behavior and the environmental fate of PCBs in environments with co-existing PCBs and MPs and for guiding the application of in situ PCB bioremediation.

Long-Term Dechlorination of Polychlorinated Biphenyls (PCBs) in Taihu Lake Sediment Microcosms: Identification of New Pathways, PCB-Driven Shifts of Microbial Communities, and Insights into Dechlorination Potential.

Microbial reductive dechlorination of polychlorinated biphenyls (PCBs) is regarded as an alternative approach for in situ remediation and detoxification in the environment. To better understand the process of PCB dechlorination in freshwater lake sediment, a long-term (108 weeks) dechlorination study was performed in Taihu Lake sediment microcosms with nine parent PCB congeners (PCB5, 12, 64, 71, 105, 114, 149, 153, and 170). Within 108 weeks, the total PCBs declined by 32.8%, while parent PCBs declined by 84.8%. PCB dechlorinators preferred to attack meta- and para-chlorines, principally para-flanked meta and single-flanked para chlorines. A total of 58 dechlorination pathways were observed, and 20 of them were not in 8 processes, suggesting the broad spectrum of PCB dechlorination in the environment. Rare ortho dechlorination was confirmed to target the unflanked ortho chlorine, indicating a potential for complete dechlorination. PCBs drove the shifts of the microbial community structures, and putative dechlorinating bacteria were growth-linked to PCB dechlorination. The distinct jump of RDase genes ardA, rdh12, pcbA4, and pcbA5 was found to be consistent with the commencement of dechlorination. The maintained high level of putative dechlorinating phylum Chloroflexi (including Dehalococcoides and o-17/DF-1), genus Dehalococcoides, and four RDase genes at the end of incubation revealed the long-term dechlorination potential. This work provided insights into dechlorination potential for long-term remediation strategies at PCB-contaminated sites.

Acceleration of polychlorinated biphenyls remediation in soil via sewage sludge amendment

Bioremediation of polychlorinated biphenyls (PCBs) is impeded by difficulties in massively cultivating bioinoculant. Meanwhile, sewage sludge is rich in pollutant-degrading microorganisms and nutrients, drawing our attention to investigate their potential to be used as a supplement for bioremediation of PCBs. Here we reported extensive microbial reductive dechlorination of PCBs by waste activated sludge (WAS) and digestion sludge (DS), which were identified to harbor multiple putative organohalide-respiring bacteria (i.e., Dehalococcoides, Dehalogenimonas, Dehalobacter, and uncultivated Dehalococcoidia) and PCB reductive dehalogenase genes (i.e., pcbA4 and pcbA5). Consequently, amendment of 1–20% (w/w) fresh WAS/DS enhanced the attenuation of PCBs by 126–544% in a soil microcosm compared with the control soil, with the fastest dechlorination of PCBs being achieved when spiked with 20% fresh WAS. Notably, dechlorination pathways of PCBs were also changed by sludge amendment. Microbial and physicochemical analyses revealed that the enhanced dechlorination of PCBs by sludge amendment was largely attributed to the synergistic effects of sludge-derived nutrients, PCB-dechlorinating bacteria, and stimulated growth of beneficial microorganisms (e.g., fermenters). Finally, risk assessment of heavy metals suggests low potential ecological risks of sludge amendment in soil. Collectively, our study demonstrates that sewage sludge amendment could be an efficient, cost-effective and environment-friendly approach for in situ bioremediation of PCBs.

Effects of iron-carbon materials on microbial-catalyzed reductive dechlorination of polychlorinated biphenyls in Taihu Lake sediment microcosms: Enhanced chlorine removal, detoxification and shifts of microbial community

Nano zero-valent iron particles (nZVI, 0.09 wt%), micro zero-valent iron particles (mZVI, 0.09 wt%), granular activated carbon (GAC, 3.03 wt%), GAC supported nZVI (nZVI/GAC, 3.12 wt%) and nZVI&GAC (nZVI 0.09 wt%, GAC 3.03 wt%) were evaluated for their effects on polychlorinated biphenyls (PCBs) anaerobic reductive dechlorination, detoxification, as well as microbial community structure in Taihu Lake (China) sediment microcosms. The results showed that all of these five materials could stimulate PCBs reductive dechlorination, especially for dioxin-like PCB congeners, and nZVI&GAC had the best removal effect on PCBs. The reduction of total PCBs increased from 13.5% to 33.2%. H2 generated by zero-valent iron corrosion was utilized by organohalide-respiring bacteria (OHRB) to enhance the dechlorination of PCBs predominantly via meta chlorine removal in the short term. The addition of ZVI had little impact on the total bacterial abundance and the microbial community structure. The adsorption of GAC and potential bioremediation properties of attached biofilm could promote the long-term removal of PCBs. GAC, nZVI/GAC, nZVI&GAC had different influences on the microbial structure. These findings provide insights into the biostimulation technique for in situ remediations of PCBs contaminated sediments.

Spatial Distribution, Bioconversion and Ecological Risk of PCBs and PBDEs in the Surface Sediment of Contaminated Urban Rivers: A Nationwide Study in China.

Surface sediments of polluted urban rivers can be a reservoir of hydrophobic persistent organic pollutants (POPs). In this study, we comprehensively assessed the contamination of two groups of POPs, that is, polychlorinated biphenyls (PCBs) and polybrominated diphenyl ethers (PBDEs), in 173 black-odorous urban rivers in China. Spatial distribution of PCBs and PBDEs showed similar patterns but very different contamination levels in surface sediments, that is, average concentrations of 10.73 and 401.16 ng/g dw for the ∑PCBs and ∑PBDEs, respectively. Tetra-/di-CBs and deca-BDE are major PCBs and PBDEs and accounted for 59.11 and 95.11 wt % of the ∑PCBs and ∑PBDEs, respectively. Compared with the persistence of PBDEs, the EF changes of chiral PCBs together with previous cultivation evidence indicated indigenous bioconversion of PCBs in black-odorous urban rivers, particularly the involvement of uncharacterized Dehalococcoidia in PCB dechlorination. Major PCB sources (and their relative contributions) included pigment/painting (25.36%), e-waste (22.92%), metallurgical industry (13.25%), and e-waste/biological degradation process (10.95%). A risk assessment indicated that exposure of resident organisms in urban river sediments to deca-/penta-BDEs could pose a high ecological risk. This study provides the first insight into the contamination, conversion and ecological risk of PCBs and PBDEs in nationwide polluted urban rivers in China.

Anaerobic condition induces a viable but nonculturable state of the PCB-degrading Bacteria Rhodococcus biphenylivorans TG9

Significant microbial removal of highly chlorinated polychlorinated biphenyls (PCBs) requires the cooperation of anaerobic and aerobic bacteria. During the sequencing process of anaerobic dechlorination and aerobic degradation of PCBs, aerobic degrading bacteria have to undergo anaerobic stress. However, the survival strategy of aerobic degrading bacteria under anaerobic condition is not well-understood. In this study, the culturable cells of Rhodococcus biphenylivorans TG9 decreased from 108 CFU/mL to values below the detection limit after 60 days of anaerobic stress while the viable cells remained 105–106 cells/mL, indicating that anaerobic condition induced TG9 entering into the viable but nonculturable (VBNC) state. Cell resuscitation was observed when oxygen was supplied further confirming the VBNC state of TG9. The results of single-cell Raman spectroscopy combined with heavy water indicated the significant decrease of metabolic activity after TG9 entering into the VBNC state. Additionally, the degradation ability of TG9 in the VBNC state was also significantly reduced, while it recovered after resuscitation. Our research proved that entering into the VBNC state is a survival strategy of TG9 under anaerobic conditions, and the limited culturability and degrading capacity could be overcome by resuscitation. The present study provides new insights for improving the remediation efficiency of PCBs contamination.