Anaerobic reductive dechlorination of chlorinated dioxins in estuarine sediments

Microbial reductive dechlorination of polychlorinated dibenzo-p-dioxins: Pathways and features unravelled via electron density

Microbial reductive dechlorination provides a promising approach for remediating sites contaminated with polychlorinated dibenzo-p-dioxins (PCDDs). Nonetheless, the overall dechlorination pathways and features remain elusive. Herein, we address these issues by quantum chemical calculations, considering the calibrations of reductive dechlorination of 15 PCDDs mediated by three Dehalococcoides strains. Chlorine substituents with lower electron density are prone to be microbially abstracted, which differentiates 72 microbe-active PCDDs from 3 nonactive analogues with a success rate of 100%. For all 256 transformation routes of 75 PCDDs, electron density differences of chlorines pinpoint 105 viable and 125 unviable pathways, corresponding a success rate of 90%. The feasibility of 26 reductive dechlorination pathways are uncertain because of the limited available experimental data. 98% (251/256) of microbial chlorine abstraction follows an order of Cl_O,Cl>Cl_Cl,Cl>Cl_H,O>Cl_H,Cl>Cl_H,H=0. PCDDs solely containing chlorines at C₁, C₄, C₆, and/or C₉ can be completely dechlorinated to non-chlorinated dioxin; while PCDDs housing chlorines at C₂, C₃, C₇, and/or C₈ can be dechlorinated to 2-MCDD or 2,7/8-DCDD as final products. These findings also support reductive dechlorination of PCDDs in mixed cultures and sediments (> 98% and 83%). These findings would promote the application of dechlorinating bacteria in targeted remediation and facilitate the respective studies on other POPs.

Identification of a Chlorodibenzo-p-dioxin Dechlorinating Dehalococcoides mccartyi by Stable Isotope Probing

Polychlorinated dibenzo-p-dioxins (PCDDs) are released into the environment from a variety of both anthropogenic and natural sources. While highly chlorinated dibenzo-p-dioxins are persistent under oxic conditions, in anoxic environments, these organohalogens can be reductively dechlorinated to less chlorinated compounds that are then more amenable to subsequent aerobic degradation. Identifying the microorganisms responsible for dechlorination is an important step in developing bioremediation approaches. In this study, we demonstrated the use of a DNA-stable isotope probing (SIP) approach to identify the bacteria active in dechlorination of PCDDs in river sediments, with 1,2,3,4-tetrachlorodibenzo-p-dioxin (1,2,3,4-TeCDD) as a model. In addition, pyrosequencing of reverse transcribed 16S rRNA of TeCDD dechlorinating enrichment cultures was used to reveal active members of the bacterial community. A set of operational taxonomic units (OTUs) responded positively to the addition of 1,2,3,4-TeCDD in SIP microcosms assimilating ¹³C-acetate as the carbon source. Analysis of bacterial community profiles of the ¹³C labeled heavy DNA fraction revealed that an OTU corresponding to Dehalococcoides mccartyi accounted for a significantly greater abundance in cultures amended with 1,2,3,4-TeCDD than in cultures without 1,2,3,4-TeCDD. This implies the involvement of this Dehalococcoides mccartyi strain in the reductive dechlorination of 1,2,3,4-TeCDD and suggests the applicability of SIP for a robust assessment of the bioremediation potential of organohalogen contaminated sites.

Redox fluctuations shape the soil microbiome in the hypoxic bioremediation of octachlorinated dibenzodioxin- and dibenzofuran-contaminated soil

The biodegradation of polychlorinated-p-dioxins and dibenzofurans (PCDD/Fs) has been recently demonstrated in a single reactor under hypoxic conditions. Maintaining hypoxic conditions through periodic aerations results in a marked fluctuation of reduction-oxidation (redox) potential. To further assess the effects of redox fluctuations, we operated two fed-batch continuously stirred tank reactors (CSTRs) with sophisticated redox controls at different anoxic/oxic fluctuations to reduce PCDD/Fs in contaminated soil. The results of long-term reactor operation showed that the CSTR with redox fluctuations at a narrow range (-63 ± 68 mV) (CSTR_A) revealed a higher substrate hydrolysis level and PCDD/F degradation rate than did the CSTR with a redox potential that fluctuated at a broad range (-13 ± 118 mV) (CSTR_B). In accordance with analyses of bacterial 16S rRNA genes, the designated hypoxic conditions with added compost supported survival of bacterial populations at a density of approximately 10⁹ copies/g slurry. The evolved core microbiome was dominated by anoxic/oxic fluctuation-adapted Bacteroidetes, Alphaproteobacteria, and Actinobacteria, with higher species diversity and functionality, including hydrolysis and degradation of dioxin-like compounds in CSTR_A than in CSTR_B. Taken together, the overall results of this study expand the understanding of redox fluctuations in association with the degradation of recalcitrant substrates in soil and the corresponding microbiome.

Dehalogenases: From Improved Performance to Potential Microbial Dehalogenation Applications

The variety of halogenated substances and their derivatives widely used as pesticides, herbicides and other industrial products is of great concern due to the hazardous nature of these compounds owing to their toxicity, and persistent environmental pollution. Therefore, from the viewpoint of environmental technology, the need for environmentally relevant enzymes involved in biodegradation of these pollutants has received a great boost. One result of this great deal of attention has been the identification of environmentally relevant bacteria that produce hydrolytic dehalogenases—key enzymes which are considered cost-effective and eco-friendly in the removal and detoxification of these pollutants. These group of enzymes catalyzing the cleavage of the carbon-halogen bond of organohalogen compounds have potential applications in the chemical industry and bioremediation. The dehalogenases make use of fundamentally different strategies with a common mechanism to cleave carbon-halogen bonds whereby, an active-site carboxylate group attacks the substrate C atom bound to the halogen atom to form an ester intermediate and a halide ion with subsequent hydrolysis of the intermediate. Structurally, these dehalogenases have been characterized and shown to use substitution mechanisms that proceed via a covalent aspartyl intermediate. More so, the widest dehalogenation spectrum of electron acceptors tested with bacterial strains which could dehalogenate recalcitrant organohalides has further proven the versatility of bacterial dehalogenators to be considered when determining the fate of halogenated organics at contaminated sites. In this review, the general features of most widely studied bacterial dehalogenases, their structural properties, basis of the degradation of organohalides and their derivatives and how they have been improved for various applications is discussed.

Dechlorinating bacteria are abundant but anaerobic dechlorination of weathered polychlorinated dibenzo-p-dioxins and dibenzofurans in contaminated sediments is limited

The potential for microbial dechlorination of the weathered polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) was determined in sediments with historical contamination by the chlorophenol wood preservative Ky-5 and its associated dimeric impurities. Sediments were collected from four sites of the Kymijoki River in South-Eastern Finland located at 0, 20, 30, and 60 km downstream from the source of contamination, and at a reference site. We examined the congener profiles of historical PCDD/Fs, including non-2,3,7,8-substituted congeners, and determined the dechlorination potential in sediments at the different sites of the river. The measured mean total concentrations for 2,3,7,8-PCDD/Fs were extremely high, 1200 mg/kg dw, at the most contaminated site, Kuusankoski. The mean concentrations for the predominant 2,3,7,8-congeners were 1,2,3,4,6,7,8-HpCDF 780 mg/kg dw, and for OCDF 380 mg/kg dw at Kuusankoski. At all other study sites of the river the mean total concentrations for 2,3,7,8-PCDD/Fs varied between 9 and 96 mg/kg dw, (6-80 mg/kg dw for 1,2,3,4,6,7,8-HpCDF, 3-13 mg/kg dw for OCDF). The sediment PCDD/F composition was similar to that of Ky-5, indicating that no or only minimal biodegradation of PCDD/F congeners has occurred in the river sediments over the last few decades since the contamination events. Microbes capable of PCDD/F dechlorination were present at all study sites based on Dehalococcoides-like Chloroflexi community determination and dechlorination of spiked 1,2,3,4-tetrachlorodibenzofuran. However, no substantial changes in the relative abundances of PCDD/Fs were observed over 2.5 years in laboratory microcosm studies, indicating that anaerobic dechlorination of weathered PCDD/Fs was limited over the course of the experiment. Therefore, concentrations of weathered PCDD/Fs in the sediments of the Kymijoki River are expected to remain at the same level for decades or centuries with further migration towards the Baltic Sea.

Forensic Analysis of Polychlorinated Dibenzo-p-Dioxin and Furan Fingerprints to Elucidate Dechlorination Pathways

Polychlorinated dibenzo-p-dioxins and -furans (PCDD/Fs) are persistent organic pollutants whose main removal process in the environment is due to biodegradation, and particularly anaerobic reductive dechlorination. Since PCDD/F congeners that are substituted in the lateral 2, 3, 7, and 8 positions are the most toxic, removal of these chlorines is advantageous, but previous studies have only demonstrated their removal under laboratory conditions. We evaluated a concentration data set of PCDD/F congeners with four or more chlorines along with all 209 polychlorinated biphenyl (PCB) congeners in surface water, treated and untreated wastewater, landfill leachate, and biosolids (NY CARP data set) to determine whether peri and peri/lateral dechlorination of PCDD/Fs occurs in these environments. Positive Matrix Factorization (PMF) applied to the data set revealed a factor indicative of the microbial dechlorination of PCBs, and this factor also contained a variety of non-2,3,7,8 substituted PCDD/F congeners. These results suggest that dechlorination of PCDD/Fs at the lateral positions is facile if not preferred in these environments. The relative lack of tetra- and penta-chlorinated PCDD/Fs suggested that dechlorination proceeds to PCDD/F congeners with less than four chlorines. The PMF results were confirmed by examining three samples that contained >90% PCB dechlorination products from the Fresh Kills Landfill and the Hudson River. Even without factor analysis, these samples demonstrated almost identical PCDD/F congener patterns. This study suggests that PCDD/Fs are reductively dechlorinated to nontoxic non-2,3,7,8 PCDD/F congeners in sewers and landfills as well as in the sediment of the Upper Hudson River.

Impact of estuarine gradients on reductive dechlorination of 1,2,3,4-tetrachlorodibenzo-p-dioxin in river sediment enrichment cultures

Polychlorinated dibenzo-p-dioxins (PCDDs) are among the most persistent organic pollutants. Although the total input of PCDDs into the environment has decreased substantially over the past four decades, their input via non-point sources is still increasing, especially in estuarine metropolitan areas. Here we report on the microbially mediated reductive dechlorination of PCDDs in anaerobic enrichment cultures established from sediments collected from five locations along the Hackensack River, NJ and investigate the impacts of sediment physicochemical characteristics on dechlorination activity. Dechlorination of 1,2,3,4-tetrachlorodibenzo-p-dioxin (1,2,3,4-TeCDD) and abundance of Dehalococcoides spp. negatively correlated with salinity and sulfate concentration in sediments used to establish the cultures. 1,2,3,4-TeCDD was dechlorinated to a lesser extent in cultures established from sediments from the tidally influenced estuarine mouth of the river. In cultures established from low salinity sediments, 1,2,3,4-TeCDD was reductively dechlorinated with the accumulation of 2-monochlorodibenzo-p-dioxin as the major product. Sulfate concentrations above 2 mM inhibited 1,2,3,4-TecDD dechlorination activity. Consecutive lateral- and peri- dechlorination took place in enrichment cultures with a minimal accumulation of 2,3-dichlorodibenzo-p-dioxin in active cultures. A Dehalococcoides spp. community was enriched and accounted for up to 64% of Chloroflexi detected in these sediment cultures.

The microbial degradation of 2,4,6-tribromophenol (TBP) in water/sediments interface: Investigating bioaugmentation using Bacillus sp. GZT

The substance 2,4,6-Tribromophenol (TBP) is used as a flame retardant in electronic and electric devices, and is a replacement for pentachlorophenol in wood preservation. TBP is a contaminant in different environmental matrices, at levels where treatment is required. This study examined the relationship between the bioaugmention of TBP degradation and the evolution of the microbial community in river water/sediment microcosms. When compared with unamended controls, bioaugmentation with Bacillus sp. GZT effectively enhanced TBP biodegradation, with approximately 40.7% of the TBP removal after a 7-week incubation period, without a lag phase (p<0.01). Amendments with 2-bromophenol, 2,6-dibromophenol, and 2,4-dibromophenol did not promote TBP biodegradation in river water/sediments (p>0.05). However, TBP biodegradation was enhanced by adding other additives, including NaCl, humic acid, sodium lactate, and sodium propionate alone, especially glucose and yeast extract. A metagenomics analysis of the total 16S rRNA genes from the treatment system with bioaugmentation showed that four microbial phyla were dominant: Proteobacteria (52.08-66.22%), Actinobacteria (20.03-5.47%), Bacteroidetes (6.68-13.68%), and Firmicutes (4.53-20.83%). This study highlights the possible benefits using bioaugmentation with GZT to remediate TBP-polluted water and sediments.

Enhancing tetrabromobisphenol A biodegradation in river sediment microcosms and understanding the corresponding microbial community

In situ remediation of contaminated sediment using microbes is a promising environmental treatment method. This study used bioaugmentation to investigate the biodegradation of tetrabromobisphenol A (TBBPA) in sediment microcosms collected from an electronic-waste recycling site. Treatments included adding possible biodegradation intermediates of TBBPA, including 2,4-dibromophenol (2,4-DBP), 2,4,6-tribromophenol (TBP), and bisphenol A (BPA) as co-substrates. Bioaugmentation was done with Ochrobactrum sp. T (TBBPA-degrader) and a mixed culture of Ochrobactrum sp. T, Bacillus sp. GZT (TBP-degrader) and Bacillus sp. GZB (BPA-degrader). Results showed that bioaugmentation with Ochrobactrum sp. T significantly improved TBBPA degradation efficiencies in sediment microcosms (P < 0.01); aerobic conditions increased the microbes' degradation activities. Co-substrates 2,4-DBP, TBP and BPA inhibited biodegradation of TBBPA. A metagenomic analysis of total 16S rRNA genes from the treated sediment microcosms showed that the following dominant genera: Ochrobactrum, Parasegetibacter, Thermithiobacillus, Phenylobacterium and Sphingomonas. The genus level of Ochrobactrum increased with increased degradation time, within 10-week of incubation. Microbes from genus Ochrobactrum are mainly linked to enhance the TBBPA biodegradation.

Effect of temperature on the reductive dechlorination of 1,2,3,4-tetrachlorodibenzofuran in anaerobic PCDD/F-contaminated sediments

The effect of temperature on the reductive dechlorination in sediments of the PCDD/F-contaminated Kymijoki River, Finland was assessed with 1,2,3,4-tetrachlorodibenzofuran (1,2,3,4-TeCDF) at various temperatures and with co-amendment of 2,3,4,6-tetrachlorophenol (2,3,4,6-TeCP) in laboratory microcosms. The dechlorination rate of 1,2,3,4-TeCDF increased with incubation temperature, with TeCDF half-lives of 2.1 y at 21°C, 3.9 y at 15°C, and 19.0 y at 4°C. Co-amendment with 2,3,4,6-TeCP reduced the TeCDF half-life to 1.8 y at 21°C. 1,2,3,4-TeCDF was dechlorinated mainly in the lateral position to 1,3,4-TrCDF and then to 1,3-DiCDF over 29 months, but incubation temperature affected the relative molar ratios of the dechlorination products. The abundance of the Dehalococcoides-like Chloroflexi community did not substantially change in microcosms over 24 months incubation at the different temperatures. The dechlorination activity of 1,2,3,4-TeCDF was significantly limited at lower temperatures, which should be considered in predicting the environmental fate of aged PCDD/Fs in sediments of the Kymijoki River.

Anaerobic reductive dechlorination of 1,2,3,4-tetrachlorodibenzofuran in polychlorinated dibenzo-p-dioxin- and dibenzofuran-contaminated sediments of the Kymijoki River, Finland

Sediments of the Kymijoki River are highly contaminated with polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs). These persistent PCDD/Fs resist biotic degradation and therefore the potential for microbial reductive dechlorination was assessed to determine how microbes impact the fate of these compounds. Anaerobic sediment microcosms of five different sites in the river were spiked with 1,2,3,4-tetrachlorodibenzofuran (1,2,3,4-TeCDF) as a model compound to determine the dechlorination potential in the sediments. Dechlorinating bacteria were active in all the study sites of the river. The extent of dechlorination over 10 and 29 months corresponded to the levels of aged PCDD/Fs, with sediments of the most contaminated site at Kuusankoski being the most active for reductive dechlorination. The dechlorination activity and levels of aged PCDD/Fs were correlated within the sediment cores at the all sites. The pathway of 1,2,3,4-TeCDF dechlorination was mainly via 1,3,4-trichlorodibenzofuran (TrCDF) to 1,3-dichlorodibenzofuran (DiCDF). Dechlorination via 1,2,4-TrCDF to further dechlorination products was also detected. Lateral reductive dechlorination would decrease the toxicity of 2,3,7,8-substituted PCDD/Fs. Our data suggest that sediments of the Kymijoki River contain indigenous microorganisms that are responsible for dechlorination of PCDD/Fs, especially at the most contaminated site.

Enriching for microbial reductive dechlorination of polychlorinated dibenzo-p-dioxins and dibenzofurans

Anaerobic enrichment cultures derived from contaminated Kymijoki River sediments dechlorinated 1,2,3,4-tetrachlorodibenzofuran (1,2,3,4-tetra-CDF), octachlorodibenzofuran (octa-CDF) and 1,2,3,4-tetrachlorodibenzo-p-dioxin (1,2,3,4-tetra-CDD). 1,2,3,4-tetra-CDF was dechlorinated via 1,2,3-, 2,3,4-, and 1,3,4/1,2,4-tri-CDFs to 1,3-, 2,3-, and 2,4-di-CDFs and finally to 4-mono-CDF. The dechlorination rate of 1,2,3,4-tetra-CDF was generally slower than that of 1,2,3,4-tetra-CDD. The rate and extent of 1,2,3,4-tetra-CDD dechlorination was enhanced by addition of pentachloronitrobenzene (PCNB) as a co-substrate. Dechlorination of spiked octa-CDF was observed with the production of hepta-, hexa-, penta- and tetra-CDFs over 6 months. Two major phylotypes of the Chloroflexi community showed an increase, one of which was identical to the Dehalococcoides mccartyi Pinellas subgroup. A set of twelve putative reductive dehalogenase (rdh) genes increased in abundance with addition of 1,2,3,4-tetra-CDF, 1,2,3,4-tetra-CDD and/or PCNB. This information will aid in understanding how indigenous microbial communities impact the fate of PCDFs and in developing strategies for bioremediation of PCDD/F contaminated sediments.

Bioremediation potential of soil contaminated with highly substituted polychlorinated dibenzo-p-dioxins and dibenzofurans: microcosm study and microbial community analysis

Highly chlorinated dibenzo-p-dioxins/dibenzofurans (DD/Fs) are main hazardous dioxins, and ubiquitously distributed in the environment. To study the feasibility of bioremediation for remedying contamination of highly chlorinated dioxins, closed microcosms were constructed with soil from a chronological site under oxygen-stimulated conditions. The results showed that high levels of near-fully and fully chlorinated DD/Fs, particularly octachlorodibenzofuran were effectually reduced without accumulation of less substituted congeners. The clone library analysis of PCR-amplified 16S rRNA gene from the octachlorodibenzofuran-degrading consortia showed that 98.3% of the detected sequences were affiliated with Proteobacteria. The obtained strains with putative aromatic dioxygenase genes and abilities to repetitively grow in octachlorodibenzofuran-containing agars were closely related to members within Actinobacteria, Firmicutes, and Proteobacteria. Among them, certain Rhodococcus, Micrococcus, Mesorhizobium and Bacillus isolates could degrade octachlorodibenzofuran with efficiencies of 26-43% within 21 days. Hierarchical oligonucleotide primer extension analysis further showed that Micrococcus, Rhizobium, Pseudoxanthomonas, and Brevudimonas populations increased largely when high concentrations of octachlorodibenzofuran were reduced. Overall, our results suggest that a distinctive microbial composition and population dynamic could be required for the enhanced degradation of highly chlorinated DD/Fs in the batch microcosm and highlight a potential of bioremediation technologies in remedying polychlorinated dioxins in the polluted sites.

Microbially mediated reductive dechlorination of weathered polychlorinated dibenzofurans in Kymijoki sediment mesocosms

Little is known about the potential for indigenous microorganisms to reductively dechlorinate weathered polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs) in contaminated sediments. The sediments of River Kymijoki, Finland are heavily contaminated with PCDFs originating from manufacture of the chlorophenol-based fungicide Ky-5. Reductive dechlorination of weathered PCDFs was monitored to examine strategies for stimulating such activities. Amendments with electron donors, a halogenated co-substrate (tetrachlorobenzene, TeCBz), and bioaugmentation with a mixed culture containing Dehalococcoides mccartyi strain 195 were used to stimulate dechlorination activity in 30 L River Kymijoki sediment mesocosms incubated from 18 °C to 21 °C. An initial onset of dechlorination of octa-, hepta- and hexa-CDFs was observed in all mesocosms in the first 2 years of incubation. During this initial 2-year period, the decrease in the mol% contribution of these PCDFs was coupled with an increase in the mol% contribution of tetra- and penta-CDFs. The ratio of 1,2,3,4,6,7,8- to 1,2,3,4,6,8,9-hepta-CDF increased significantly. Subtle differences were observed between amended and unamended mesocosms. For penta-CDFs, a decreasing mol% ratio of peri vs. total chlorines and increasing mol% ratio of lateral vs. total chlorines was observed in mesocosms amended with TeCBz, suggesting that the amendments may affect pathways of dechlorination. Analysis of congener patterns using principal components analysis supported the observation that dechlorination was most pronounced during the first 2 years. Polymerase chain reaction and denaturing gradient gel electrophoresis (PCR-DGGE) analysis of 16S rRNA genes revealed a diverse Chloroflexi community. This study showed evidence for dechlorination of weathered PCDFs in Kymijoki sediment mesocosms mediated by indigenous microorganisms.

Carbon isotope fractionation during dechlorination of 1,2,3,4-tetrachlorodibenzo-p-dioxin by a Dehalococcoides-containing culture

Carbon isotope fractionation was observed during dechlorination of 1,2,3,4-tetrachlorodibenzo-p-dioxin (1,2,3,4-TeCDD) by a mixed culture containing Dehalococcoides ethenogenes strain 195. Fractionation was examined when 1,2,3,4-TeCDD was added as the only chlorinated compound and when 1,2,3,4-TeCDD was added with a known growth substrate, tetrachloroethene (PCE). The 1,2,3,4-TeCDD was dechlorinated to 1,2,4-trichlorodibenzo-p-dioxin (1,2,4-TrCDD) which was enriched in (13)C relative to 1,2,3,4-TeCDD with isotope separation factors, epsilon(C), of 1.3+/-0.2 per thousand and 1.7+/-0.4 per thousand (average+/-95% confidence interval (CI)) in cultures with and without PCE, respectively. The 1,2,4-TrCDD was further dechlorinated to 1,3-dichlorodibenzo-p-dioxin (1,3-DCDD) which was depleted in (13)C relative to 1,2,4-TrCDD with epsilon(C) of -2.4+/-0.4 per thousand and -2.9+/-0.8 per thousand (average+/-95% CI) in cultures with and without PCE, respectively. This demonstrates carbon isotope fractionation during sequential reductive dechlorination of PCDDs, where isotope fractionation during dechlorination of the intermediate was substantial and a (13)C depleted lightly chlorinated PCDD congener was ultimately formed during dechlorination of more highly chlorinated PCDD congeners. Despite reproducible, statistically significant differences between isotope compositions of the parent, 1,2,3,4-TeCDD and daughter, 1,2,4-TrCDD and 1,3-DCDD congeners in triplicate bottles of both treatments, fractionation factors for 1,2,3,4-TeCDD could not be determined for all replicates by regression analysis of the plot of the Rayleigh equation. It is possible that dissolution of 1,2,3,4-TeCDD imposed a kinetic limitation on dechlorination, thus masking isotope fractionation during its dechlorination.

Properties of a trichlorodibenzo-p-dioxin-dechlorinating mixed culture with a Dehalococcoides as putative dechlorinating species

An anaerobic mixed culture enriched over 16 transfers (1/10) from Saale river sediment reductively dehalogenated 1,2,4- and 1,2,3-trichlorodibenzo-p-dioxin (TrCDD) to di- and monochlorinated congeners in the presence of pyruvate (or lactate) and fumarate as cosubstrates. Besides TrCDD, tetrachloroethene and 1,2,3-trichlorobenzene were dechlorinated. Dioxin dehalogenation was sensitive to pasteurization, but was not remarkably influenced by inhibitors of methanogens, sulfate-reducing bacteria or Gram-positive bacteria. The rate of 1,3-dichlorodibenzo-p-dioxin formation increased with rising initial concentrations of 1,2,4-TrCDD (1-250 microM) from 0.05 to 5.4 micromol l(-1) day(-1). Two isolates, belonging to Sulfurospirillum and Trichococcus, did not show reductive dehalogenation. 16S rDNA-targeted methods further revealed the presence of Acetobacterium, Desulfitobacterium, Desulfuromonas and Dehalococcoides. Nested polymerase chain reaction (PCR) indicated the presence of Dehalococcoides in highest most probable number (MPN) dilutions that were positive for dioxin dechlorination.

Modeling the reductive dechlorination of polychlorinated dibenzo-p-dioxins: kinetics, pathway, and equivalent toxicity

Reductive dechlorination of polychlorinated dibenzo-p-dioxins (PCDDs) involves 256 reactions linking 76 congeners with highly variable toxicities, so it is challenging to assess the overall effect of this process on the environmental impact of PCDD contamination. This study describes a quantitative solution to this problem that allows calculation of the toxic equivalent quantity (TEQ) of the mixture of PCDD congeners predicted by a linear free-energy relationship (LFER) that relates rate constants of dechlorination to calculated reduction potentials. The reduction potentials were derived from Gibbs free energies, calculated using density functional theory (DFT) and vapor pressures and solubilities for individual congeners. The LFER was calibrated with rate constants that we recently reported for PCDD dechlorination by nano-zerovalent iron (nZVI). Simulation done with this model predicts that more than 100 years would be required for complete dechlorination of octachlorinated dibenzo-p-dioxin (OCDD) to dibenzo-p-dioxin (DD) under conditions representative of treatment with nZVI and that the TEQ of the mixture of intermediates during this process increases 10-fold, peaking at around 3-6 years, mainly because of the higher toxicity of 2,3,7,8-substituted congeners.

Bioremediation of PCDD/Fs-contaminated municipal solid waste incinerator fly ash by a potent microbial biocatalyst

Removal of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) from fly ash poses a serious problem. In the study presented here, we used a microbial biocatalyst which is a mixture of 4 bacterial and 5 fungal dioxin-degrading strains. The ability of this biocatalyst to bioremediate PCDD/Fs from contaminated municipal solid waste incinerator (MSWI) fly ash was examined by solid-state fermentation under laboratory conditions. Treatment of MSWI fly ash with the microbial biocatalyst for 21 days resulted in a 68.7% reduction in total toxic PCDD/Fs. Further analyses revealed that the microbial biocatalyst also removed 66.8% of the 2,3,7,8-substituted congeners from the fly ash. During the treatment period, the presence of the individual strains composing the microbial biocatalyst was monitored by the amplification of strain-specific DNA sequences followed by denaturing gradient gel electrophoresis (DGGE). This analysis showed that all of the bacterial and fungal strains composing this dioxin-degrading microbial mixture maintained under the dioxin treatment conditions. These results demonstrate that this microbial biocatalyst could potentially be used in the bioremediation of PCDD/Fs from contaminated fly ash.

Microbial degradation of chlorinated dioxins

Polychlorinated dibenzo-p-dioxins (PCDD) and polychlorinated dibenzofurans (PCDF) were introduced into the biosphere on a large scale as by-products from the manufacture of chlorinated phenols and the incineration of wastes. Due to their high toxicity they have been the subject of great public and scientific scrutiny. The evidence in the literature suggests that PCDD/F compounds are subject to biodegradation in the environment as part of the natural chlorine cycle. Lower chlorinated dioxins can be degraded by aerobic bacteria from the genera of Sphingomonas, Pseudomonas and Burkholderia. Most studies have evaluated the cometabolism of monochlorinated dioxins with unsubstituted dioxin as the primary substrate. The degradation is usually initiated by unique angular dioxygenases that attack the ring adjacent to the ether oxygen. Chlorinated dioxins can also be attacked cometabolically under aerobic conditions by white-rot fungi that utilize extracellular lignin degrading peroxidases. Recently, bacteria that can grow on monochlorinated dibenzo-p-dioxins as a sole source of carbon and energy have also been characterized (Pseudomonas veronii). Higher chlorinated dioxins are known to be reductively dechlorinated in anaerobic sediments. Similar to PCB and chlorinated benzenes, halorespiring bacteria from the genus Dehalococcoides are implicated in the dechlorination reactions. Anaerobic sediments have been shown to convert tetrachloro- to octachlorodibenzo-p-dioxins to lower chlorinated dioxins including monochlorinated congeners. Taken as a whole, these findings indicate that biodegradation is likely to contribute to the natural attenuation processes affecting PCDD/F compounds.

Dechlorination and detoxification of 1,2,3,4,7,8-hexachlorodibenzofuran by a mixed culture containing Dehalococcoides ethenogenes Strain 195

Toxic polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) with chlorines substituted at the lateral 2, 3, 7, and 8 positions are of great environmental concern. We investigated the dechlorination of 1,2,3,4,7,8-hexachlorodibenzofuran (1,2,3,4,7,8-HxCDF) and 1,2,3,4,6,7,8,9-octachlorodibenzo-p-dioxin (OCDD) by a mixed culture containing Dehalococcoides ethenogenes strain 195. The 1,2,3,4,7,8-HxCDF was dechlorinated to 1,3,4,7,8-pentachlorodibenzofuran and 1,2,4,7,8-pentachlorodibenzofuran and further to two tetrachlorodibenzofuran congeners, which were identified as 1,3,7,8-tetrachlorodibenzofuran and 1,2,4,8-tetrachlorodibenzofuran. Because no 2,3,7,8-substituted congeners were formed as dechlorination products from 1,2,3,4,7,8-HxCDF, this dechlorination represents a detoxification reaction. Tetrachloroethene (PCE) and 1,2,3,4-tetrachlorobenzene (1,2,3,4-TeCB) were added as additional halogenated substrates to enhance the degree of 1,2,3,4,7,8-HxCDF dechlorination. The 1,2,3,4-TeCB enhanced the extent of dechlorination of 1,2,3,4,7,8-HxCDF approximately 3-fold compared to PCE or no additional substrate amendment. No dechlorination products were detected from OCDD. Bioremediation of PCDD/Fs by bacterial reductive dechlorination should address the pathway of dechlorination to ensure detoxification.

Microbial dehalogenation of trichlorinated dibenzo-p-dioxins by a Dehalococcoides-containing mixed culture is coupled to carbon isotope fractionation

An anaerobic enrichment culture reductively dehalogenated 1,2,4- and 1,2,3-trichlorodibenzo-p-dioxin (TrCDD) almost exclusively at peripheral positions forming the main products 1,3-dichloro-(DiCDD) and 2-monochlorodibenzo-p-dioxin (MCDD) from 1,2,4-TrCDD and 2,3-DiCDD from 1,2,3-TrCDD. Dehalococcoides was monitored in the mixed culture by quantitative real-time PCR. A yield of 2.5 x 10(8) to 2.75 x 10(8) copies of 16S rRNA genes per micromole of chloride released suggested growth by dehalorespiration with dibenzo-p-dioxins. For the analysis of carbon isotope fractionation, the dioxin congeners were isolated by solid-phase microextraction (SPME) from the headspace of the cultures. The delta13C composition of 1,2,4-TrCDD did not change remarkably during the course of reductive dehalogenation; however, the intermediate 1,3-DiCDD became enriched, and the final product 2-MCDD significantly depleted in 13C with a discrimination of 2.5-3.6 per thousand between 1,3-DiCDD and 2-MCDD. 1,2,3-TrCDD and its main product 2,3-DiCDD became slightly enriched in 13C, whereas the formed low concentrations of 2-MCDD were depleted in 13C by 5.5-4.8 per thousand. This study demonstrates carbon isotope fractionation during sequential reductive dehalogenation of chlorinated dibenzo-p-dioxins, whereby isotope fractionation upon dehalogenation of the intermediate was substantial. This can provide a basis for the development of a new method to monitor the fate of dioxins in the environment using compound specific stable isotope analyses.

Comparison of anaerobic microbial communities from Estuarine sediments amended with halogenated compounds to enhance dechlorination of 1,2,3,4-tetrachlorodibenzo-p-dioxin

A suite of experiments were conducted to ascertain whether dehalogenation of a model dioxin compound could be stimulated in marine sediments by supplementation with halogenated analogues to enrich for dehalogenating bacteria and if growth by members of the Chloroflexi-like group was associated with dioxin removal. Five halogenated compounds (tetrachlorobenzene, tetrachloroanisole, tetrachlorophenol, tetrachlorobenzoic acid and trichloroacetophenone) were added with 1,2,3,4-tetrachlorodibenzo-p-dioxin (TeCDD) to estuarine sediments from four sites in San Diego Bay and the coast of southern New Jersey to test for dioxin dehalogenation. Most of the halogenated additives were found to stimulate dechlorination of the model dioxin. Molecular analysis of the bacterial population using 16S rRNA and reductive dehalogenase genes indicated that distinct microbial populations were enriched with each halogenated co-amendment. Additionally, Chloroflexi-like ribosomal genes associated with dehalogenation were detected. For example, quantitative real-time PCR analysis of 16S rRNA and reductive dehalogenase gene copy number in the microcosms showed a positive correlation with 1,2,3,4-TeCDD reductive dechlorination in coastal sediments amended with different halogenated additives. These results suggest that specific Chloroflexi-like microorganisms related to Dehalococcoides are involved in 1,2,3,4-TeCDD reductive dechlorination.

Phylogenetic characterization of a polychlorinated-dioxin- dechlorinating microbial community by use of microcosm studies

Microcosms capable of reductive dechlorination of polychlorinated dibenzo-p-dioxins/dibenzofurans (PCDD/Fs) were constructed in glass bottles by seeding them with a polluted river sediment and incubating them anaerobically with an organic medium. All of the PCDD/F congeners detected were equally reduced without the accumulation of significant amounts of less-chlorinated congeners as the intermediate or end products. Alternatively, large amounts of catechol and salicylic acid were produced in the upper aqueous phase. Thus, the dechlorination of PCDD/Fs and the oxidative degradation of the dechlorinated products seemed to take place simultaneously in the microcosm. Denaturing gel gradient electrophoresis and clone library analyses of PCR-amplified 16S rRNA genes from the microcosm showed that members of the phyla Firmicutes, Proteobacteria, and Bacteroidetes predominated. A significant number of Chloroflexi clones were also detected. Quantitative real-time PCR with specific primer sets showed that the 16S rRNA genes of a putative dechlorinator, "Dehalococcoides," and its relatives accounted for 0.1% of the total rRNA gene copies of the microcosm. Most of the clones thus obtained formed a cluster distinct from the typical "Dehalococcoides" group. Quinone profiling indicated that ubiquinones accounted for 18 to 25% of the total quinone content, suggesting the coexistence and activity of ubiquinone-containing aerobic bacteria. These results suggest that the apparent complete dechlorination of PCDD/Fs found in the microcosm was due to a combination of the dechlorinating activity of the "Dehalococcoides"-like organisms and the oxidative degradation of the dechlorinated products by aerobic bacteria with aromatic hydrocarbon dioxygenases.

Biodegradation of xenobiotics by anaerobic bacteria

Xenobiotic biodegradation under anaerobic conditions such as in groundwater, sediment, landfill, sludge digesters and bioreactors has gained increasing attention over the last two decades. This review gives a broad overview of our current understanding of and recent advances in anaerobic biodegradation of five selected groups of xenobiotic compounds (petroleum hydrocarbons and fuel additives, nitroaromatic compounds and explosives, chlorinated aliphatic and aromatic compounds, pesticides, and surfactants). Significant advances have been made toward the isolation of bacterial cultures, elucidation of biochemical mechanisms, and laboratory and field scale applications for xenobiotic removal. For certain highly chlorinated hydrocarbons (e.g., tetrachlorethylene), anaerobic processes cannot be easily substituted with current aerobic processes. For petroleum hydrocarbons, although aerobic processes are generally used, anaerobic biodegradation is significant under certain circumstances (e.g., O(2)-depleted aquifers, oil spilled in marshes). For persistent compounds including polychlorinated biphenyls, dioxins, and DDT, anaerobic processes are slow for remedial application, but can be a significant long-term avenue for natural attenuation. In some cases, a sequential anaerobic-aerobic strategy is needed for total destruction of xenobiotic compounds. Several points for future research are also presented in this review.

Detection and characterization of a dehalogenating microorganism by terminal restriction fragment length polymorphism fingerprinting of 16S rRNA in a sulfidogenic, 2-bromophenol-utilizing enrichment

Terminal restriction fragment length polymorphism analysis of reverse-transcribed 16S rRNA during periods of community flux was used as a tool to delineate the roles of the members of a 2-bromophenol-degrading, sulfate-reducing consortium. Starved, washed cultures were amended with 2-bromophenol plus sulfate, 2-bromophenol plus hydrogen, phenol plus sulfate, or phenol with no electron acceptor and were monitored for substrate use. In the presence of sulfate, 2-bromophenol and phenol were completely degraded. In the absence of sulfate, 2-bromophenol was dehalogenated and phenol accumulated. Direct terminal restriction fragment length polymorphism fingerprinting of the 16S rRNA in the various subcultures indicated that phylotype 2BP-48 (a Desulfovibrio-like sequence) was responsible for the dehalogenation of 2-bromophenol. A stable coculture was established which contained predominantly 2BP-48 and a second Desulfovibrio-like bacterium (designated BP212 based on terminal restriction fragment length polymorphism fingerprinting) that was capable of dehalogenating 2-bromophenol to phenol. Strain 2BP-48 in the coculture could couple reductive dehalogenation to growth with 2-bromophenol, 2,6-dibromophenol, or 2-iodophenol and lactate or formate as the electron donor. In addition to halophenols, strain 2BP-48 appears to use sulfate, sulfite, and thiosulfate as electron acceptors and is capable of simultaneous sulfidogenesis and reductive dehalogenation in the presence of sulfate.

Dehalococcoides ethenogenes strain 195 reductively dechlorinates diverse chlorinated aromatic pollutants

Dehalococcoides ethenogenes strain 195 dechlorinates tetrachloroethene to vinyl chloride and ethene, and its genome has been found to contain up to 17 putative dehalogenase gene homologues, suggesting diverse dehalogenation ability. We amended pure or mixed cultures containing D. ethenogenes strain 195 with 1,2,3,4-tetrachlorodibenzo-p-dioxin, 2,3,7,8-tetrachlorodibenzo-p-dioxin, 2,3-dichlorodibenzo-p-dioxin, 1,2,3,4-tetrachlorodibenzofuran, 2,3,4,5,6-pentachlorobiphenyl, 1,2,3,4-tetrachloronaphthalene, various chlorobenzenes, or a mixture of 2-, 3-, and 4-chlorophenol to determine the dehalogenation ability. D. ethenogenes strain 195 dechlorinated 1,2,3,4-tetrachlorodibenzo-p-dioxin to a mixture of 1,2,4-trichlorodibenzo-p-dioxin and 1,3-dichlorodibenzo-p-dioxin. 2,3,4,5,6- Pentachlorobiphenyl was dechlorinated to 2,3,4,6- and/or 2,3,5,6-tetrachlorobiphenyl and 2,4,6-trichlorobiphenyl. 1,2,3,4-Tetrachloronaphthalene was dechlorinated primarily to an unidentified dichloronaphthalene congener. The predominant end products from hexachlorobenzene dechlorination were 1,2,3,5-tetrachlorobenzene and 1,3,5-trichlorobenzene. We did not detect dechlorination daughter products from monochlorophenols, 2,3-dichlorodibenzo-p-dioxin or 2,3,7,8- tetrachlorodibenzo-p-dioxin. D. ethenogenes strain 195 has the ability to dechlorinate many different types of chlorinated aromatic compounds, in addition to its known chloroethene respiratory electron acceptors. Remediation of sediments contaminated with aromatic halogenated organic pollutants such as polychlorinated biphenyls and polychlorinated dibenzo-p-dioxins could require billions of dollars in the coming years. Harnessing microorganisms such as Dehalococcoides spp. that detoxify these compounds via removal of halogens may lead to cost-effective biotechnological approaches for remediation.

Anaerobic Dehalogenation of Organohalide Contaminants in the Marine Environment

Microbially mediated dehalogenation processes contribute to the global cycling of both biogenic and anthropogenic halogenated organic compounds. Detailed information on biodegradation mechanisms for a variety of organohalides and on the microorganisms mediating these processes has greatly increased our understanding of the cycling and fate of these unique and widespread compounds in our environment. The marine environment appears to be a particularly rich source of dehalogenating microorganisms. It is well established by laboratory and field studies that anaerobic dehalogenation of sediment contaminants, such as PCBs, pesticides, and dioxins, occurs intrinsically and can be enhanced via various methods. Specific dehalogenating bacterial populations can be enriched on various organohalides. Biodehalogenation processes are likely to be significantly affected by the prevailing terminal electron-accepting condition, and thus, biotransformation of organohalide contaminants in marine and estuarine environments will vary as a function of the redox conditions within the sediment profile. Fundamental knowledge of the activities and interactions of dehalogenating microorganisms is providing a strong basis for development of new bioremediation technologies for removal of harmful halogenated compounds from our environment.

Anaerobic biotransformation of tetrabromobisphenol A, tetrachlorobisphenol A, and bisphenol A in estuarine sediments

Biotransformation of the flame retardants tetrabromobisphenol A and tetrachlorobisphenol A, and their ultimate biodehalogenation product, bisphenol A, was examined in anoxic estuarine sediments. Dehalogenation of tetrabromobisphenol A and tetrachlorobisphenol A was examined under conditions promoting either methanogenesis or sulfate reduction as the primary terminal electron-accepting process. Complete dehalogenation of tetrabromobisphenol A to bisphenol A with no further degradation of bisphenol A, was observed under both methanogenic and sulfate-reducing conditions. Dehalogenation of tetrachlorobisphenol A under both methanogenic and sulfate-reducing conditions resulted in the accumulation of a persistent dichlorinated bisphenol A isomer, while no bisphenol A was formed. Co-amendment of sediment enrichments with either 2,6-dibromo- or 2,6-dichlorophenol did not affect the extent of dehalogenation as compared to sediments that were amended only with the flame retardants. Sediment cultures pre-acclimated on 2-bromophenol dehalogenated the flame retardants in a manner similar to that of fresh sediments. No loss of bisphenol A was observed in separate incubations within 162 days under conditions promoting either methanogenesis, sulfate-reduction, iron(III)-reduction, or nitrate-reduction. Furthermore, identical enrichments that readily degraded 4-hydroxybenzoate, a structural analogue of bisphenol A, did not exhibit bisphenol A degradation. The dehalogenation of tetrabromo- and tetrachlorobisphenol A and the potential for accumulation of bisphenol A in anoxic sediments is significant given the widespread use of these chemicals.