1,2-DCA biodegradation potential of an aquifer assessed in situ and in aerobic and anaerobic microcosms

BACKGROUND

1,2-dichloroethane (1,2-DCA) biodegradation can occur through aerobic or anaerobic pathways that can be exploited in bioremediation strategies. Bioremediation interventions are site specific and generally based on anaerobic pathways, nevertheless expanding knowledge on proper conditions favoring the biodegradation and especially on 1,2-DCA degrading microorganisms is crucial. In this work the intrinsic biodegradation potential of an aquifer impacted by Chlorinated Aliphatic Hydrocarbons (mainly 1,2-DCA) was evaluated by characterizing the aquifer microbiome across space and time and by setting up biostimulation treatments in microcosms under different aerobic and anaerobic conditions, in parallel.

RESULTS

The microbial profiling of the aquifer revealed noticeable alpha and beta diversity across the sampling sites within the aquifer and strong fluctuations over time. Surprisingly both the anaerobic and aerobic biostimulation treatments led to the successful removal of 1,2-DCA in microcosms, the enrichment of known 1,2-DCA degraders and the detection of reductive or hydrolytic dehalogenases. Ancylobacter and Starkeya were enriched in aerobic microcosms. Desulfovibrio and Desulfuromonas, known as perchloroethylene degraders, were enriched in anaerobic microcosms, suggesting they could be yet unknown 1,2-DCA respirers.

CONCLUSIONS

Our results demonstrate the occurrence of both aerobic and anaerobic bioremediation potential in the aquifer despite its negative redox potential. Due to the feasibility of direct oxidation with oxygen insufflation, we propose that an enhanced bioremediation strategy based on direct oxidation of 1,2-DCA could be applied to the contaminated aquifer as an ecofriendly, efficient and cost-effective approach as an alternative to anaerobic biodegradation.

Sulfadiazine chlorination disinfection by-products in constructed wetlands: Identification of biodegradation products and inference of transformation pathways

Disinfection by-products (DBPs) formed from chlorination of antibiotics have greater toxicity than their parent compounds. Herein, this study investigated the biotransformation process of sulfadiazine Cl-DBPs in constructed wetlands (CWs). Results showed that, S atom on sulfonyl group, and N atoms on primary and secondary amine groups were the most reactive sites of sulfadiazine molecule. S1-N4 and S1-C8 of sulfadiazine are the most vulnerable bonds to cleave, followed by C14-N4 and C11-N5 bonds. In the chlorination process, sulfadiazine went through C-N bond cleavage, N-reductive alkylation, halogenation, and desulfonation to produce two aromatic Cl-DBPs. In the biodegradation process in CWs, sulfadiazine Cl-DBPs went through processes mainly including dechlorination, S-N bond cleavage, aniline-NH2 oxidation, desulfonation, phenol-OH oxidation, benzene ring cleavage, C-N bond cleavage, and β-oxidation of fatty acids under the action of a variety of oxidoreductases and hydrolases, during which a total of ten biodegradation products was identified. Moreover, sulfadiazine affected the biodegradation rather than the adsorption process in CWs. The two aromatic sulfadiazine Cl-DBPs had much higher bioaccumulation potentials than their parent sulfadiazine, but for the ten biodegradation products of sulfadiazine Cl-DBPs in CWs, 70% and almost 100% of them had lower bioaccumulation potentials than sulfadiazine and their parent sulfadiazine Cl-DBPs, respectively. The CWs were effective in reducing the environmental risk of sulfadiazine Cl-DBPs.

Microbial debromination of hexabromocyclododecanes

Hexabromocyclododecanes (HBCDs), a new sort of brominated flame retardants (BFRs), are globally prevalent and recalcitrant toxic environmental pollutants. HBCDs have been found in many environmental media and even in the human body, leading to serious health concerns. HBCDs are biodegradable in the environment. By now, dozens of bacteria have been discovered with the ability to transform HBCDs. Microbial debromination of HBCDs is via HBr-elimination, HBr-dihaloelimination, and hydrolytic debromination. Biotic transformation of HBCDs yields many hydroxylated and lower brominated compounds which lack assessment of ecological toxicity. Bioremediation of HBCD pollution has only been applied in the laboratory. Here, we review the current knowledge about microbial debromination of HBCDs, aiming to promote the bioremediation applied in HBCD contaminated sites. KEY POINTS: • Microbial debromination of HBCDs is via hydrolytic debromination, HBr-elimination, and HBr-dihaloelimination. • Newly occurred halogenated contaminants such as HBCDs hitch the degradation pathway tamed by previously discharged anthropogenic organohalides. • Strategy that combines bioaugmentation with phytoremediation for bioremediation of HBCD pollution is promising.

Characterizing natural degradation of tetrachloroethene (PCE) using a multidisciplinary approach

A site in mid-western Sweden contaminated with chlorinated solvents originating from a previous dry cleaning facility, was investigated using conventional groundwater analysis combined with compound-specific isotope data of carbon, microbial DNA analysis, and geoelectrical tomography techniques. We show the value of this multidisciplinary approach, as the different results supported each interpretation, and show where natural degradation occurs at the site. The zone where natural degradation occurred was identified in the transition between two geological units, where the change in hydraulic conductivity may have facilitated biofilm formation and microbial activity. This observation was confirmed by all methods and the examination of the impact of geological conditions on the biotransformation process was facilitated by the unique combination of the applied methods. There is thus significant benefit from deploying an extended array of methods for these investigations, with the potential to reduce costs involved in remediation of contaminated sediment and groundwater.

Isolation, characterization and bioaugmentation of an acidotolerant 1,2-dichloroethane respiring Desulfitobacterium species from a low pH aquifer

A Desulfitobacterium sp. strain AusDCA of the Peptococcaceae family capable of respiring 1,2-dichloroethane (1,2-DCA) to ethene anaerobically with ethanol or hydrogen as electron donor at pH 5.0 with optimal range between pH 6.5-7.5 was isolated from an acidic aquifer near Sydney, Australia. Strain AusDCA is distant (94% nucleotide identity) from its nearest phylogenetic neighbor, D. metallireducens, and could represent a new species. Reference gene-based quantification of growth indicated a doubling time of 2 days in cultures buffered at pH 7.2, and a yield of 7.66 (± 4.0) × 106 cells µmol-1 of 1,2-DCA. A putative 1,2-DCA reductive dehalogenase was translated from a dcaAB locus and had high amino acid identity (97.3% for DcaA and 100% for DcaB) to RdhA1B1 of the 1,2-DCA respiring Dehalobacter strain WL. Proteomic analysis confirmed DcaA expression in the pure culture. Dehalogenation of 1,2-DCA (1.6 mM) was observed in batch cultures established from groundwater at pH 5.5 collected 38 days after in situ bioaugmentation but not in cultures established with groundwater collected at the same time from wells not receiving bioaugmentation. Overall, strain AusDCA can tolerate lower pH than previously characterized organohalide respiring bacteria and remained viable in groundwater at pH 5.5.

Diverse dechlorinators and dechlorination genes enriched through amendment of chlorinated natural organic matter fractions

In uncontaminated environments, chlorinated natural organic matter (Cl-NOM) can act as an electron acceptor for organohalide-respiring bacteria. It is unknown, however, whether different types of Cl-NOM are preferentially dechlorinated or whether enrichment with Cl-NOM affects the ability of bacteria to dechlorinate contaminants. In this research NOM was extracted from sediment, fractionated based on hydrophobicity, and either amended to polychlorinated biphenyl-contaminated soil directly or chlorinated and then amended to soil. Amendments of the least hydrophobic Cl-NOM fraction were dechlorinated most rapidly, followed by the moderately hydrophobic Cl-NOM fraction. Soil that had been enriched on the moderately hydrophobic fraction of Cl-NOM was also capable of faster dechlorination of the contaminants trichloroethene and tetrachlorobenzene. Community analysis of the soil during enrichment showed that some known organohalide-respiring bacteria were present and may have played a role in dechlorination; nevertheless, many bacteria appeared to be enriched during both Cl-NOM and contaminant dechlorination. In addition, the quantities of two haloalkane dehalogenase genes increased during enrichment on Cl-NOM. These results show for the first time that Cl-NOM can prime contaminant dechlorination and also suggest that hydrolytic dechlorination processes were involved in both Cl-NOM and contaminant dechlorination.

Presence, Diversity, and Enrichment of Respiratory Reductive Dehalogenase and Non-respiratory Hydrolytic and Oxidative Dehalogenase Genes in Terrestrial Environments

Organohalide-respiring bacteria have been linked to the cycling and possible respiration of chlorinated natural organic matter (Cl-NOM) in uncontaminated soils and sediments. The importance of non-respiratory hydrolytic/oxidative dechlorination processes in the cycling of Cl-NOM in terrestrial soil and sediment, however, is still not understood. This research analyzes the dechlorination potential of terrestrial systems through analysis of the metagenomes of urban lake sediments and cultures enriched with Cl-NOM. Even with the variability in sample type and enrichment conditions, the potential to dechlorinate was universal, with reductive dehalogenase genes and hydrolytic or oxidative dehalogenase genes found in all samples analyzed. The reductive dehalogenase genes detected grouped taxonomically with those from organohalide-respiring bacteria with broad metabolic capabilities, as opposed to those that obligately respire organohalides. Furthermore, reductive dehalogenase genes and two haloacid dehalogenase genes increased in abundance when sediment was enriched with high concentrations of Cl-NOM. Our data suggests that both respiratory and non-respiratory dechlorination processes are important for Cl-NOM cycling, and that non-obligate organohalide-respiring bacteria are most likely involved in these processes.

Abundance of Chlorinated Solvent and 1,4-Dioxane Degrading Microorganisms at Five Chlorinated Solvent Contaminated Sites Determined via Shotgun Sequencing

Shotgun sequencing was used for the quantification of taxonomic and functional biomarkers associated with chlorinated solvent bioremediation in 20 groundwater samples (five sites), following bioaugmentation with SDC-9. The analysis determined the abundance of (1) genera associated with chlorinated solvent degradation, (2) reductive dehalogenase (RDases) genes, (3) genes associated with 1,4-dioxane removal, (4) genes associated with aerobic chlorinated solvent degradation, and (5) D. mccartyi genes associated with hydrogen and corrinoid metabolism. The taxonomic analysis revealed numerous genera previously linked to chlorinated solvent degradation, including Dehalococcoides, Desulfitobacterium, and Dehalogenimonas. The functional gene analysis indicated vcrA and tceA from D. mccartyi were the RDases with the highest relative abundance. Reads aligning with both aerobic and anaerobic biomarkers were observed across all sites. Aerobic solvent degradation genes, etnC or etnE, were detected in at least one sample from each site, as were pmoA and mmoX. The most abundant 1,4-dioxane biomarker detected was Methylosinus trichosporium OB3b mmoX. Reads aligning to thmA or Pseudonocardia were not found. The work illustrates the importance of shotgun sequencing to provide a more complete picture of the functional abilities of microbial communities. The approach is advantageous over current methods because an unlimited number of functional genes can be quantified.