Degradation of BDE-47 in mangrove sediments with amendment of extra carbon sources

Isolation and characterization of Rhodococcus sp. GG1 for metabolic degradation of chloroxylenol

The coronavirus disease 2019 (COVID-19) pandemic has significantly increased the demand of disinfectant use. Chloroxylenol (para-chloro-meta-xylenol, PCMX) as the major antimicrobial ingredient of disinfectant has been widely detected in water environments, with identified toxicity and potential risk. The assessment of PCMX in domestic wastewater of Macau Special Administrative Region (SAR) showed a positive correlation between PCMX concentration and population density. An indigenous PCMX degrader, identified as Rhodococcus sp. GG1, was isolated and found capable of completely degrading PCMX (50 mg L-1) within 36 h. The growth kinetics followed Haldane's inhibition model, with maximum specific growth rate, half-saturation constant, and inhibition constant of 0.38 h-1, 7.64 mg L-1, and 68.08 mg L-1, respectively. The degradation performance was enhanced by optimizing culture conditions, while the presence of additional carbon source stimulated strain GG1 to alleviate inhibition from high concentrations of PCMX. In addition, strain GG1 showed good environmental adaptability, degrading PCMX efficiently in different environmental aqueous matrices. A potential degradation pathway was identified, with 2,6-dimethylhydroquinone as a major intermediate metabolite. Cytochrome P450 (CYP450) was found to play a key role in dechlorinating PCMX via hydroxylation and also catalyzed the hydroxylated dechlorination of other halo-phenolic contaminants through co-metabolism. This study characterizes an aerobic bacterial pure culture capable of degrading PCMX metabolically, which could be promising in effective bioremediation of PCMX-contaminated sites and in treatment of PCMX-containing waste streams.

Methanol promotes the biodegradation of 2,2',3,4,4',5,5'-heptachlorobiphenyl (PCB 180) by the microbial consortium QY2: Metabolic pathways, toxicity evaluation and community response

As one of the most frequently detected PCB congeners in human adipose tissue, 2,2',3,4,4',5,5'-heptachlorobiphenyl (PCB 180) has attracted much attention. However, PCB 180 is difficult to be directly utilized by microorganisms due to its hydrophobicity and obstinacy. Herein, methanol (5 mM) as a co-metabolic carbon source significantly stimulated the degradation performance of microbial consortium QY2 for PCB 180 (51.9% higher than that without methanol addition). Six metabolic products including low-chlorinated PCBs and chlorobenzoic acid were identified during co-metabolic degradation, denoting that PCB 180 was metabolized via dechlorination, hydroxylation and ring-opening pathways. The oxidative stress and apoptosis induced by PCB 180 were dose-dependent, but the addition of methanol effectively promoted the tolerance of consortium QY2 to resist unfavorable environmental stress. Additionally, the significant reduction of intracellular reactive oxygen species (ROS) and enhancement of cell viability during methanol co-metabolic degradation proved that the degradation was a detoxification process. The microbial community and network analyses suggested that the potential PCB 180 degrading bacteria in the community (e.g., Achromobacter, Cupriavidus, Methylobacterium and Sphingomonas) and functional abundance of metabolic pathways were selectively enriched by methanol, and the synergies among species whose richness increased after methanol addition might dominate the degradation process. These findings provide new insights into the biodegradation of PCB 180 by microbial consortium.

Systematic characterization of sediment microbial community structure and function associated with anaerobic microbial degradation of PBDEs in coastal wetland

As the widely used flame retardant, polybrominated diphenyl ethers (PBDEs) have been ubiquitously detected in wetland sediments. Microbial degradation is the importantly natural attenuation process for PBDEs in sediments. In this study, the microbial degradation of PBDEs and inherent alternation of microbial communities were explored in anaerobic sediments from coastal wetland, North China. BDE-47 and BDE-153 could be degraded by the indigenous microbes, with biodegradation following pseudo-first-order kinetic. In sediments, the major genera for BDE-47 and BDE-153 degradation were Paeisporosarcina and Gp7, respectively, in single exposure. However, Marinobacter was dominant genera in the combined exposure to BDE-47 and BDE-153, and competition against Marinobacter existed between BDE-47 and BDE-153 degradation. Analysis of bacterial metabolic function indicated that membrane transport, amino acid and carbohydrate metabolism were included in degradation. This study provides the systematic characterization of the sediment microbial community structure and function associated anaerobic microbial degradation of PBDEs in coastal wetland.

Role of extracellular polymeric substances in metal sequestration during mangrove restoration

Extracellular polymeric substances (EPS) are widely observed in aquatic ecosystems, however the potential function of EPS on metal sequestration in mangrove wetlands is unclear. Thus, an ecological restoration area (including Sonneratia apetala, Kandelia obovata and unvegetated mudflat) was employed to assess the effect of mangrove reforestation on metal sequestration and the underlying roles played by EPS. The results showed that mangrove restoration directly promoted metal accumulation (e.g., Cr, Cu, Ni, Pb, and Zn) in sediments. However, alleviated metal bioavailability was detected after mangrove reforestation. The changes in metal accumulation and bioavailability were highly correlated with EPS and microbial composition. Mangrove restoration (especially for K. obovata reforestation) also significantly promoted EPS production, in which multiple metal-chelating functional groups (e.g., hydroxyl, carboxyl, and imino) were identified by Fourier infrared spectra. Moreover, the contents of EPS were positively correlated with metal accumulation but negatively correlated with metal bioavailability. The present data further illustrated that the enhancements of Gammaproteobacteria, Bacteroidia, Desulfobulbia, and Desulfobacteria might be important for EPS production. In summary, this is the first study to reveal that the presence of artificial mangroves might act as an efficient barrier in metal sequestration and immobilization by enhancing inherent microbial EPS.

Toxicity Effects of Combined Mixtures of BDE-47 and Nickel on the Microalgae Phaeodactylum tricornutum (Bacillariophyceae)

Nickel and 2,2’,4,4’-tetrabromodiphenyl ether (BDE-47) are two environmental pollutants commonly and simultaneously present in aquatic systems. Nickel and BDE-47 are individually toxic to various aquatic organisms. However, their toxicity mechanisms are species-dependent, and the toxic effects of combined mixtures of BDE-47 and nickel have not yet been investigated. The present study investigated the toxic effects of combined mixtures of BDE-47 and nickel in the diatom Phaeodactylum tricornutum. BDE-47 and nickel mixtures significantly decreased cell abundance and photosynthetic efficiency, while these cells’ reactive oxygen species (ROS) production significantly increased. The EC₅₀-72 h for BDE-47 and mixtures of BDE-47 and nickel were 16.46 ± 0.93 and 1.35 ± 0.06 mg/L, respectively. Thus, combined mixtures of the two pollutants enhance their toxic effects. Interactions between BDE-47 and nickel were evaluated, revealing synergistic interactions that contributed to toxicity in P. tricornutum. Moreover, transcriptomic analyses revealed photosynthesis, nitrogen metabolism, the biosynthesis of amino acids, the biosynthesis of secondary metabolites, oxoacid metabolism, organic acid metabolism, carboxylic acid metabolism, and oxidation-reduction processes were considerably affected by the mixtures. This study provides evidence for the mechanisms of toxicity from combined BDE-47 and nickel exposure while also improving our understanding of the ecological risks of toxic chemicals on microalgae.

Persistent organic pollutants (POPs) in coastal wetlands: A review of their occurrences, toxic effects, and biogeochemical cycling

Coastal wetlands, such as mangroves, seagrass beds, and salt marshes, are highly threatened by increasing anthropic pressures, including chemical pollution. Persistent organic pollutants (POPs) have attracted attention in these particularly vulnerable ecosystems, due to their bioaccumulative, pervasive, and ecotoxic behavior. This article reviews and summarizes available information regarding current levels, biogeochemical cycling, and effects of POPs on coastal wetlands. Sediment POP levels were compared with international quality guidelines, revealing many areas where compounds could cause damage to biota. Despite this, toxicological studies on some coastal wetland plants and microorganisms showed a high tolerance to those levels. These taxonomic groups are likely to play a key role in the cycling of the POPs, with an active role in their accumulation, immobilization, and degradation. Toxicity and biogeochemical processes varied markedly along three main axes; namely species, environmental conditions, and type of pollutant. While more focused research on newly and unintentionally produced POPs is needed, mainly in salt marshes and seagrass beds, with the information available so far, the environmental behavior, spatial distribution, and toxicity level of the studied POPs showed similar patterns across the three studied ecosystems.

Effects of methanol on the performance of a novel BDE-47 degrading bacterial consortium QY2 in the co-metabolism process

2,2',4,4'-tetrabrominated diphenyl ether (BDE-47), frequently detected in the environment, is arduous to be removed by conventional biological treatments due to its persistence and toxicity. Herein effects of methanol as a co-metabolic substrate on the biodegradation of BDE-47 was systematically studied by a functional bacterial consortium QY2, constructed through long-term and successive acclimation from indigenous microorganisms. The results revealed that BDE-47 (0.25 mg/L) was completely removed within 7 days in the 2.5 mM methanol treatment group, and its degradation efficiency was 3.26 times higher than that without methanol treatment. The addition of methanol dramatically accelerated the debromination, hydroxylation and phenyl ether bond breakage of BDE-47 by QY2. However, excessive methanol (>5 mM) combined with BDE-47 had strong stress on microbial cells, including significant (p < 0.05) increase of reactive oxygen species level, superoxide dismutase activity, catalase activity and malondialdehyde content, even causing 20.65% cell apoptosis and 11.27% death. It was worth noting that the changes of QY2 community structure remained relatively stable after adding methanol, presumably attributed to the important role of the genus Methylobacterium in maintaining the functional and structural stability of QY2. This study deepened our understanding of how methanol as co-metabolite substances stimulated the biodegradation of BDE-47 by microbial consortium.