Transformation of HBCDs by Rhodococcus sp. stu-38

Distribution of microbial taxa and genes degrading halogenated organic pollutants in the mangroves

Anthropogenic activities have led to serious contamination of halogenated organic pollutants (HOPs), such as PCBs, PBDEs, and HBCDs, in the mangrove wetland. Biodegradation of HOPs is generally driven by environmental microorganisms harboring dehalogenase genes. However, little is known if HOPs can affect the distributions of HOPs-degrading bacteria and dehalogenase genes in the mangrove wetlands. Historical data suggested that HOPs contamination has been persistent and even deteriorated in the mangrove wetlands in China. We found that the organohalides-respiring bacteria Dehalococcoidia and reductive dehalogenase genes were more prevalent in the subsurface layer sediments (20-30 cm depth; 1.935-9.876 % relative abundance; 71-286 contigs) than the surface layer (0-5 cm depth; 0.174-2.020 % relative abundance; 7-130 contigs). While the genes of haloacid and haloalkane dehalogenases were more abundant in the surface layer (30-100 and 18-138 contigs) than the subsurface layer (22-56 and 50-101 contigs). The abundance of HOPs-degrading genes of reductive dehalogenase, haloacid dehalogenases, AtzA, AtzB, TrzA, TrzN, PcpB, were determined by GeoChip 5.0. Their total abundance ranged from 444.760 to 880.909. Their distributions were mainly associated with the contamination levels of HOPs and strength of anthropogenic activities around the mangrove wetlands. Therefore, the distribution of bacterial taxa and genes involved in HOPs degradation was related to the depth of sediments and affected by the selective stress from HOPs.

Biotransformation of HBCDs by the microbial communities enriched from mangrove sediments

1,2,5,6,9,10-Hexabromocyclododecanes (HBCDs) are a sort of persistent organic pollutants (POPs). This research investigated 12 microbial communities enriched from sediments of four mangroves in China to transform HBCDs. Six microbial communities gained high transformation rates (27.5-97.7%) after 12 generations of serial transfer. Bacteria were the main contributors to transform HBCDs rather than fungi. Analyses on the bacterial compositions and binning genomes showed that Alcanivorax (55.246-84.942%) harboring haloalkane dehalogenase genes dadAH and dadBH dominated the microbial communities with high transformation rates. Moreover, expressions of dadAH and dadBH in the microbial communities and Alcanivorax isolate could be induced by HBCDs. Further, it was found that purified proteins DadAH and DadBH showed high conversion rates on HBCDs in 36 h (91.9 ± 7.4 and 101.0 ± 1.8%, respectively). The engineered Escherichia coli BL21 strains harbored two genes could convert 5.7 ± 0.4 and 35.1 ± 0.1% HBCDs, respectively, lower than their cell-free crude extracts (61.2 ± 5.2 and 56.5 ± 8.7%, respectively). The diastereoisomer-specific transforming trend by both microbial communities and enzymes were γ- > α- > β-HBCD, differed from α- > β- > γ-HBCD by the Alcanivorax isolate. The identified transformation products indicated that HBCDs were dehalogenated via HBr elimination (dehydrobromination), hydrolytic and reductive debromination pathways in the enriched cultures. Two enzymes converted HBCDs via hydrolytic debromination. The present research provided theoretical bases for the biotransformation of HBCDs by microbial community and the bioremediation of HBCDs contamination in the environment.