Desulfitobacterium contributes to the microbial transformation of 2,4,5‐T by methanogenic enrichment cultures from a Vietnamese active landfill

The herbicide 2,4,5‐trichlorophenoxyacetic acid (2,4,5‐T) was a major component of Agent Orange, which was used as a defoliant in the Vietnam War. Little is known about its degradation under anoxic conditions. Established enrichment cultures using soil from an Agent Orange bioremediation plant in southern Vietnam with pyruvate as potential electron donor and carbon source were shown to degrade 2,4,5‐T via ether cleavage to 2,4,5‐trichlorophenol (2,4,5‐TCP), which was further dechlorinated to 3,4‐dichlorophenol. Pyruvate was initially fermented to hydrogen, acetate and propionate. Hydrogen was then used as the direct electron donor for ether cleavage of 2,4,5‐T and subsequent dechlorination of 2,4,5‐TCP. 16S rRNA gene amplicon sequencing indicated the presence of bacteria and archaea mainly belonging to the Firmicutes, Bacteroidetes, Spirochaetes, Chloroflexi and Euryarchaeota. Desulfitobacterium hafniense was identified as the dechlorinating bacterium. Metaproteomics of the enrichment culture indicated higher protein abundances of 60 protein groups in the presence of 2,4,5‐T. A reductive dehalogenase related to RdhA3 of D. hafniense showed the highest fold change, supporting its function in reductive dehalogenation of 2,4,5‐TCP. Despite an ether‐cleaving enzyme not being detected, the inhibition of ether cleavage but not of dechlorination, by 2‐bromoethane sulphonate, suggested that the two reactions are catalysed by different organisms.