A switch of chlorinated substrate causes emergence of a previously undetected native Dehalobacter population in an established Dehalococcoides -dominated chloroethene-dechlorinating enrichment culture

Abstract

Chlorobenzenes are soil and groundwater pollutants of concern which can be reductively dehalogenated by organohalide-respiring bacteria from the genera Dehalococcoides and Dehalobacter. The bioaugmentation culture KB-1 harbours Dehalococcoides mccartyi spp. that reductively dehalogenate trichloroethene to ethene. It contains more than 30 reductive dehalogenase genes; some of them are highly similar to genes found in the chlorobenzene-respiring Dehalococcoides mccartyi strain CBDB1. We explored the chlorobenzene dehalogenation capability of the KB-1 enrichment culture using 1,2,4-trichlorobenzene (1,2,4-TCB). We achieved adaptation of KB-1 to 1,2,4-TCB which is dehalogenated to a mixture of dichlorobenzenes, and subsequently to monochlorobenzene and benzene. Surprisingly, a native Dehalobacter population, and not a Dehalococcoides population, couples the dechlorination of 1,2,4-TCB to growth achieving an average yield of 1.1 ± 0.6 × 10¹³ cells per mole of Cl⁻ released. Interestingly, the dechlorination of 1,2,4-TCB occurs alongside the complete dechlorination of trichloroethene to ethene in cultures fed both electron acceptors. Dehalobacter was not previously identified as a major player in KB-1, but its ecological niche was favoured by the introduction of 1,2,4-TCB. Based on 16S rRNA phylogeny, Dehalobacter populations seem to cluster into specialized clades, and are likely undergoing substrate specialization as a strategy to reduce competition for electron acceptors.

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