Novel uncultured Chloroflexi dechlorinate perchloroethene to trans‐dichloroethene in tidal flat sediments

Summary The marine environment represents a rich source of bio‐ and geogenically produced organohalogens, including the common pollutant perchloroethene (PCE). However, diversity and function of marine chloroethene‐dechlorinating microorganisms are largely unknown. Here, we have studied the activity and composition of a tidal flat sediment bacterial and archaeal community from the North Sea exposed to low concentrations of PCE. After 2 weeks of incubation, PCE was rapidly dechlorinated via trichloroethene to dichloroethene (DCE). Unexpectedly, these microcosms produced 3.5‐fold more trans ‐DCE than cis ‐DCE. The actively dechlorinating microbial populations were traced by stable isotope probing of rRNA with 13 C‐labelled acetate for 4 days. Terminal restriction fragment length polymorphism fingerprinting and clone libraries of isotopically enriched, ‘heavy’ 13 C‐labelled bacterial 16S rRNA revealed the populations potentially involved in reductive dechlorination. Major clone groups belonged to the Proteobacteria (50.0%; 22.4% δ‐, 12.1% γ‐, 6.9% α‐, 6.9% β‐ and 1.7% ε‐subgroup) and Chloroflexi (29.3%). Populations represented by the two dominant terminal restriction fragments were affiliated with the Dehalococcoidetes (subphylum II of the Chloroflexi ), and were exclusively detected in the heavy fraction of the PCE‐dechlorinating incubation. The phylogenetically novel, larger population, designated Tidal Flat Chloroflexi Cluster, was closely related to the recently discovered PCE‐dechlorinating Lahn Cluster bacteria from anoxic river sediment but more distantly related to canonical Dehalococcoides spp. (92–94% sequence identity). The second population was closely related to ‘ Dehalobium chlorocoercia DF‐1’. Both populations appear to be responsible for reductive dechlorination of highly chlorinated ethenes to predominantly trans ‐DCE in tidal flat sediment incubations.