Dechlorination of three tetrachlorobenzene isomers by contaminated harbor sludge-derived enrichment cultures follows thermodynamically favorable reactions

Yue Lu,Steffi Herrmann,Pieter Vandermeeren,Javier Ramiro-Garcia,Hauke Smidt,Danuta Cichocka,Dirk Springael,Siavash Atashgahi

doi:10.1007/s00253-016-8004-8

Abstract

Dechlorination patterns of three tetrachlorobenzene isomers, 1,2,3,4-, 1,2,3,5-, and 1,2,4,5-TeCB, were studied in anoxic microcosms derived from contaminated harbor sludge. The removal of doubly, singly, and un-flanked chlorine atoms was noted in 1,2,3,4- and 1,2,3,5-TeCB fed microcosms, whereas only singly flanked chlorine was removed in 1,2,4,5-TeCB microcosms. The thermodynamically more favorable reactions were selectively followed by the enriched cultures with di- and/or mono-chlorobenzene as the main end products of the reductive dechlorination of all three isomers. Based on quantitative PCR analysis targeting 16S rRNA genes of known organohalide-respiring bacteria, the growth of Dehalococcoides was found to be associated with the reductive dechlorination of all three isomers, while growth of Dehalobacter, another known TeCB dechlorinator, was only observed in one 1,2,3,5-TeCB enriched microcosm among biological triplicates. Numbers of Desulfitobacterium and Geobacter as facultative dechlorinators were rather stable suggesting that they were not (directly) involved in the observed TeCB dechlorination. Bacterial community profiling suggested bacteria belonging to the phylum Bacteroidetes and the order Clostridiales as well as sulfate-reducing members of the class Deltaproteobacteria as putative stimulating guilds that provide electron donor and/or organic cofactors to fastidious dechlorinators. Our results provide a better understanding of thermodynamically preferred TeCB dechlorinating pathways in harbor environments and microbial guilds enriched and active in anoxic TeCB dechlorinating microcosms.

Highlights

Chlorobenzenes (CBs) are aromatic chlorinated compounds with a benzene ring substituted with one to six chlorine atoms
After a short lag phase of 1–8 days, dechlorination of TeCBs was observed in all microcosms (Fig. 2) indicating that long-term storage at cold temperature did not negatively affect the dechlorination activity of the microcosms. 1,2,4-TCB was observed as a transient intermediate during the dechlorination of all three TeCB isomers, whereas 1,3,5-TCB was only detected as an intermediate product of 1,2,3,5-TeCB-dechlorination
Dechlorination of 1,2,3,4, 1,2,3,5, and 1,2,4,5-TeCB isomers to DCB and/or MCB was observed in anoxic microcosms derived from harbor sludge, and enriched consortia showed selectivity in mediating thermodynamically more favorable steps in the reductive dechlorination of TeCB isomers (Fig. 4)

Summary

Introduction

Chlorobenzenes (CBs) are aromatic chlorinated compounds with a benzene ring substituted with one to six chlorine atoms. Partial reductive dechlorination of highly chlorinated benzenes has been shown in a broad range of sub-oxic/anoxic environments such as soil (Jiang et al 2015; Ramanand et al 1993), riverbed sediment (Nowak et al 1996; Taş et al 2010; Taş et al 2011), sewage sludge (Fathepure et al 1988; Fennell et al 2004), and drainage ditch (Nelson et al 2014) with lower chlorinated benzenes as end products. Preference for thermodynamically more favorable CB reductive dechlorination pathways was shown before by a microbial consortium originating from lake sediment (Beurskens et al 1994), but this selectivity was not observed in other microcosms derived from contaminated freshwater sediment samples (Vandermeeren et al 2014; Zhou et al 2015)

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