Role of “Dehalococcoides” spp. in the Anaerobic Transformation of Hexachlorobenzene in European Rivers

Abstract

The diffuse pollution by chlorinated organic compounds in river basins is a concern, due to their potential adverse effects on human health and the environment. Organohalides, like hexachlorobenzene (HCB), are recalcitrant to aerobic microbial degradation, and "Dehalococcoides" spp. are the only known microorganisms capable of anaerobic transformation of these compounds coupled to their growth. In this study, sediments from four European rivers were studied in order to determine their HCB dechlorination capacities and the role of Dehalococcoides spp. in this process. Only a weak correlation was observed between Dehalococcoides species abundance and HCB transformation rates from different locations. In one of these locations, in the Ebro River sediment, HCB dechlorination could be linked to Dehalococcoides species growth and activity by 16S rRNAbased molecular methods. Furthermore, HCB dechlorination activity in this sediment was found over the full range of ambient temperatures that this sediment can be exposed to during different seasons throughout the year. The sediment contained several reductive dehalogenase (rdh) genes, and analysis of their transcription revealed the dominance of cbrA, previously shown to encode a trichlorobenzene reductive dehalogenase. This study investigated the role of Dehalococcoides spp. in HCB dechlorination in river sediments and evaluated if the current knowledge of rdh genes could be used to assess HCB bioremediation potential.Production of hexachlorobenzene (HCB)-containing pesticides is banned in most of the world due to HCB's toxic and carcinogenic nature. However, production and emission of the compound still occurs as an intermediate in chemical processes and from natural sources, such as volcanoes (5, 17). In aquatic environments, HCB is mainly deposited in the sediment due to its hydrophobicity. Bacterial reductive dechlorination plays an important role in the degradation of chlorinated aromatic contaminants like HCB in these anaerobic environments (15,19). Under anaerobic conditions, HCB is used by some bacteria in their energy metabolism by coupling reductive dehalogenation to electron transport phosphorylation (36). This is the only known pathway, so far, for the microbial transformation of HCB that is linked to microbial growth. Reductive dechlorination of HCB and its lesser chlorinated derivatives was previously reported in several river sediments (8,11,18,19). Nevertheless, in none of these studies were the microorganisms responsible for the process characterized.Three strains of bacteria capable of degrading HCB via reductive dechlorination have been isolated so far. "Dehalococcoides" sp. strain CBDB1 and "Dehalococcoides ethenogenes" strain 195 dechlorinate HCB to 1,3-dichlorobenzene (DCB), 1,4-DCB, and 1,3,5-trichlorobenzene (TCB) (3,14). Additionally, production of 1,2-DCB was also observed with strain 195. A distant relative of Dehalococcoides spp., "Dehalobium chlorocoercia" DF-1, dechlorinates HCB only to 1,3,5-TCB (48). Besides HCB, strain CBDB1 can also degr...

Dechlorination and organohalide-respiring bacteria dynamics in sediment samples of the Yangtze Three Gorges Reservoir

“…There are several groups of bacteria which are able to dehalogenate chlorinated aliphatic and aromatic hydrocarbons under anaerobic conditions. Remarkably, similar groups of bacteria have been reported to be capable of dechlorination of CBs (Field and Sierra-Alvarez 2008;Taş et al 2011), PCBs (Bedard 2008), and chlorinated ethenes (Löffler et al 2000;Schmidt et al 2006). In this study, chloroethenes were used as model compounds.…”

Role of Microbes and Nanomaterials in the Removal of Pesticides from Wastewater

“…The CS5 strain of Achromobacter xylosoxidans, which was isolated form activated sludge, was reported for its capability to degrade α-endosulfan and βendosulfan by more than 0.0248 g/L and 0.0105 g/L, respectively, after eight days of incubation [23]. Hexachlorobenzene was observed to be anaerobically dechlorinated by Dehalococcoides sp., and the trichlorobenzene reductive dehalogenase enzyme was reported to be crucial in degradation which was the product of the cbrA gene [24]. Two closely related species of Citrobacter amalonaticus were isolated, and their genome was sequenced by Chaussonnerie et al [25], and they reported their potential of biotransformation and remediation of chlordecone by these two isolates [25].…”

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

“…As a result of massive use and uncontrolled disposal on one hand and high persistence and bioaccumulation in ecosystems on the other hand, broad distribution of these pollutants has been found in a range of different environments (Barber et al 2005 ; Malcolm et al 2004 ; Wang et al 2010 ). 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. Even though monochlorobenzene (MCB) was considered to be recalcitrant to biotransformation in anoxic environments (Field and Sierra-Alvarez 2008 ), several studies reported further dechlorination of MCB to benzene in anaerobic enrichment cultures originating from freshwater sediment samples (Fung et al 2009 ; Nowak et al 1996 ; Zhou et al 2015 ).…”