Reductive Dehalogenation of Chlorophenols by Anaerobic Microbial Consortia

Abstract

Chlorophenols are recognized to be carcinogenic to rats, potentially carcinogenic to humans and are especially resistant to degradation due to the stability induced by their chlorine substituents. However, anaerobic microorganisms can sequentially remove these chlorine constituents from these compounds through the process of reductive dehalogenation, which renders them more amenable to subsequent aerobic degradation and ultimate mineralization. These microorganisms are able to utilize halogenated compounds for energy synthesis by coupling reductive dehalogenation to energy metabolism. In this research, 20 samples from both natural (i.e. soils and sediments) and engineered (i.e. sludge from treatment plants) systems were collected from various locations in Singapore, China, Malaysia and Indonesia and were used as inocula for studies on their capability to dechlorinate pentachlorophenol (PCP) and 2,4,6-trichlorophenol (2,4,6-TCP). Of the 20 samples, only a bacterial consortium, D12, grapidly dechlorinate 2,4,6-TCP. Both cultures LWN and RIV completely dechlorinated 2,4,6-TCP to 4-CP in 5 days while culture SC took 12 days. Cultures LWN, RIV and SC were then tested for the presence of possible dehalogenators within the 3 bacterial consortia. A common chlorophenol-dechlorinating bacterium from the genus Desulfitobacterium was discovered in culture RIV while culture SC contained Dehalococcoides-like bacteria, which was never reported to have been able to completely dechlorinate 2,4,6-TCP. DNA sequencing results showed an even more interesting finding with the predominance of Sedimentibacter-like bacteria in culture LWN since Sedimentibacter have never been previously shown to dehalogenate any form of halogenated compounds. Halogenated compounds other than chlorophenols were also subjected to reductive dechlorination by cultures LWN, RIV and SC. Extensive debromination of polybrominated diphenyl ethers (PBDE) was shown to be possible by culture RIV. Meanwhile, culture