Sustained dechlorination of vinyl chloride to ethene inDehalococcoides-enriched cultures grown without addition of exogenous vitamins and at low pH

Abstract

ABSTRACTTrichloroethene (TCE) is a ubiquitous groundwater pollutant. Successful TCE bioremediation has been demonstrated at field sites using specialized microbial consortia harboring TCE-respiringDehaloccocoideswhose growth is cobalamin (vitamin B12)-dependent. Bioaugmentation cultures grown ex situ with ample exogenous vitamins in the medium and at neutral pH may become vitamin-limited or inhibited by acidic pH once injected into field sites, resulting in incomplete TCE dechlorination and accumulation of more toxic vinyl chloride (VC). Here, we report growth of theDehalococcoides-containing bioaugmentation culture KB-1 in a TCE-amended mineral medium devoid of vitamins and in a VC-amended mineral medium at low pH (6.0 and 5.5). In cultures grown without exogenous vitamins or cobalamin,Acetobacterium, which can synthesize 5,6-dimethylbenzimidazole (DMB), the lower ligand of cobalamin, andSporomusaare the dominant acetogens. At neutral pH, a growingAcetobacteriumpopulation supports complete TCE dechlorination byDehalococcoidesat millimolar levels with a substantial increase in the amount of measured cobalamin (~20-fold). Sustained dechlorination of VC to ethene was achieved at a pH as low as 5.5, yet at low pHAcetobacteriumis less abundant, potentially affecting the production of DMB and/or cobalamin. However, dechlorination activity at very low pH (< 5.0) was not stimulated by DMB supplementation, but was restored by raising pH to neutral. Assays in cell extracts revealed that vinyl chloride reductase (VcrA) activity declines significantly below pH 6.0 and is undetectable below pH 5.0. This study highlights the roles of and interplay between vitamin-producing populations and pH in microbial dechlorinating communities, and their importance for successful chlorinated ethenes bioremediation at field sites.