Dynamics of reductive TCE dechlorination in two distinct H 2 supply scenarios and at various temperatures A.C. Heimann*, A.K. Friis, C. Scheutz & R. Jakobsen Institute of Environment & Resources, Bygningstorvet, Technical University of Denmark, Building 115, DK- 2800, Lyngby, Denmark (*author for correspondence: e-mail: axh@er.dtu.dk) Accepted 8 February 2006 Key words: bioenergetics, dechlorination, fermentation, hydrogen, methanogenesis, temperature Abstract Anaerobic microbial dechlorination of trichloroethene (TCE) by a mixed, Dehalococcoides containing culture was investigated at different temperatures (4–60 °C) using propionate and lactate as a slow- and fast-releasing hydrogen (H 2 ) source, respectively. Distinct temperature-dependent dynamics of substrate fermentation and H 2 levels could explain observed patterns of dechlorination. While varying the temper- ature caused changes in rate, the overall pattern of dechlorination was characteristic of the supplied electron donor. Feeding cultures with a rapidly fermentable substrate such as lactate generally resulted in high H 2 concentrations and fast and complete dechlorination accompanied by rapid methanogenesis. In contrast, low H 2 release rates resulting from fermentation of propionate were associated with 2 to 3)fold longer time frames necessary for complete dechlorination at intermediate temperatures (15–30 °C). A lag- phase prior to dechlorination of cis-dichloroethene (cDCE), together with a characteristic build-up of H 2 and methane, was consistently observed at slow H 2 supply. At temperatures of 10 °C and lower, the system remained in this lag phase and no dechlorination past cDCE was observed within the experimental time frame. However, when lactate was the substrate, complete dechlorination of TCE occurred within 74 days at 10 °C, accompanied by methane production. The choice of fermentable substrate decisively influenced the rate and degree of dechlorination at an electron donor/TCE ratio as high as 666:1. Temperature- dependent H 2 levels resulting from fermentation of different substrates could be satisfactorily explained through thermodynamic calculations of the Gibbs free energy yield assuming a constant metabolic energy threshold of )20 kJ/(mol reaction). Introduction Due to their adverse health effects chlorinated eth- enes pose a serious problem at many contaminated sites (Wiedemeier et al. 1999). Chlorinated ethenes such as trichloroethene (TCE) can be microbially reduced yielding non-toxic compounds like ethene (DiStefano et al. 1991; Freedman & Gossett 1989). As we currently understand it, complete microbial dechlorination is restricted to the bacterial genus Dehalococcoides which require molecular hydrogen (H 2 ) as their ultimate electron donor (Lo¨ffler et al. 2003). In mixed microbial communities this can lead to competition for H 2 produced by fermenta- tion of organic substrates reflected by low aqueous H 2 levels of a few nmol per liter (Lo¨ ffler et al. 1999; Yang & McCarty 1998). A wide variety of organic compounds can be used as substrates in dechlorinating consortia ei- ther by producing H 2 via fermentation or as direct electron donors. Various studies have elucidated the role and applicability of these different electron donors to microbial dechlorination encompassing short-chain acids (formate, acetate, propionate, lactate, butyrate, isobutyrate), alcohols (methanol, ethanol), and carbohydrates (glucose) (Aulenta Biodegradation (2007) 18:167–179 Ó Springer 2006 DOI 10.1007/s10532-006-9052-z