Correspondence Comment on “Methane As a Product of Chloroethene Biodegradation under Methanogenic Conditions” SIR: Bradley and Chapelle have recently presented evidence that CH4 and CO2 can be endproducts of the anaerobic biodegradation of chlorinated ethylenes such as vinyl chloride (1). In their paper, the emphasis was on the formation of CH4; but in my opinion, the formation of CO2 is at least as interesting. Anaerobic oxidation of vinyl chloride to H2/CO2 with water as oxidant is counterintuitive, but thermodynamical calculations show that this is an exergonic reaction (see below). This applies even more for the di-, tri-, and tetrachlorinated ethylenes. Thus, thermodynamic calculations also help to rationalize previous reports, by the same laboratory, on the anaerobic oxidation of 1,2-dichloroethylene under Mn(IV)-reducing conditions (2). Anaerobic degradation processes can be conveniently depicted as series of decarboxylations and redox reactions (3, 4). Application of this paradigm to ethylene yields:
C2H4 + 4H2O f 2CO2 + 6H2
∆G°′ ) 91.8 kJ/mol
Under standard conditions (25 °C; C2H4, CO2, and H2 in the gaseous state), this reaction is endergonic; but at lower partial pressures of H2 the reaction will become exergonic (Figure 1). One way to achieve this would be by coupling this reaction to H2 removal via methanogenesis. In methanogenic ecosystems, the hydrogen partial pressure is generally between 3 and 30 Pa (5). Under these conditions, oxidation of ethene is exergonic. Nevertheless methanogenic degradation of ethylene has never been observed (6, 7). In contrast to the positive ∆G°′ value for the oxidation of ethylene to H2/CO2, the oxidation of vinyl chloride to H2/ CO2 plus HCl is exergonic, even under standard conditions:
C2H3Cl + 4H2O f 2CO2 + 5H2 + HCl ∆G°′ ) -62.5 kJ/mol This is because part of the reducing equivalents is funneled into a reductive dechlorination step. Reductive dechlorination of vinyl chloride is a highly exergonic reaction:
C2H3Cl + H2 f C2H4 + HCl
∆G°′ ) -154.3 kJ/mol
FIGURE 1. Effect of the partial pressure of hydrogen on the change in Gibbs free energy for some reactions potentially involved in vinyl chloride degradation in methanogenic ecosystems: (A) ethylene + 4H2O f 2CO2 + 6H2; (B) vinyl chloride +4H2O f 2CO2 + 5H2 + HCl; (C) vinyl chloride + H2 f ethylene + HCl; (D) vinyl chloride + H2O f 0.5ethylene + 2HCl + 0.5CH3COOH. Calculations are based on Gibbs free energy data listed in Thauer et al. (8) and Dolfing and Janssen (9). 2302
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TABLE 1. Change in Gibbs Free Energy (kJ/mol of Substrate) Values for the Degradation of Chlorinated Ethylenes to Various Combinations of Endproductsa substrate ethene vinyl chloride cis-1,2-dichloroethylene trans-1,2-dichloroethylene 1,1-dichloroethylene trichloroethylene tetrachloroethylene
H2/CO2/HClb C2H4/CO2/HClc CH4/CO2/HCld 91.8 -62.5 -206.3 -208.5 -206.2 -372.9 -544.5
-139.0 -267.5 -269.8 -267.4 -418.8 -575.1
-104.4 -226.0 -337.1 -339.3 -337.0 -471.0 -609.9
a Gibbs free energy values were taken from Thauer et al. (8) and from Dolfing and Janssen (9). All calculations are for standard conditions: 1 molar concentrations, gases at 1 atm, pH ) 7, and 25 °C. b Stoichiometry according to C H Cl 2 a 4-a + 4H2O f 2CO2 + (a + 2)H2 + (4-a)HCl. c Stoichiometry according to: 6C2HaCl4-a + (16-4a)H2O f (a+2)C2H4+ (8-2a)CO2 + (24-6a)HCl. d Change in Gibbs free energy value of the H2/CO2/HCl generating reaction plus (a+2) × 32.7 (with 32.7 being the change in Gibbs free energy value for the reaction H2 + 0.25CO2 f 0.25CH4 + 0.5H2O in kJ/mol of H2).
Thus, depicting complete oxidation of vinyl chloride as a combination of vinyl chloride dechlorination and ethylene oxidation gives rise to the hypotheses that (i) vinyl chloridecan be fermented according to
6C2H3Cl + 4H2O f 5C2H4 + 2CO2 + 6HCl ∆G°′ ) -139.0 kJ/mol and (ii) that organisms may exist that can actually derive energy for growth from this reaction. However, in the experiments of Bradley and Chapelle, roughly equal amounts of CH4 and CO2 were formed from vinyl chloride (1). This suggests that fermentation of vinyl chloride proceeded via acetate as an intermediate:
2C2H3Cl + 2H2O f C2H4 + CH3COOH + 2HCl ∆G°′ ) -153.6 kJ/mol The observed stoichiometry in the labeling pattern makes the alternative hypothesis that vinyl chloride was completely oxidized to H2/CO2 in one organism and that these compounds were subsequently converted into methane by a second organism rather unlikely.
Reductive dechlorination of chlorinated compounds such as chloroethylenes is a highly exergonic reaction. Each dechlorination step yields about 150 kJ/mol of Cl. And since reductive dechlorination is an integral part of all degradation pathways discussed above, it follows that the amount of energy that is available from the various pathways in all cases increases with increasing degree of chlorination of the parent compound (Table 1). This implies that the conclusions on the potential degradation pathways of vinyl chloride that can be drawn from the calculations presented above are typical for the potential degradation pathways of all chlorinated ethylenes. The thermodynamic approach presented above offers a framework that leads to the formulation of various hypotheses that can, and undoubtedly will, be tested with regard to the mechanism(s) behind the highly interesting observation of Bradley and Chapelle that methane is a product of chloroethene biodegradation under methanogenic conditions. Furthermore, the framework implicitly suggests various types of enrichment cultures that could be started. Bradley and Chapelle have carefully abstained from speculating, but I feel that the above sample calculations, which are not exhaustive, help to better position methane formation from chloroethane biodegradation among the concerted actions of microbes in methanogenic and other anaerobic ecosystems.
Literature Cited (1) Bradley, P. M.; Chapelle, F. H. Environ. Sci. Technol. 1999, 33, 653-656. (2) Bradley, P. M.; Landmeyer, J. E.; Dinicola, R. S. Appl. Environ. Microbiol. 1998, 64, 1560-1562. (3) Zehnder, A. J. B.; Stumm, W. In Biology of Anaerobic Microorganisms; Zehnder, A. J. B., Ed.; Wiley-Interscience: New York, 1988; pp 1-38. (4) Dolfing, J. In Biology of Anaerobic Microorganisms; Zehnder, A. J. B., Ed.; Wiley-Interscience: New York, 1988; pp 417-468. (5) Dolfing, J. FEMS Microbiol. Ecol. 1992, 101, 183-187. (6) Schink, B. FEMS Microbiol. Ecol. 1985, 31, 63-68. (7) Koene-Cottaar, F. H. M.; Schraa, G. FEMS Microbiol. Ecol. 1998, 25, 251-256. (8) Thauer, R. K.; Jungermann, K.; Decker, K. Bacteriol. Rev. 1977, 41, 100-180. (9) Dolfing, J.; Janssen, D. B. Biodegradation 1994, 5, 21-28.
Jan Dolfing AB-DLO Wageningen, The Netherlands ES9902625
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