Environ. Sci. Technol. 1999, 33, 2304
Response to Comment on “Methane As a Product of Chloroethene Biodegradation under Methanogenic Conditions” SIR: The intense focus in recent years on the environmental fate of chloroethene compounds has led to a growing appreciation that microbial degradation of these contaminants is not only possible but, under certain circumstances, favorable. From the early insight that some highly reduced metabolic components could “accidentally” catalyze reductive dechlorination, we have progressed to a recognition that microbial reduction of chloroethenes can be beneficial to the responsible microorganisms. This changing perspective is best illustrated by the growing list of halorespirers shown to derive energy for growth and metabolism from reductive dechlorination of chloroethenes (1, 2). A number of recent studies have demonstrated that indigenous microorganisms can also catalyze the net oxidation of cis-1,2-dichloroethene and vinyl chloride to CO2 (36). Oxidative degradation of these chloroethenes has been demonstrated under several distinct terminal electron accepting conditions, in aquifer systems as well as surface water environments, and at numerous sites throughout the United States (3-6). In a recent paper (7), we reported that CH4 could also be a significant product of chloroethene biodegradation under methanogenic conditions. As was the case with degradation of chloroethenes to CO2, this degradation of VC to CH4 was observed to occur in sediments collected from two geographically distinct sites. The widespread occurrence of these microbial transformations suggests that these processes are commonplace and may be beneficial to the responsible microorganisms. In the preceding comment, Dolfing provides thermodynamic calculations that indicate that microbial degradation of chloroethenes to CO2 or CO2 and CH4 is not only favorable but sufficiently so as to raise the possibility that microorgansims may derive energy for growth from these processes. Thus, Dolfing’s conclusions are quite consistent with the occurrence of these processes in various sites and environments (3-6). Several thermodynamically favorable reactions are hypothesized to explain the results of previous studies
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(3-7) and one of these suggests that acetate may be an intermediate in the production of CO2 and CH4 from VC (7). It is further suggested that the thermodynamics of acetate production are enhanced if coupled to reduction of VC to ethene. The results of a followup study (conducted in this lab and currently in submission to this Journal) indicate that the mechanism of CH4 production during VC biodegradation is acetotrophic methanogenesis (8). However, the temporal separation between CO2/CH4 production and ethene production and the coincidence of the onset of ethene production and cessation in CO2/CH4 production do not appear consistent with the proposed coupling of acetate production and VC reduction to ethene (7). Given the potential energy gain from such a coupling, the proposed mechanism merits continued consideration. The growing empirical evidence for microbial degradation of chloroethenes to CO2 or CO2 and CH4 (3-8) and the compelling demonstration by Dolfing that such processes potentially are quite energetic suggest that, like halorespiration, these processes may reflect a highly selected and environmentally important microbial capability.
Literature Cited (1) Maymo-Gatell, X.; Chien, Y.-T.; Gossett, J. M.; Zinder, S. H. Science 1997, 276, 1568-1571. (2) Rosner, B. M.; McCarty, P. L.; Spormann, A. M. Appl. Environ. Microbiol. 1997, 63, 4139-4144. (3) Bradley, P. M.; Chapelle, F. H. Environ. Sci. Technol. 1996, 30, 2084-2086. (4) Bradley, P. M.; Chapelle, F. H. Anaerobe 1998, 4, 81-87. (5) Bradley, P. M.; Chapelle, F. H.; Lovley, D. R. Appl. Environ. Microbiol. 1998, 64, 3102-3105. (6) Bradley, P. M.; Landmeyer, J. E.; Dinicola, R. S. Appl. Environ. Microbiol. 1998, 64, 1560-1562. (7) Bradley, P. M.; Chapelle, F. H. Environ. Sci. Technol. 1999, 33, 653-656. (8) Bradley, P. M.; Chapelle, F. H. Environ. Sci. Technol. Submitted for publication.
Paul M. Bradley* and Francis H. Chapelle U.S. Geological Survey 720 Gracern Road, Suite 129 Columbia, South Carolina 29210-7651 ES9920112
10.1021/es9920112 Not subject to U.S. copyright. Publ. 1999 Am. Chem.Soc. Published on Web 05/13/1999
