Correspondence Comment on “Competition for Hydrogen within a Chlorinated Solvent Dehalogenating Anaerobic Mixed Culture” SIR: Yang and McCarty (1) recently raised the question how to favor dehalogenating microorganisms over other possible H2-utilizing microorganisms within a methanogenic mixed culture. This is a very relevant topic. In environments where reductive dehalogenation is an issue such as contaminated soils and sediments reducing equivalents are generally in short supply. Adding reducing equivalents to enhance dehalogenation processes is then an option, but in order to do this cost effectively information is needed on the conditions that favor dehalogenating organisms over other hydrogen-using microorganisms in their quest for H2. Indeed H2 appears to be the preferred source of reducing equivalents (2, 3). Yang and McCarty found that microbially catalyzed reductive dechlorination of chlorinated ethenes is still operative at H2 concentrations in the range of 2-11 nM, i.e. below the threshold for H2 uptake by methanogens (1). For a classical microbiologist working with chemostats (4, 5) this result immediately suggests a strategy to select for dechlorinators, viz. by working at low dilution rates at which the H2 concentration will be too low to sustain growth of hydrogenotrophic methanogens. Reducing equivalents can be added in any form: one of the characteristics of methanogenic degradation processes is their sequential nature (6). Complex substrates are degraded by a series of microorganisms, and the flux of electrons goes completely via H2 and acetate, the direct substrates for the methanogens (6). The chemostat approach is however not directly applicable to in-situ bioremediations. Another approach, which would be applicable in situ, would be to add a source of reducing equivalents that is not attractive for hydrogenotrophic methanogens. Methane itself would be an interesting candidate. From a theoretical thermodynamic point of view methane could serve as an exergonic source of reducing equivalents for dechlorinating organisms (7, 8). From a more practical point of view this seems impossible in the cultures studied by Yang and McCarty: at a H2 level of 2 nM methane oxidation to H2/CO2 is endergonic. This leaves the option that methane is used directly by the dechlorinating organisms, which is highly unlikely. A more promising candidate therefore is acetate. There are various potential mechanisms through which acetate could serve as a source of reducing equivalents for dechlorinating organisms. The first is of course direct use of acetate by the dechlorinators. This possibility has been documented: Dechlorinating Bacterium 1, nowadays known as Desulfomonile tiedjei (9), for example, is known to use acetate as an alternative source of reducing equivalents if H2 is not available (3). Furthermore Krumholz has described an anaerobic bacterium, Desulfuromonas chloroethenica, which can actually grow on the coupling of acetate oxidation to tetrachloroethylene oxidation (10, 11). Another possibility is the indirect use of the small amounts of H2 that are released during acetoclastic methanogenesis (12-14). There are indications that this may have occurred in a mixed culture * Corresponding author phone: 31-317-474658; fax: 31-317424812; e-mail:
[email protected]. 10.1021/es990128k CCC: $18.00 Published on Web 04/24/1999
1999 American Chemical Society
that dechlorinated 2,4,6-trichlorophenol (15, 16). A third route for the generation of reducing equivalents from acetate may be its oxidation to H2/CO2 (17). Recent results suggest that this process can indeed occur and be selected for under conditions that are inimicus to the average methanogen (18). Close scrutiny of the data presented by Young and McCarty (1) suggests that acetate indeed was used as a source of reducing equivalents by the dechlorinating organisms in their culture. The justification for the current comment is to highlight this unrecognized by very relevant observation. In Yang and McCarty’s culture benzoate was given as a source of reducing equivalents. When benzoate and H2 had been consumed dechlorination still continued. Acetate was the only plausible source of reducing equivalents at that stage. Methanogenesis from acetate did apparently not occur, at least not when the cultures were fed chlorinated ethylenes, but acetate was converted into methane in cultures that were incubated without chlorinated ethylenes. This combination of observations suggests the use of acetate as a specific source of reducing equivalents for reductive dechlorination of chlorinated ethenes. It remains to be seen, of course, whether this apparent specificity also applies to soils and sediments. In fact the mixed culture with which Yang and McCarty worked originated from digested sewage sludge (1). Another relevant point is whether the dechlorinators use acetate directly or after it has been oxidized to H2/CO2.
Literature Cited (1) Yang, Y.; McCarty, P. L. Environ. Sci. Technol. 1998, 32, 35913597. (2) Dolfing, J.; Tiedje, J. M. FEMS Microbiol. Ecol. 1986, 38, 293298. (3) Dolfing, J.; Tiedje, J. M. Arch. Microbiol. 1991, 156, 356-361. (4) Veldkamp, H. Adv. Microb. Ecol. 1977, 1, 59-94. (5) Gottschal, J. C. In Methods in Microbiology; Norris, J. R., Grigorova, R., Eds.; Academic Press: London, UK, 1990; Vol. 22, pp 87-124. (6) Zehnder, A. J. B. In Water Pollution Microbiology; Mitchell, R., Ed.; Wiley: New York, NY, 1978; Vol. 2, pp 349-376. (7) Dolfing, J.; Harrison, B. K. Environ. Sci. Technol. 1992, 26, 22132218. (8) Dolfing, J.; Beurskens, J. E. M. Adv. Microb. Ecol. 1995, 14, 143-206. (9) DeWeerd, K. A.; Mandelco, L.; Tanner, R. S.; Woese, C. R.; Suflita, J. M. Arch. Microbiol. 1990, 154, 23-30. (10) Krumholz, L. R.; Sharp, R.; Fishbain, S.; Appl. Environ. Microbiol. 1996, 62, 4108-4113. (11) Krumholz, L. R. Int. J. Syst. Bacteriol. 1997, 47, 1262-1263. (12) Lovley, D. R.; Ferry, J. G. Appl. Environ. Microbiol. 1984, 49, 247-249. (13) Phelps, T. J.; Conrad, R.; Zeikus, J. G. Appl. Environ. Microbiol. 1985, 50, 589-594. (14) Krzycki, J. A.; Morgan, J. B.; Conrad, R.; Zeikus, J. G. FEMS Microbiol. Ecol. 1987, 40, 1193-1198. (15) Perkins, P. S.; Komisar, S. J.; Puhakka, J. A.; Ferguson, J. F. Water Res. 1994, 28, 2101-2107. (16) Dolfing, J. Water Res. 1995, 29, 1811-1812. (17) Schnu ¨ rer, A.; Schink, B.; Svensson, B. H. Int. J. Syst. Bacteriol. 1996, 46, 1145-1152. (18) Schnu ¨ rer, A.; Houwen, F. P.; Svensson, B. H. Arch. Microbiol. 1994, 162, 70-74.
Jan Dolfing* Research Institute for Agrobiology and Soil Fertility (AB-DLO) Wageningen, The Netherlands ES990128K VOL. 33, NO. 12, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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