Environ. Sci. Technol. 1995,29, 552-552
SIR: We welcome critical comment on our review article and will attempt to address the points raised by Wackett and Allan. At the outset, we want to emphasize that the principal point of our review (I) was that the current literature (51 references cited in the review) provides “ ... a strong scientific basis to support field demonstrations of in situ degradation of toxicants in the rhizophere”. Wackett and Allan are skeptical of this conclusion, citing an unpublished study of theirs and stating many of the papers cited in the review ... show that ... rhizosphere soils degraded less of the toxicant than non-rhizosphere soils ...”. In fact, the only papers that suggest this are two contradictory papers, one co-authored by Wackett (21, showing that atrazine is degraded in the rhizosphere, and one by Seibert et al. (3)that can be interpreted either way. The Seibert et al. paper (3) states that no significant influence of growing plants on atrazine degradation was observed, as measured by I4CO, evolution. In the same paper, Seibert et al. also report an increase in hydroxylated metabolites of atrazine in the rhizosphere when compared to the non-rhizosphere region. Thus, the basis of the objection by Wackett and Allan appears to rest on whether formation of hydroxylated metabolites of atrazine in the rhizosphere contributes to bioremediation or whether mineralization is the only end point that can be accepted for bioremediation. We believe that criteria other than mineralization contribute to bioremediation (4). Wackett and Allan make a curious calculation based on one of the articles we reviewed, suggesting inappropriately that TCE mineralization in rhizosphere soils would require over 31 000 years. This calculation is invalid for a very important reason-the calculations are made from experimental data in which rhizosphere soils were NOT IN CONTACT WITH LMNG PLANTS! The approach of making such a calculation based on data from laboratory experiments without plants to predict degradation rates in the rhizosphere under field conditions with plants is not valid. For example, in studies of the fate of [14C]TCE in the rhizospheres of five plant species (live plants, in contact with soil), Anderson (5) found that mineralization of [14C]TCEto 14C02ranged from two to six times greater in soils with plant roots present than in soils without plants. At a minimum, Anderson’s findings underscore the importance of plants as a potential influence on processes occurring in the rhizosphere. Wackett and Allan further offer the opinion that the difference in degradation rates between rhizosphere and non-rhizosphere soils is “trivial”when compared to methanogenic conversion rates reported in an article co-authored by Wackett (6).Wackett and Allan’s calculation for methanogenic conversion of 70 pmol of TCE in 2 s is confusing. Although we could not find these data in the paper cited, rates of TCE degradation are reported for purified soluble methane monooxygenase. However,the authors conclude that “ ...no information is available concerningthe presence of the soluble methane monooxygenase in environmental samples”. Even if we accept without question the converI‘,..
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sion of 70 pmol of TCE12 s, the persistence of TCE in surface soils is a well-documented phenomenon, and any success at increasingdegradation rates under field conditions would be noteworthy. Thus, the potential for degradation of TCE in the rhizosphere should not be dismissed on the basis of the sMMO data cited by Wackett and Allan. In summary, it is our opinion that the current literature suggests, as our review claims, “that plants, in conjunction with their associated microbial communities, offer a potentially important treatment strategy for in situ biological remediation of chemically contaminated soils”. Moreover, we believe that field studies are especially appropriate given the difficulties of accuratelysimulatingthe complexity of field ecosystems in the laboratory. By making this assertion, however, we do not wish to diminish the overall importance of molecular or mechanistic studies. Indeed, the final sentence of our review states that “A better understanding of the mechanisticinteraction between plant roots and their surrounding microbial communities w ill favor successful field demonstrations and permit effective selection and management of vegetation to achieve in situ bioremediation”.
Literature Cited (1) Anderson, T. A.; Guthrie, E. A.; Walton, B. T. Environ. Sci. Technol. 1993, 27, 2630-2636. (2) Mandelbaum, R. T.; Wackett, L. P.; Man, D. L. Appl. Enuiron. Microbiol. 1993, 59, 1695-1701. (3) Seibert, K.; Fuehr, F.; Cheng, H. H. In Theoryand Practical Use ofsoil-Applied Herbicides Symposium; Eurpean Weed Resource Society: Paris, 1981; pp 137-144. (4) Walton, B. T.; Hoylman, A. M.; Perez, M. M.; Anderson, T. A.; Johnson, T. R.; Guthrie, E. A.; Christman, R. F. Rhizosphere microbial communities as a plant defense against toxic substances in soils. In Bioremediation through Rhizosphere Technology; Anderson, T. A., Coats, J. R., Eds.;ACS Symposium Series 563;American Chemical Society: Washington, DC, 1994; pp 82-92. (5) Anderson, T. A. Comparative plant uptake and microbial degradation of trichloroethylene in the rhizospheres of five plant species-Implications for bioremediation of contaminated surface soils. Ph.D. Dissertation, University of Tennessee, Knoxville, TN, 1991, 187 pp. (6) Fox, B. G.; Borneman, J. G.; Wackett, L. P.; Lipscomb, J. D. Biochemistry 1990,29, 6419.
Barbara T. Walton* Oak Ridge National Laboratory Oak Ridge, Tennessee 37831-6253
Todd A. Anderson Iowa State University Ames, Iowa 50011-3140
Elizabeth A. Guthrie University of North Carolina Chapel Hill,North Carolina 27599-7400 ES940954N
0013-936x/95/0929-0552$09.00/0
Q 1995 American Chemical Society
