Comment on "Bioremefion in the Rhnosphere" SIR: The December 1993 issue of Environ. Sci. Technol. contained a critical review on biodegradation of organic pollutants in the rhizosphere (1). This review reflects increasing interest in rhizosphere-enhanced bioremediation, as indicated by presentations at national symposia held in 1993 and sponsored by the American Chemical Society, the American Society of Agronomy, and the National Institutes of Environmental Health Sciences.We have read the literature and conducted experimentsrelating to this topic because of our respective research interests. One of us (L.P.W.)has worked in the field of biodegradation; the other (D.L.A.)has worked on rhizosphere effectsrelating to plant nutrition. Our own experience leads us to disagree with the main conclusionof the Environ.Sci. Technol.review that "studies provide a strong scientific basis to support field demonstrations of in situ degradation of toxicants in the rhizosphere". In general, we feel that rhizosphereenhancement effects are being overstated. In many of the papers cited in the review article, rhizosphere soils degraded less of the toxicant than nonrhizosphere soils (e.g.,ref 2) or supported insignificantrates of biodegradation. As an example of the latter case, a paper often cited by the authors describes biodegradation of trichloroethylenein rhizosphere soils (3). In that study, 50 g of rhizosphere soil was exposed to 26.7 pmol of [14C]trichloroethyleneand produced 70 pmol of [14ClCOz.Thus, < 0.0003%of the added trichloroethylenewas transformed to COZover 30 days. To put this in terms of bioremediation, a linear stoichiometric conversion of trichloroethylene to COZ at this rate would take 31 350 years. Consider that 1 mg of a suitable methanogenic bacterium would oxidize 70 pmol of trichloroethylene in less than 2 s (4). Why is ref 3 cited as supporting rhizosphere environment of biodegradation? It is because non-rhizosphere soils, in the same study, transformed trichloroethyleneat only 40% the rate of rhizosphere soils. Both rates, however, are trivial. In a less dramatic fashion, other references cited for support of rhizosphere-enhanced degradation are also unconvincing. Siebert et al. (5) observed "no significant influence of growing plants on atrazine degradation". At 5 ppm [l4C1atrazine,mineralization increased in only one of two soils,from 3% [ l4C]COzto 5%[14ClCOzin the presence of decomposing (not living) roots. In our studies (6),2025% of 100 ppm [l4C1atrazinewas mineralized to ['*C]COz by inoculation of non-rhizosphere soil with a mixed bacterial culture obtained from non-rhizosphere soils. In total, the literature adds up to a less than compelling case for going to the trouble and expense of using plants in trying to stimulate microbially based bioremediation.
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0 1995 American Chemical Society
The logic that plants should theoretically stimulate biodegradation must also be subjected to a scientific discussion. Plant roots exude carbohydrates,organic acids, and many other excellent bacterial carbon sources. Fifty years of microbiological research indicate that catabolic pathways are tightlyregulated,with sugars and tricarboxylic acid cycle intermediates being utilized preferentially over compounds we call pollutants. Biodegradation in soil depends on the growth and survival of microorganisms possessing the appropriate genes and regulatory mechanisms to express catabolic enzymes. Many factors determine whether a pollutant in soil is metabolized by a suitable organism and enzyme. These include the temperature, pH, water content, oxygen tension, soil adsorptivity, and many other parameters. Plants wiu affect these parameters in ways that are beneficial, neutral, or detrimental to the biodegradation of particular compounds by specific organisms. Thus, all possible outcomes of rhizosphere effects on biodegradation (suppression,no effect,or enhancement) would be anticipated and are, in fact, observed. Logically, it is most effective to focus bioremediation efforts directly on providing optimal conditions to promote the growth and metabolism of the specific microorganismsresponsible for the biodegradation reactions of interest. Studies on the rhizosphere are of great interest and directlyimpact agricultural and ecological sciences. While some specific combinations of roots, microbes, and pollutants might show enhancement of biodegradation, we believe the connection between the rhizosphere and bioremediation is overstated. Rather than randomly searching plant-soil-toxicant combinations, a more productive research agenda would be to postulate specific plantmicrobe interactions likely to enhance biodegradation.
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.; Allan, D . L.Appl. Environ. Microbiol. 1993, 59, 1695-1701. (3) Walton, B.T.;Anderson, T.A. Appl. Environ. Microbiol. 1990, 56, 1012-1016. (4) Fox, B. G.; Borneman, J. G.; Wackett, L. P.; Lipscomb, J. D. Biochemistry 1990,29, 6419. (5) Seibert, K.; Fuehr, F.; Cheng, H. H. In TheoryundPructicul Use of Soil-Applied Herbicides Symposium; European Weed Resource Society: Paris,1981; pp 137-146. (6) Mandelbaum, R. T.;Allan, D. L.; Wackett, L. P. Unpublished data.
Lawrence P. Wackett* and Deborah L. Allan University of Minnesota St. Paul, Minnesota 55108 ES940037H
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