INFLUENCE OF PH ON THE TOXIC EFFECTS OF ZINC, CADMIUM, AND PENTACHLOROPHENOL ON PURE CULTURES OF SOIL-MICROORGANISMS

In this study the effect of the acidification of soil pore water on th e uptake and toxicity of cationic and anionic pollutants was measured in an experimental model system. The influence of pH on the toxic effe cts of zinc, cadmium, and pentachlorophenol was studied in buffered su spensions of pure cultures of soil microorganisms. In this system the speciation of the toxicant, the pH, and the biomass are defined, const ant, and thus easier to study than in a system with the solid soil mat rix and pore water. The mineralization of [C-14]acetate to (CO2)-C-14 was used to measure the toxic effects of pollutants on a fungus (Asper gillus niger CBS 121.49), an actinomycete (Streptomyces lividans 66), two Gram-negative Pseudomonas putida strains (MT-2 and DSM 50026), and a Gram-positive strain (Rhodococcus erythropolis A177). Large differe nces in sensitivity were observed between the species. For pentachloro phenol the highest EC50 was 81 mg/L for Pseudomonas putida at pH 8, wh ereas the lowest was 0.13 mg/L for Aspergillus niger at pH 6. Aspergil lus niger was not sensitive to 1,000 mg Zn/L, whereas Pseudomonas puti da at pH 7.8 showed the lowest EC50, 0.14 mg Zn/L. When pH was increas ed, pentachlorophenol became less toxic and showed less sorption to th e biomass, whereas zinc and cadmium became more toxic and showed more sorption to the biomass. The results indicate that higher pore-water c oncentrations due to acidification of zinc- and cadmium-polluted soils may not be accompanied by increased toxic effects on microorganisms b ecause of the relatively low toxicity of these metals in pore water at low pH.