Abstract-In this study the effect of the acidification of soil pore water on the uptake and toxicity of cationic and anionic pollutants was measured in an experimental model system. The influence of pH on the toxic effects of zinc, cadmium, and pentachlorophenol was studied in buffered suspensions of pure cultures of soil microorganisms. In this system the speciation of the toxicant, the pH, and the biomass are defined, constant, and thus easier to study than in a system with the solid soil matrix and pore water. The mineralization of [ 14 C]acetate to 14 CO 2 was used to measure the toxic effects of pollutants on a fungus (Aspergillus 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 differences in sensitivity were observed between the species. For pentachlorophenol the highest EC50 was 81 mg/L for Pseudomonas putida at pH 8, whereas the lowest was 0.13 mg/L for Aspergillus niger at pH 6. Aspergillus niger was not sensitive to 1,000 mg Zn/L, whereas Pseudomonas putida at pH 7.8 showed the lowest EC50, 0.14 mg Zn/L. When pH was increased, pentachlorophenol became less toxic and showed less sorption to the biomass, whereas zinc and cadmium became more toxic and showed more sorption to the biomass. The results indicate that higher pore-water concentrations due to acidification of zinc-and cadmium-polluted soils may not be accompanied by increased toxic effects on microorganisms because of the relatively low toxicity of these metals in pore water at low pH.
The mineralization of [14C]acetate or [14C]glucose was studied in aseptic subsoil samples at 10°C. At 1 μg/L and 100 μg/L, the mineralization showed first‐order kinetics preceded by a short lag phase. For the sandy subsurface soils, the half‐lives of acetate or glucose varied from 0.5 until 21 d, whereas a peat subsoil and a sandy surface soil produced half‐lives of 11 and 1.6 h, respectively.
The EC50 is the toxicant concentration that causes 50% reduction of the percentage substrate mineralized at a certain incubation time and increases with time. The time‐independent toxicant concentration IC50 gives 50% inhibition of the mineralization rate and is equal to the EC50 at short incubation times. The IC50 of pentachlorophenol varied between 0.1 and 880 mg/kg subsoil. The aerobic mineralization of acetate in surface sand showed an IC50 of 28 mg pentachlorophenol/kg. Hydrochloric acid showed IC50 values ranging from pH 6.2 to pH 2.8. For chlorite and cadmium, the IC50 values were 34 and 59 mg/kg, respectively.
When soils were incubated with 100 mg acetate/L, exponential growth kinetics were observed. Under these conditions the inhibitory effect of pentachlorophenol was relatively small because the resistant part of the acetate‐mineralizing microflora could grow and replace the inhibited part. The kinetics show why toxicity tests using high substrate concentrations are less sensitive than tests using more natural low substrate concentrations. The model predicts that slow‐growing microorganisms, which are vital to the functioning of the ecosystem, would recover only very slowly from the toxic effects of environmental pollution.
The results of toxicity tests can be used to calculate the potentially affected fraction (PAF) of species in an ecosystem at a given pollutant concentration using statistical extrapolation methods. The PAF curve indicates the fraction of species from the original community that may become inhibited at each elevated pollutant concentration and is a measure of the ecotoxicological risk. Pollution-induced community tolerance (PICT) is a true community response that is measured under controlled conditions in the laboratory, using organisms from contaminated field sites. Microorganisms from experimental field plots with added Zn were exposed to various concentrations of Zn in the laboratory and the mineralization of 14C acetate was monitored. Microorganisms from plots with Zn concentrations above 124 mg/kg showed a significant increase in the effect concentration 10% (EC10) and, therefore, had a significant PICT. The pore-water concentrations of Zn in these field soils were in the same magnitude as the EC10 of the microorganisms from these soils. The PAF curve was calculated from previously reported toxicity tests with five different microbial species using the average and the standard deviation of the logarithmically transformed EC10 values. The average sensitivity of this PAF curve was similar to the EC50 of the acetate mineralization curve from the field plot without added Zn2+, but the PAF curve was less steep. Our experiments indicated that 27 to 84% of the original microbial species were inhibited at Zn concentrations from 334 to 1,858 mg/kg soil, respectively. Our results suggest that the PICT method can now also be used to quantify the fraction of the original species composition that is inhibited at a specific pollutant concentration.
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