Sensitivity of bacterioplankton in the Rhine river to various toxicants measured by thymidine incorporation and activity of exoenzymes

Abstract

Natural populations of bacteria in water samples from the lower Rhine River were tested for their sensitivity to selected toxicants. Bacterial growth, measured as [3H-methyl] thymidine incorporation, was affected by additions of 5 μg Cu L−1; these concentrations were similar to the copper concentrations found in the river. Concentrates of the organic fraction from river water, obtained via XAD-columns, affected thymidine incorporation in samples of river water when added in concentrations corresponding to ambient levels. The exoenzymatic activities of phosphatase and protease decreased systematically at concentrations higher than 5 μg Cu L−1. Potassium dichromate (K2Cr2O7) at a concentration of approximately 3 mg L−1 and a detergent tetrapropylbenzene sulfonate (TPBS) at a concentration of 5 mg L−1 had detectable effects on microbial activity. Atrazine affected the growth rate but not the enzymatic activity of bacterial populations when this herbicide was added in concentrations of 50 to 200 μg L−1. Thymidine incorporation was more sensitive to all test compounds than the enzymatic activities. It is concluded that the present pollution levels, although decreased, are still likely to affect microbial processes in the river.