MEASUREMENT OF HEAVY-METAL TOLERANCE OF SOIL BACTERIA USING THYMIDINE INCORPORATION INTO BACTERIA EXTRACTED AFTER HOMOGENIZATION CENTRIFUGATION

Thymidine incorporation into soil bacteria DNA decreased with increasing Cu pollution in an artificially contaminated sandy loam (0-400 mug Cu as CuSO4 g-1 soil) stored for 2 yr. The ATP content was affected to almost the same degree as thymidine incorporation. No effects on total or viable bacterial counts were found, To study tolerance of the bacterial community, bacteria were first extracted by homogenization-centrifugation from the differently polluted soils. To these bacterial solutions a range of Cu concentrations (as CuSO4) was then added and the thymidine incorporation rate was measured. By comparing with a control (no Cu added to the solutions), IC50 values (giving 50% reduction compared to the control) for each bacterial solution could be calculated. There were no differences in these IC50 values for bacterial solutions from soils with 0 to 120 mug added Cu g-1 soil. With more severe pollution higher IC50 values were found, indicating that a more Cu tolerant bacterial community had evolved. Bacteria extracted from the most polluted soils had IC50 values 10 times higher than bacteria from nonpolluted soils. Using the same technique, an increased tolerance to Ag, Zn, and Cd was also evident in a Cu polluted soil compared to a nonpolluted one, while no effect on the Pb tolerance of the bacteria was found.