Effects of media components on toxicity of Cd to rhizobia

Numerous studies have attempted to relate the ability of microorganisms to tolerate heavy metals added to artificial media to metal concentrations found in the environment from which the microbes were originally isolated. Organic and inorganic components of artificial media, however, may bind and chelate metals, thus reducing the biological activity/potency of these metals. We varied the concentration of various components of HM (HEPES — MES) medium and determined their effects on Cd toxicity to Rhizobium leguminosarum biovar trifolii. Varying the concentrations of Ca, Mg and agar in the medium had no effect on Cd toxicity. Increasing additions of HEPES, MES, phosphate, and yeast extract significantly reduced the observed severity of Cd toxicity. When 3 mg L^?1 phosphate was added to the medium, the Cd maximum resistance level (MRL) was 2 gmg mL^?1. When, however, the addition of phosphate was increased to 660 mg mL^?1, the MRL to Cd was 18 μg mL^?1. These results clearly demonstrate that Cd toxicity to microorganisms is significantly influenced by the composition of the media used in toxicity testing.