Assessing the Toxic Effects of Nickel, Cadmium and EDTA on Growth of the Plant Growth-Promoting Rhizobacterium Pseudomonas brassicacearum

Plant growth-promoting rhizobacteria (PGPR) play an important role in the biodegradation of natural and xenobiotic organic compounds in soil. They can also alter heavy metal bioavailability and contribute to phytoremediation in the presence or absence of synthetic metal chelating agents. In this study, the inhibitory effect of Cd2+ and Ni2+ at different concentrations of Ca2+ and Mg2+, and the influence of the widely used chelator EDTA on growth of the PGPR Pseudomonas brassicacearum in a mineral salt medium with a mixture of four main plant exudates (glucose, fructose, citrate, succinate) was investigated. Therefore, the bacteriostatic effect of Cd2+, Ni2+ and EDTA on the maximum specific growth rate and the determination of EC50 values was used to quantify inhibitory impact. At high concentrations of Ca2+ (800 mu mol L-1) and Mg2+ (1,250 mu mol L-1), only a small inhibitory effect of Cd2+ and Ni2+ on growth of P. brassicacearum was observed (EC50 Cd2+, 18,849 +/- 80 mu mol L-1; EC50 Ni2+, 3,578 +/- 1,002 mu mol L-1). The inhibition was much greater at low concentrations of Ca2+ (25 mu mol L-1) and Mg2+ (100 mu mol L-1) (EC50 Cd2+, 85 +/- 0.5 mu mol L-1 and EC Ni2+, 62 +/- 1.8 mu mol L-1). For the chosen model system, a competitive effect of the ions Cd2+ and Ca2+ on the one hand and Ni2+ and Mg2+ on the other hand can be deduced. However, the toxicity of both, Cd2+ and Ni2+, could be significantly reduced by addition of EDTA, but if this chelating agent was added in stoichiometric excess to the cations, it also exhibited an inhibitory effect on growth of P. brassicacearum.