Metal sensitivity of some symbiotic N2-fixing bacteria and Pseudomonas strains.

An investigation was carried out to determine the sensitivity of different soil microbes (Rhizobium, Bradyrhizobium and Pseudomonas) to various metals (Cu2+, Zn2+, Co2+, Mn2+, Mo2+ and Fe2+) in vitro. Sulphate and chloride forms of these microelements were used (except Mo2+ as Na2Mo04) in 0.1, 1.0 and 10 micrograms/ml concentrations in modified YEM and nutrient broth. Growth (optical density, OD550 and OD640) of bacterium inoculated (approx. 10(6) CFU/ml) tubes, was measured spectrophotometrically after 48 h of incubation of 28 degrees C in a rotary shaker (150 rpm). Data of triplicate samples are shown as percent of control tubes (inoculated, free from treatments) and after an analysis of variance SE was calculated. Strains of Rhizobium leguminosarum proved to be the most sensitive to Cu2+, Zn2+ and Co2+. The slow growing Bradyrhizobium and plant growth promoting (PGPR) Pseudomonas isolates, however, were affected only at the highest (10 micrograms/ml) dose of these elements. In contrast Mn2+, Mo2+ and Fe2+ microelements were stimulatory for the growth of all investigated soil microbes. Sulphate forms of the most harmful Cu2+ and Zn2+ cations were more toxic than the chloride forms. An especially high diversity was found among the R. leguminosarum bv. viceae isolates. Monitoring the sensitivity of these microbes has a primary importance for selection of ecologically diverse isolates, as potential inocula in heavy-metal affected soils.