A model bioassay for influence of heavy metal salts toxicity on the viability of nodule-forming bacteria Rhizobium meliloti

The study consists of a bioassay of a medium experimentally contaminated with heavy metals using Rhizobium meliloti microorganisms as a test object. In vitro, the viability reaction of clover and alfalfa nodule bacteria to lead, cadmium, copper and zinc was established. The effect of the studied metals on the number of colonies depends on the concentration of the chemical element, physiological action of the microelement and biological characteristics of the strains used. The studies performed have determined a direct relationship of reducing the growth in the rhizobia colonies number of clover and alfalfa with an increase in the concentration of salts of heavy metals. Inhibition of the colonies number had a high regularity when the environment was contaminated with lead, copper and cadmium at a concentration of 0,3%; with a subsequent increase, complete death of microorganisms was observed. With the introduction of zinc into the nutrient medium in small concentrations, a positive resistance tendency of the clover and alfalfa bacteria was observed: thus, at 0,01–0,1% of the zinc salt content, the number of grown colonies was above the control level and amounted to 714–987 pcs. at the control value of 578 pcs. In an environment with a concentration of zinc 0,5%, a significant decrease in the growth of clover rhizobia was noted up to 65 pieces of colonies. At the same time, in the alfalfa rhizobia colonies in these concentrations a lower degree of survival was observed and the environment of zinc 0,3% became critical.