Abstract

Effects of heavy metals and pesticides on cowpea (Vigna unquiculata) rhizoplane microbial community were assessed in vitro via dehydrogenase activity. The microbial community was exposed to various concentrations of heavy metals and pesticides in a nutrient broth-glucose-2,3,5-triphenyl chloride (TTC) medium. At 0.2 mM, iron and cadmium stimulated the dehydrogenase activity of the microbial community. For all the metal ions, there was progressive inhibition with each successive increase in the concentration of metal ion, reaching near 100% at 0.6, 0.8, 1.2, 0.12 and 12 mM for cobalt, cadmium, iron, mercury and nickel, respectively. Between 0.2 and 0.4 mM, zinc sharply inhibited dehydrogenase activity and at concentration above 0.4 mm, inhibition of dehydrogenase activity became less pronounced. The order of toxicity is Hg²⁺ > Co²⁺ > Cd²⁺ > Zn²⁺ > Fe²⁺ > Ni²⁺. The herbicides Cotrazine (Atrazine 80W) and Northrin^®10EC stimulated dehydrogenase activity of the microbial community at 0.2% and inhibited it at higher concentrations. The median inhibitory concentrations (IC₅₀s) of Cotrazine (Atrazine 80W) and Northrin^®10EC were 0.552 ± 0.028 and 0.593 ±0.051%, respectively. The dehydrogenase activity varied significantly (p < 0.05) with the type and concentrations of metals or pesticides. The result indicates that the heavy metals and pesticides are potentially toxic to V. unquiculata root surface microorganisms. In soil, this toxicity may affect nitrogen fixation processes and by extrapolation affect crop yield.

Key words: Dehydrogenase activity, rhizosplane bacteria, atrazine, cypermethrin, heavy metals, cowpea.