The present study assessed the experimental and predicted toxicity of pesticides and heavy metals binary mixtures to Rhizobium species. The experimental toxicity response was assessed using the inhibitory effect of the individual and binary mixtures of glyphosate (Gly) with cadmium (Cd (II) and lead (Pb (II)); and 2, 2 Dichlorovinyl dimethyl phosphate (DDVP) with Cd (II) and Pb (II) to Rhizobium species total dehydrogenase activity using 2,3,5 triphenyltetrazolium chloride (TTC) as the artificial electron acceptor. The binary mixtures were composed using fixed percentage ratios of 50% : 50%, 60% : 40% and 80% : 20% mixtures of Gly + Cd (II), Gly + Pb (II) and mixtures of DDVP + Cd (II), DDVP + Pb (II). The Half maximal effective concentration (EC50) of the toxicants were estimated using monotonic logistic dose-response model. Prediction of binary mixture toxicity effect was carried out using Concentration Addition (CA) and Independent Action (IA) models. Results obtained showed that Cd (II) and Pb (II) ions inhibited Rhizobium sp. dehydrogenase activity in a logistic concentration dependent manner; glyphosate and DDVP exhibited a biphasic toxicity effect to Rhizobium sp. Glyphosate and DDVP at concentrations below 5000mg/L and 1000mg/L respectively stimulated the total dehydrogenase activity of the isolate. Using EC50 ranking, the order of toxicity was Cd (II) >Pb (II) > DDVP > glyphosate. The binary mixtures of glyphosate and Pb (II) in ratios of Gly (50%) : Pb (II) (50%) and Gly (80%) : Pb (II) (20%) stimulated total dehydrogenase activity of Rhizobium sp. at concentrations below 1000 mg/L and 500mg/l for the respective mixtures. Prediction of binary mixtures toxicity using the CA and IA model differed in their outcome of toxicity interactions. The CA and IA model predicted additive interactions for the mixtures of Gly 50% + Cd (II) 50%, DDVP 60% + Cd (II) 40%, DDVP 50% + Cd (II) 50% and DDVP 80% + Cd (II) 20%, which differed from experimentally observed synergism. The CA model underestimated the toxicity of binary mixtures of DDVP + Cd (II) and glyphosate + Pb (II), while overestimating the toxicity of the DDVP + Pb (II) and glyphosate + Cd (II) binary mixtures. Furthermore, the IA model predicted mostly additive interaction for binary mixtures of glyphosate + Cd (II) and DDVP + Cd (II) ions mixture across the ratios; which was a gross overestimation of toxicity in most of the binary mixtures studied. The joint action of the mixtures on test organism predicted with concentration addition (CA) and independent action (IA) models may present varying interactions dependent on the relative amount of the heavy metals present in the binary mixtures and their relative mode of action.

Keywords : Prediction, Toxicity, Pesticides, Heavy Metals, Concentration Addition, Independent Action