Synergistic toxicities of binary and ternary mixtures of an anionic surfactant and divalent metals to Lysinibacillus fusiformis isolated from a vegetable farm

The toxicities of the heavy metals (Pb, Cd, Ni, Zn, and Co) and their ternary mixtures with Sodium Dodecyl Sulfate (SDS) to Lysinibacillus fusiformis isolated from Talinum fruticosum farms irrigated with Otamiri River water in Owerri, Imo State, Nigeria, were assessed using dehydrogenase activity (DHA) restriction as an endpoint. Fixed ratio mixtures (arbitrary concentration ratio (ABCR) and equi-effect concentration ratio (EECR) mixtures) were formulated to evaluate the combined toxicities of these toxicants. Toxicities were predicted with concentration addition (CA) and independent action (IA) models and compared with the experimentally observed toxicities. The response of the bacterium to the toxicants’ toxicities was concentration-dependent and gradually inhibited the DHA as the concentration increased, with percentage inhibitions greater than 95% at 0.5 mM for Zn, 1 mM for Ni, 0.3 mM for Pb, 0.08 mM for Cd, 0.7 mM for Co, as well as 10 mM for SDS. The 50% effective concentrations (EC50S) of the individual toxicants differed significantly from one another (P < 0.05). All the dose-response relationships of the ABCR and EECR mixtures and the individual toxicants could be described by a logistic function. In most binary mixtures, predicted toxicities from the CA and IA models were significantly different from the observed toxicities. In ABCR1 mixture ratio of SDS + Cd2+ mixtures, CA and IA models correctly predicted the experimental data at different points, while the IA model correctly predicted the experimental data in the EECR50 mixture ratio of SDS + Pb2+ mixture. In SDS + Co2+ mixtures, EC50S predicted by both models were identical. The effects of the mixtures interactions showed both weak and strong synergism, as well as additive against the soil bacterium. Similarly, in all but ABCR1 and ABCR2 mixture ratios of SDS + Cd + Zn ternary mixtures, the experimentally observed EC50, CA- and IA-predicted EC50S were significantly different from one another (P < 0.05). Furthermore, both models greatly underestimated the mixture toxicity at all tested mixture ratios and were strongly synergistic against the soil bacterium. The use of such contaminated water for irrigation could negatively affect the soil bacterial community and, by extension, soil fertility, going by the possible interaction between heavy metals and SDS.