Assessing the toxicities of the binary mixtures of sodium dodecyl sulfate and heavy metals to Serratia marcescens (SerEW01) from Otamiri River

Toxicities of sodium dodecyl sulfate (SDS) + Pb(II), SDS + Cd(II), SDS + Ni(II), SDS + Zn(II), and SDS + Co(II) binary mixtures to Serratia marcescens (SerEW01) isolated from Otamiri river water, Owerri, Imo State, Nigeria were undertaken, using dehydrogenase activity as a response. Inhibitions of dehydrogenase activity by the individual toxicants were concentrationdependent, increasing steadily as the concentration increases. The observed EC50S ranged from 0.046 ± 0.003 mM for Zn(II) to 2.329 ± 0.092 mM for SDS. Duncan tests indicated that the EC50S of the individual toxicants differed significantly from each other. The order of decreasing toxicities was Zn(II) > Cd(II) > Co(II) > Ni(II) > Pb(II) > SDS. Fixed ratio mixtures [Arbitrary concentration ratio (ABCR) and EC50 equieffect concentration ratio (EECR 50)] were used to study the joint action of the binary mixtures. The mixtures progressively inhibited dehydrogenase activity in S. marcescens as the concentration increases. However, SDS 98.08% + Co(II) 1.92% mixture ratio was biphasic. The effects of the mixtures on the dehydrogenase activity were assessed using Toxic Index, Model Deviation Ratio and Isobolographic analyses. In addition, the toxicities of the mixtures were predicted with concentration addition (CA) and independent action (IA) models. In SDS+Ni(II) binary mixture, both models predicted similar toxicities. In all binary mixtures, both models greatly underestimated the mixture toxicities compared to the experimentally-observed data. Similarly, both the experimentally-observed, CA and IA-predicted EC50S were statistically different from each another. Furthermore, the binary mixtures were generally synergistic against S. marcescens (SerEW01). This demonstrates the potential danger of co-contamination of the aquatic system by SDS and heavy metals