Abstract

Toxicity of Zn²⁺ on four planktonic bacteria isolated from New Calabar River water was assessed via dehydrogenase assay. Pure cultures of the bacterial strains were exposed to various Zn²⁺ concentrations (0.2 - 2.0 mM) in a nutrient broth amended with glucose and TTC. The responses of the bacterial strains to Zn²⁺ is concentration-dependent. At 0.2 mM, Zn²⁺ stimulated dehydrogenase activity inProteus sp. PLK2 and Micrococcus sp. PLK4. In all strains, dehydrogenase activity was progressively inhibited at concentrations greater than 0.2 mM.  The IC₅₀ ranges from 0.236 ± 0.044 to 0.864 ± 0.138 mM. Total inhibition occurred at concentrations ranging from 1.283 ± 0.068 to 2.469 ± 0.045 mM. The order of zinc tolerance is: Micrococcus sp. PLK4 > Proteus sp. PLK2 > Pseudomonassp. PLK5 > Escherichia sp. PLK1. The  result of the in vitro study indicated that the bacterial strains are sensitive to Zn²⁺ stress. Therefore, Zn²⁺contamination would pose serious threat to their metabolism in natural environments.

Key words: New Calabar River, zinc toxicity, planktonic bacteria, dehydrogenase.