Nweke et al.:Toxicity of zinc to heterotrophic bacteria froma tropical river sediment APPLIED ECOLOGY AND ENVIRONMENTAL RESEARCH 5(1): 123-132 http://www.ecology.uni-corvinus.hu ● ISSN 1589 1623  2007, Penkala Bt., Budapest, Hungary - 123 - TOXICITY OF ZINC TO HETEROTROPHIC BACTERIA FROM A TROPICAL RIVER SEDIMENT C. O. NWEKE 1, * – C. S. ALISI 2 – J.C. OKOLO 1 – C. E. NWANYANWU 1 1 Department of Microbiology, Federal University of Technology, P.M.B.1526, Owerri, Nigeria. 2 Department of Biochemistry, Federal University of Technology, P.M.B. 1526, Owerri, Nigeria. e-mail: xrisokey@yahoo.com (Received 31 th Jan 2007; accepted 15 th May 2007) Abstract. Tolerance to Zn 2+ by pure cultures of Bacillus, Salmonella and Arthrobacter species isolated from New Calabar River sediment was assessed through dehydrogenase assay. The cultures were exposed to Zn 2+ concentrations of 0.2 to 2.0 mM in a nutrient broth-glucose-TTC medium. The responses of the bacterial strains varied with Zn 2+ concentration. In Salmonella sp. SED2, Zn 2+ stimulated dehydrogenase activity at 0.2 mM. In Bacillus sp. SED1 and Arthrobacter sp. SED4, dehydrogenase activity was progressively inhibited with increasing Zn 2+ concentration. The IC 50 ranges from 0.206 ± 0.030 to 0.807 ± 0.066 mM. Total inhibition of dehydrogenase activity was observed at concentrations ranging from 1.199 ± 0.042 to 1.442 ± 0.062. The order of zinc tolerance is: Salmonella sp. SED2 > Arthrobacter sp. SED4 > Bacillus sp. SED1. The result of the in vitro study indicated that Zinc is potentially toxic to sediment bacteria and could pose serious threat to their metabolism in natural environments. Keywords: dehydrogenase activity, sediment bacteria, New Calabar River. Introduction Bacteria and other microorganisms densely colonize freshwater and marine sediments. In these environments, bacteria constitute the primary agents of early transformation of organic matter and regeneration of nutrients and also serve as food source for higher trophic level. Microorganisms are vital for the efficient functioning of any ecosystem, hence factors that affect their metabolism, composition and abundance are of great concern. Monitoring microbial responses has been recommended as an early warning indicator of ecosystem stress as microbes respond promptly to environmental perturbations [12, 29]. Measurement of microbial enzyme activity is used in the assessment of ecotoxicological impacts of environmental substrates. In this regard, dehydrogenase activity has been widely used. The dehydrogenase assay is an effective primary test for assessing the potential toxicity of metals to soil microbial activities [1, 7, 18, 31] and bioavailability of metal in a beach sediment [9]. The pollution of New Calabar River is due to anthropogenic activities along its bank [25, 27, 28]. The heavy metal content, seasonal variations in the population of heavy metal resistant bacteria and toxicity of heavy metals to bacteria isolated from the New Calabar River have been reported [23, 25, 26]. Moreso, Odokuma and Abah reported bioaccumulation of selected heavy metals by bacteria isolated from this river [24]. Although, these works focused on pure cultures of bacterial isolates, they did not consider the inhibition of dehydrogenase enzyme activity in these bacteria. This study was aimed at assessing the effects of zinc on the dehydrogenase activities of Bacillus, Salmonella and Arthrobacter species isolated from New Calabar River sediment.