Acute toxicity of the natural algicide, cyanobacterin, to Daphnia magna

Cyanobacterin is a potent, photosynthetic inhibitor produced as a secondary metabolite by the filamentous, freshwater cyanobacterium Scytonema hofmanni. With a spectrum of activity encompassing species of cyanobacteria and eucaryotic algae as well as higher plants, cyanobacterin might be utilized as a commercial algicide for algae waterbloom control. As a component of the preliminary environmental risk/hazard evaluation of this proposed application, the acute toxicity of cyanobacterin to the planktonic crustacean, Daphnia magna, was assessed. Based on five trials, the 48-hr LC50 for D. magna was determined to be approximately 1.37 micrograms/ml (range: 0.78-2.58 micrograms/ml). In addition, a significant percentage of the surviving daphnids were completely immobile at the end of the 48-hr exposure period. These results give strong indication that cyanobacterin and/or its residues exhibit some nontarget organism lethality at projected use-concentration level. Furthermore, estimates of partition coefficients and bioconcentration factor, generated through quantitative structure-activity relationships, suggest that sediment, suspended particulates, and biota may serve as major compartments of cyanobacterin partitioning in an aquatic ecosystem. Before cyanobacterin should be further considered for application as a commercial algicide, a rigorous evaluation of nontarget organism lethality and environmental fate is required.