Toxicity and environmental requirements of a strain of Aphanizomenon flos-aquae (L.) Ralfs

Abstract

A toxic strain of Aphanizomenon flos-aquae was isolated and cultured in a defined medium (ASM-1). This strain fixes nitrogen, has a pH optimum of 7.5, and has maximum growth rates at 5000 lux and 26 °C. Phosphorus levels of 1.0 μM readily supported populations of 1 × 105 cells/ml with stimulation of growth evident at a concentration of 0.1 μM P. Tris is inhibitory to growth of this alga at concentrations above 2.5 mM.Toxin production is related to age of culture, temperature, and light intensity but not nitrogen source.Intraperitoneal injection of toxic extracts into Fundulus heteroclitus, Cyprinodon variegatus, and white mice gave a LD100 of 0.5 mg/kg, 0.5 mg/kg, and 8 mg/kg, respectively. Assays with Notemigonus crysoleucas and Bosmina longirostris indicate that the toxin from naturally occurring bloom populations (1 × 106 cells/ml), if released all at once, would be capable of killing certain species of fish and microcrustaceans. Daphnia catawba, however, was much more resistant to toxin (LD100 = 1.0 mg/ml) while cyclopoid copepods, ostracods, and chydorid cladocerans were completely unaffected by toxin concentrations of 2.0 mg/ml. The possible ecological significance of this toxic alga in relation to fish kills is also discussed.