Effect of pH, irradiance and population size on the toxicity of Furadan to two species of Anabaena

Abstract

In two selected nitrogen-fixing cyanobacterial species Anabaena fertilissima and Anabaena variabilis pH, irradiance and different inocula sizes considerably modified the toxic effect of 50% effective concentration (EC 50) dose of the pesticide Furadan (carbofuran 75 DB). Maximum growth and chlorophyll (Chl) a content of A. fertilissima was observed in the pH range of 8 - 9 and that of A. variabilis at pH 7-8, while at acidic pH (5-6) and at pH above 9 these parameters were considerably retarded. Toxicity of the EC 50 dose of Furadan was increased further at pH 5-6, whereas reduction in the toxicity to the test cyanobacteria was observed at pH 7.8-9.0. The experimental organisms grew comparatively better and synthesized higher amount of Chl a at an irradiance of 12.5 than at 7.5 or 2.5 W m-2. The toxicity of EC 50 dose of the pesticide gradually decreased with the increasing irradiance. The toxic effect of Furadan was larger when the initial cyanobacterial population concentration was low and vice versa.