Toxicity of two carbamate insecticides to the cyanobacterium Anabaena PCC 7120 and computations of partial lethal concentrations by the probit method.

Toxicity studies of two commercial carbamate insecticides, carbaryl and carbofuran with the nitrogen-fixing filamentous cyanobacterium Anabaena PCC 7120, are described. Under nitrogen-fixing conditions and with calcium nitrate supplementation, 100 and 120 ppm carbaryl were the respective lethal concentrations (LC100), while 20 to 80 ppm (nitrogen-fixing conditions) and 20 to 100 ppm (with nitrate supplementation) were the partial lethal doses (<LC100). Under nitrogen-fixing conditions and nitrate supplementation, 100 to 1,000 ppm and 100 to 1,200 ppm were the respective partial lethal concentrations, whereas 1,500 ppm carbofuran was the LC100 for both conditions. In agar media, the highest permissive insecticide concentrations were 60 ppm for carbaryl and 250 ppm for carbofuran; minimum inhibitory concentrations were 10 and 25 ppm; and the LC100 were 80 and 300 ppm, respectively. Computations of percentage lethal data yielded LC25, LC50 and LC75 values by the probit method. The number of vegetative cells between two successive heterocysts decreased. The N-content of the cultures in nitrogen-fixing medium determined by the micro-Kjeldahl method, was affected significantly by both insecticides. Carbofuran was less hazardous than carbaryl to the cyanobacterium.