Differential responses of two freshwater cyanobacteria, Anabaena variabillis and Nostoc commune, to sulfonylurea herbicide bensulfuron-methyl

The effect of bensulfuron-methyl on the non-targeted cyanobacteria was greater on A. variabillis than N. commune. Both A. variabillis and N. commune were initially able to utilize low concentrations of the herbicide, bensulfuron-methyl, whereas higher concentrations of bensulfuron-methyl or the hydrolyzed products of the herbicide were found to be toxic. Growth and photosynthesis inhibitions of over 50% were observed, when 8 to 10 ppm of the herbicide was applied. Nitrogenase activities of the cyanobacteria were decreased by 94-98% in A. variabillis and by 85-86% in N. commune after 24 h of incubation with 10 ppm and 20 ppm of bensulfuron-methyl. Nitrogenase activities were also inhibited by the addition of ammonium salts as low as 0.05 mM. Furthermore, the toxic effect of the herbicide was the highest at pH 4-6, showing approximately 42-60% toxicity, whereas much lower toxicity (9-28%) was observed at higher pH of 7-10, due to base-catalyzed hydrolysis of bensulfuron-methyl.