Toxicity of carbamate pesticides to cells, heterocysts and akinetes of the cyanobacterium Cylindrospermum sp.

Abstract

Toxicity-studies of two carbamate insecticides carbaryl and carbofuran and three carbamate fungicides ziram, zineb and mancozeb (commercial formulations) with the N 2 -fixing filamentous cyanobacterium (blue-green alga) Cylindrospermum sp. are described. Toxicity to vegetative cells, heterocysts and nitrogen fixation, as well as akinete-formation and - germination are recorded. Under N 2 -fixing condition and calcium nitrate supplementation, the levels 100 and 120 ppm carbaryl were the respective lethal concentration 100 or effective concentration 100 , while the levels 20 to 80 ppm (under N 2 -fixing condition) and 20 to 100 ppm (with nitrate supplementation) were the partial lethal concentrations (less than LC 100 ). Under N 2 -fixing condition and nitrate supplementation, the levels 2000 and 3000 ppm carbofuran were the LC 100 , respectively, whereas levels 100 to 1500 ppm and 100 to 2000 ppm were respective partial lethal doses. The level of 25 ppm carbofuran had growth enhancement in both conditions, i.e., without and with nitrate supplementation. The fungicide ziram had growth stimulatory effect on the cyanobacterium at 0.01 ppm level in both nitrogen fixing and nitrate supplemented conditions. The partial lethal range for both growth conditions was 0.5 to 0.13 and LC 100 levels were 0.17 and 0.2 ppm for N 2 -fixing and nitrate supplemented media, respectively. The fungicide zineb had no growth stimulation at the lowest concentration used; partial lethal ranges were 1 to 15 ppm in N 2 -fixing and 5 to 20 ppm in nitrate supplemented growth media, and the LC 100 levels were 20 and 30 ppm in respective conditions. The fungicide mancozeb had no growth stimulation at the lowest dose used; partial lethal levels were 1 to 7 ppm mancozeb and LC 100 levels were 9 ppm in both growth conditions. Average numbers of vegetative cells between two polar heterocysts were affected by all used pesticides. In agar media, highest permissive concentrations (HPC) were 80 ppm for carbaryl and 50 ppm for carbofuran; LC 100 were 100 and 600 ppm for the respective insecticides. HPC values of ziram, zineb and mancozeb were 0.15, 5 and 5 ppm, respectively and LC 100 values of these fungicides were 0.17, 10, and 7 ppm in both N 2 -fixing and nitrate supplemented agar media. Values of no observed effective concentration (NOEC), minimum inhibitory concentration (MIC) or least observed effective concentration (LOEC), HPC and LC 100 of the five pesticides were recorded. Dehydrogenase activity of heterocysts (reduction of 2,3,5-triphenyl tetrazolium chloride or TTC) was affected by these pesticides at levels much higher than the respective LC 100 levels, but N-contents of cultures in N 2 -fixing medium in presence of individual pesticides were affected at lower levels of respective LC 100 levels, as determined by micro-kjeldahl method. Formation and germination of akinetes were impaired at all the levels below the HPC in agar media of each pesticide. It was observed that these three fungicides had damaging rolls on vegetative cells, heterocyst activity and N 2 -fixation than these two insecticides; and ziram was most toxic among the fungicides used.