The toxicity of a commercial formulation of the insecticide parathion-methyl to the N 2 -fixing filamentous cyanobacterium (blue-green alga) Cylindrospermum sp. was studied. A concentration of parathion-methyl of 0.5 ppm caused growth increase in liquid growth media. The minimum inhibitory concentration of parathion-methyl for both types (N 2 -fixing and nitrate supplemented) of liquid and solid media was 1.0 ppm. LC 50 values were: 4.4 ppm (liquid, N 2 -fixing), 5.5 ppm (liquid, nitrate supplemented), 3.3 ppm (agar, N 2 -fixing) and 4.0 ppm (agar, nitrate supplemented). LC 100 values for N 2 -fixing liquid and both types of agar media were 10.0 ppm, while for the liquid nitrate supplemented medium the LC 100 was 12.0 ppm. Both akinete (spore) formation and germination were inhibited below the highest permissive concentration of 8.0 ppm, with the insecticide incorporated in the agar media. In soil, the LC 50 and LC 100 values for parathion-methyl were 13.6 and 30 ppm, respectively. Both the dehydrogenase activity of heterocysts (monitored by 2,3,5-triphenyl tetrazolium chloride reduction) and the nitrogen concentration of cultures (estimated by the micro-Kjeldahl method) were affected by the insecticide, but the latter (N 2 -fixation) was more sensitive. The Kruskal-Wallis H test on the numbers of vegetative cells in the filaments revealed that the insecticide significantly affected the division of vegetative cells. The cyanobacterium could detoxify the growth medium containing high levels (30 and 40 ppm) of the insecticide in short-term exposures at the expense of cell viability.
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