Toxicity-data of two carbamate insecticides, carbaryl and carbofuran, and three fungicides, ziram, zineb and mancozeb with rice-field N 2 -fixing cyanobacterium Cylindrospermum sp., obtained by in vitro growth and at soil-water interface, were analyzed by the probit method. Growth enhancing concentration, no-observed effective concentration, minimum inhibitory concentration, the highest permissive concentration and lethal concentration 100 (LC 100 ) were determined experimentally. The LC 50 values of carbaryl, carbofuran, ziram, zineb and mancozeb in N 2 -fixing liquid medium were 56.2, 588.8, 0.07, 4.2 and 3.4 μg/mL, respectively, whereas the corresponding LC 100 values were 100.0, 1500.0, 0.17, 25.0 and 9.0 μg/mL, respectively. The LC 50 values of these pesticides in succession in N 2 -fixing agar medium were 44.7, 239.9, 0.07, 1.8 and 2.3 μg/mL, respectively, whereas the corresponding LC 100 values were 100.0, 600.0, 0.17, 10.0 and 7.0 μg/mL, respectively. Similar results with nitrate supplemented liquid and agar media indicated that nitrate supplementation had toxicity reducing effect. The LC 50 and LC 100 values of toxicity in the N 2 -fixing liquid medium at soil-water interface were 91.2 and 200.0 μg/mL for carbaryl, 2 317 and 6 000 μg/mL for carbofuran, 0.15 and 0.50 μg/mL for ziram, 16.4 and 50.0 μg/mL for zineb, and 7.2 and 25.0 μg/mL for mancozeb, respectively. Each LC 100 value at soil-water interface with a pesticide was significantly higher than its corresponding LC 100 value at liquid/agar media. It can be concluded that, under the N 2 -fixing conditions, the cyanobacterium tolerated higher levels of each pesticide at soil-water interface.
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