Probit Analysis of Carbamate- Pesticide-Toxicity at Soil-Water Interface to N2-Fixing Cyanobacterium Cylindrospermum sp.

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) (LC100) were determined experimentally. The LC50 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 mu g/mL, respectively, whereas the corresponding LC100 values were 100.0, 1500.0, 0.17, 25.0 and 9.0 mu g/mL, respectively. The LC50 values of these pesticides in succession in N-2-fixing agar medium were 44.7, 239.9, 0.07, 1.8 and 2.3 mu g/mL, respectively, whereas the corresponding LC100 values were 100.0, 600.0, 0.17, 10.0 and 7.0 mu g/mL, respectively. Similar results with nitrate supplemented liquid and agar media indicated that nitrate supplementation had toxicity reducing effect. The LC50 and LC100 values of toxicity in the N-2-fixing liquid medium at soil-water interface were 91.2 and 200.0 mu g/mL for carbaryl, 2 317 and 6 000 mu g/mL for carbofuran, 0.15 and 0.50 mu g/mL for ziram, 16.4 and 50.0 mu g/mL for zineb, and 7.2 and 25.0 mu g/mL for mancozeb, respectively. Each LC100 value at soilwater interface with a pesticide was significantly higher than its corresponding LC100 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.