| In this paper, according to OECD Algal Growth Inhibition, 30 fungicides and insecticides were tested to build a kind of model organism for bioassay screening pesticides using four algae firstly in China .The results showed as follow:1 Four algae could be used for screening fungicides and insecticides as model organism, Scenedesmus quadricauda was more sensitive than the other three algae and it was selected firstly as target organism during pesticides screening. Next was the Chlorella vulgaris, Selenastrum capricornutum and Scenedesmus obliqnus.2 The optimum combination of condition for cultivating Scenedesmus quadricauda were liquid HB-4 medium, at 25 ± 1℃ , light intensity of 5000 lx/cm2, illumination continuous and cultivating for 96 hours. At 25±1℃, during continuous illumination , each growth rate of algae reached the highest value.3 The optimum biological determination index was absorbance value at 680nm,which could reflect well the growth rate of algae and was measured quickly and accurately .4 There were correlated relationship between absorbance at 680nm and cell density or Chla concentration. Their Regression Equation were C(lxl05cell/mL)= 41.236XA (r2=0.9725, p=0.0000) and Chla(mg/L)= 12.109X^(r2=0.9948, p=0.0000).5 The toxicity of acetone and ethyl isobutyl acetate was the lowst in supplying the solvents , and their EC50 value were 1.52% and 1.82% respectively . Especially acetone had dissolving capability that itself possessed well. Acetone and ethyl isobutyl acetate were determined as the optimum solvent.
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