Toxic Effects of Butachlor and Bensulfuron Methyl on Cyanobacterium Spirulina platensis

The toxic effects of butachlor and bensulfuron methyl on S. platensis were studied using shaking flash method. It showed that significant dosage effect of butachlor on algal growth(expressed in OD560) existed. The growth of S. platensis exposed under 64 mg·L-1 butachlor was severely inhibited. Specific growth rate was 0.111 d-1. It was only 24% of the control. During the 7 d experimental period, EC50 decreased exponentially. The EC50 in 96 h was 45.4 mg·L-1, and it dropped to 15.2 mg·L-1 in day 7. Like butachlor, dosage effect of bensulfuron methyl on growth inhabitation on S. platensis was also significant. Specific growth rate under 44.8 mg·L-1 treatment was 0.458 d-1. It was 82% of the control. During the 8 d experimental period, EC50 value increased exponentially. EC50 in 96 h was 51.3 mg·L-1, and EC50 in day 8 was 215 mg·L-1. When treated with these two herbicides, algal filaments of S. platensis became shorter, and some of the spiraled filaments changed into a linear form. Percentage of linear algal filaments increased while herbicides concentration increased. Nearly all algal filaments changed into linear form with much lighter color in the algal culture with the highest concentration of herbicides. Although toxicity of the two herbicides to S. platensis was moderate, butachlor exhibited higher toxicity than bensulfuron methyl to the alga according to 96 h-EC50. With longer exposure time, toxicity of butachlor increased while that of bensulfuron methyl decreased gradually. Lots of spiraled filaments of S. platensis switched to linear ones under high concentration treatments, and some filaments even disconnected and became much shorter.