Effects of three pesticides on the growth, photosynthesis and photoinhibition of the edible cyanobacterium Ge-Xian-Mi (Nostoc)

Effects of butachlor, bensulfuron-methyl, and dimethoate on the growth, photosynthesis, and photoinhibition of the edible cyanobacterium Ge-Xian-Mi were examined in order to gain insight into the relationship between its productivity reduction and the abusive use of pesticides in the field. Severe inhibition of growth was found in the presence of four- to six-fold field concentration of butachlor and very high concentrations of bensulfuron-methyl and dimethoate. Mild stimulation of photosynthesis was observed over a limited range of low concentrations of these three pesticides. We found that PSII and PSI were, respectively, the inhibitory sites of 150 μM butachlor and 150 μM bensulfuron-methyl. However, the inhibitory site of 2000 μM dimethoate seems to be situated at the terminal of the whole chain or dark reaction. The colonies exposed to 150 μM butachlor were more sensitive to high light than control cells and those exposed to bensulfuron-methyl, dimethoate, or low butachlor concentration. Dim light-induced rapid recovery of photoinhibited colonies was observed for the control, 10 μM butachlor, bensulfuron-methyl, and dimethoate treatments. However, the maximal PSII photochemical efficiency of photoinhibited colonies treated with 150 μM butachlor was maintained at a relatively stable value in low light. Our findings suggest that the abusive utilization of butachlor might be an important factor limiting the productivity of Ge-Xian-Mi in the field.