Effect of Dimethoate on Photosynthesis and Pigment Fluorescence ofSynechocystissp. PCC 6803

The organophosphorus (OP) insecticides are powerful inhibitors of esterases, and their toxic actions are commonly explained in terms of acetylcholinesterase (EC 3.1.1.7) inhibition but their phytotoxic effects remain unexplained. In this study the effects of an OP insecticide, dimethoate, on cyanobacterial photosynthesis and respiration were measured using the cyanobacteriumSynechocystissp. PCC 6803 as test organism. The insecticide caused enhancement of respiratory O2consumption at all tested concentrations (10–300 μM) while photosynthesis was found to be significantly affected at concentrations ≥50 μM.From fluorescence emission analysis, oxygen exchange measurement, and determination of14CO2incorporation, it was found that dimethoate caused inhibition of photosynthetic electron transport, resulting in increase of PS II fluorescence and reduction in photosynthetic carbon fixation. An increase of nonphotochemical quenching was caused by the insecticide through the increase in acidity of the thylakoid lumen. Furthermore, detachment of phycobilisomes (PBS) from the PS II reaction centers was observed in term of increase in PBS fluorescence in treated cultures. This detachment is expected to be caused by membrane fluidity changes. The fluorescence enhancement of PS II was more than that of the PBS.