Effect of dimethoate on photosynthesis and pigment fluorescence of Synechocystis sp. 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 cyanobacterium Synechocystis sp. PCC 6803 as test organism. The insecticide caused enhancement of respiratory O2 consumption at all tested concentrations (10-300 microM) while photosynthesis was found to be significantly affected at concentrations > or = 50 microM. From fluorescence emission analysis, oxygen exchange measurement, and determination of 14CO2 incorporation, 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 terms 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.