EFFECT OF DIMETHOATE ON PHOTOSYNTHESIS AND PIGMENT FLUORESCENCE OF SYNECHOCYSTIS SP PCC-6803

The organophosphorus (OP) insecticides are powerful inhibitors of este rases, and their toxic actions are commonly explained in terms of acet ylcholinesterase (EC 3.1.1.7) inhibition but their phytotoxic effects remain unexplained, In this study the effects of an OP insecticide, di methoate, on cyanobacterial photosynthesis and respiration were measur ed using the cyanobacterium Synechocystis sp. PCC 6803 as test organis m, The insecticide caused enhancement of respiratory O-2 consumption a t all tested concentrations (10-300 mu M) while photosynthesis was fou nd to be significantly affected at concentrations greater than or equa l to 50 mu M. From fluorescence emission analysis, oxygen exchange mea surement, and determination of (CO2)-C-14 incorporation, it was found that dimethoate caused inhibition of photosynthetic electron transport , resulting in increase of PS II fluorescence and reduction in photosy nthetic carbon fixation, An increase of nonphotochemical quenching was caused by the insecticide through the increase in acidity of the thyl akoid lumen, Furthermore, detachment of phycobilisomes (PBS) from the PS II reaction centers was observed in terms of increase in PBS fluore scence in treated cultures, This detachment is expected to be caused b y membrane fluidity changes, The fluorescence enhancement of PS II was more than that of the PBS. (C) 1997 Academic Press.