Effects of an organophosphorus insecticide dimethoate (O,O-dimethyl-S-(N-methyl carbamoylmethyl) phosphorodithioate) and herbicide triazine (2-chloro-4-ethylamino-6- isopropylamino -S-triazine) were measured separately and also in combination on growth and pigment fluorescence of the cyanobacterium Anabaena doliolum with 4 days exposure. Both the pesticides reduced the growth and chl-a content of the cultures but enhanced the fluorescence from phycobilisomes (PBS), PS II and PS I. PS II fluorescence at 580 nm excitation was increased more than that of PBS and PS I and atrazine more effectively accelerated PS II fluorescence than that of dimethoate. The EC50s were 33.17±1.85 μM and 11.94±0.83 μM for dimethoate and atrazine, respectively, determined from a significant first order polynomial relation between PS II fluorescence and pesticide concentrations. The predicted EC50s, calculated from concentration addition equation, ranged between 12.76 and 28.16 μM and the observed EC50 in combination of the pesticides ranged between 12.62±0.93 μM and 28.74±1.39 μM. It was found to have additive effects in the combination.
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