SIMAZINE-INDUCED INHIBITION IN PHOTOACCLIMATED POPULATIONS OF ANABAENA-CIRCINALIS (CYANOPHYTA)

The effects of the triazine herbicide, simazine, on photosynthetic oxygen evolution and growth rate in photoaclimated populations of Anabaena circinalis Rabenhorst were investigated. Chemostat populations were acclimated to photon flux densities (PFDs) of 50, 130, and 230-mu-mol.m-2.s-1 of photosynthetic active radiation (PAR). Decreases in chlorophyll a (Chl a), c-phycocyanin (CPC), and total carotenoid (TCar) contents and CPC : Chl a and CPC : TCar ratios of populations coincided with increasing PFD. Polynomial regression models that characterize inhibition of photosynthesis for populations acclimated to 50 and 130-mu-mol photons.m-2.s-1 PAR were distinct from the model for populations acclimated to 230-mu-mol photons.m-2.s-1 PAR. Simazine concentrations that depressed oxygen evolution 50% compared to controls decreased with increasing PFD. Increases and decreases in both biomass and growth rate coincided with increasing PFD and simazine concentration, respectively. Simazine concentrations that depressed growth rate 50% compared to controls increased with decreasing PFD. The differences in photosynthetic and growth inhibition among photoacclimated populations indicate that sensitivity to photosystem II inhibitors is affected by alterations in pigment contents.