ACCLIMATION OF PHOTOSYSTEM-II IN A CYANOBACTERIUM AND A EUKARYOTIC GREEN-ALGA TO HIGH AND FLUCTUATING PHOTOSYNTHETIC PHOTON FLUX DENSITIES,SIMULATING LIGHT REGIMES INDUCED BY MIXING IN LAKES

Photoacclimation of Scenedesmus protuberans Fritsch and Microcystis ae ruginosa Kutzing emend. Elenkin to high and fluctuating PPFD was studi ed in continuous cultures with computer-controlled variable light regi mes. The aim of the work was to provide a better understanding of spec ies-specific acclimation to high PPFD (as encountered by cyanobacteria in surface waterblooms), and of suppression of the growth of colony-f orming cyanobacteria during periods of prolonged mixing in lakes. The dynamics of a set of variables was followed during the light period, i ncluding pigment composition, maximum rate, efficiency and minimum qua ntum requirement of photosynthesis, PS II cross-sections, and fluoresc ence variables. Both the green alga and the cyanobacterium displayed s trong photo-inhibition of photosynthesis in the sinusoidal light regim e, which simulated a natural light regime in the absence of mixing. P- max, alpha, QR and the ratio of variable to maximum fluorescence decli ned, and the number of inactive PS II centres and PS IIbeta centres in creased towards midday. Introduction of oscillations in the diurnal li ght regime, simulating different intensities of wind-induced mixing in lakes, mitigated photo-inhibition. Microcystis showed a prompt non-ph otochemical quenching of fluorescence in all light regimes, even at lo w to moderate PPFD. The sustained presence of zeaxanthin in Microcysti s possibly induced instant, thermal dissipation of excitation energy f rom the antenna. Microcystis also exhibited a more reluctant acclimati on to fluctuating PPFD. Growth rate of Scenedesmus was higher in all l ight regimes. This implied that if (known) differences in loss process es were ignored, Scenedesmus would outcompete Microcystis in lakes. Th e results underlined the importance of buoyancy regulation in increasi ng the daily light dose of cyanobacteria (but at the same time prevent ing over-excitation), and ultimately in the success in Microcystis in stable lakes.