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The metalimnion of lake Zürich is dominated by the red coloured cyanobacterium Planktotrix rubescens, where it lives in an extremely low light environment. Photosynthesis of the organism was studied using oxygen evolution and variable fluorescence. After transfer to 2 m depth in the epilimnion, simulating an entrainment event that normally occurs in late summer, photoacclimation was followed.The metalimnetic population had a very high photosynthetic efficiency, and the minimum quantum requirement varied between 7 -12 photons (mol O 2 ) -1 . Upon transfer to high light the quantum requirement increased, which was interpreted as inactivation of a number of photosystem II (PSII) units. At the first entrainment we observed only inactivation of PSII, causing a decrease in the slope of the photosynthesis light curve (both for oxygen evolution and photosynthetic electron transport by PSII (ETR)), without affecting the maximal rate of oxygen evolution (P B max ), suggesting no effect on CO 2 -fixation. During the 2 nd entrainment experiment we observed both an inactivation of the number of PSII as well as a decrease in P B max . In both cases the ETRbased estimates of oxygen evolution (P B ) overestimated the measured rate of P B , indicating the operation of alternative electron sinks. Especially at high light respiratory processes were seemingly stimulated.

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