Changes in photosynthetic properties measured by oxygen evolution and variable chlorophyll fluorescence in a simulated entrainment experiment with the cyanobacterium Planktothrix rubescens

The metalimnion of lake Zurich is dominated by the red coloured cyanobacter ium Planktotrix rubescens, where it lives in an extremely low light environ ment. Photosynthesis of the organism was studied using oxygen evolution and variable fluorescence. After transfer to 2 in depth in the epilimnion. sim ulating an entrainment event that normally occurs in late summer, photoaccl imation 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 in terpreted as inactivation of a number of photosystem II (PSII) units. At th e first entrainment we observed only inactivation of PSII, causing a decrea se in the slope of the photosynthesis light curve (both for oxygen evolutio n and photosynthetic electron transport by PSII (ETR)), without affecting t he maximal rate of oxygen evolution (PBmaxB), suggesting no effect on CO2-f ixation. During the 2(nd) entrainment experiment we observed both an inacti vation of the number of PSII as well as a decrease in P-max(B). In both cas es the ETR-based estimates of oxygen evolution (P-B) overestimated the meas ured rate of P-B, indicating the operation of alternative electron sinks. E specially at high light respiratory processes were seemingly stimulated.