LIGHT ADAPTATION IN THE CYANOBACTERIUM SYNECHOCOCCUS SP PCC-7942 MEASURED BY THE DUAL-MODULATION FLUOROMETER

In photosynthetic organisms, the fluorescence emission competes for ex citation energy with photochemistry. An increase in the fluorescence y ield reflects limitation of photochemistry and vice versa. This relati onship is widely used in photosynthesis research to monitor photosynth etic activity of plants by noninvasive fluorescence measurements. Plan ts respond to the changes in irradiance by photosynthetic adaptation p rocesses that allow concerted operation of the two photosystems in a v ariable light environment. Here, we demonstrate the power of the dual- modulation fluorometry using an example of the light adaptation in the cyanobacterium Synechococcus sp. PCC 7942. It is shown that upon an e xposure to a strong white light, the photosynthetic apparatus undergoe s a sequence of adaptation processes including photoinhibition and red uction of the Photosystem II effective absorption cross-section (anten na size) that modify the fluorescence yield. The adaptation also inclu des exchange of two forms of the Photosystem II reaction center protei n D1 differing in their primary structure.