EFFECTS OF STRONG LIGHT IRRADIATION ON ANTENNAE AND REACTION CENTERS OF THE CYANOBACTERIUM ANABAENA-VARIABILIS - A TIME-RESOLVED FLUORESCENCE STUDY

Photoinhibition and photobleaching as a function of time at high fluence rates were studied using picosecond time-resolved emission spectra (TRES) and fluorescence induction in the cyanobacterium Anabaena variabilis. Photoinhibition and photobleaching can be distinguished using these techniques as phenomena which operate on different parts of the photosynthetic apparatus with very different induction periods. Photoinhibition is a short-term irradiation effect (time scale of hours) under our irradiation conditions and operates on photosystem II reaction centres in agreement with earlier studies. On photoinhibition the minimum fluorescence intensity F0 is increased and the maximum fluorescence Fmax is decreased. Both effects lead to a decrease in the variable fluorescence yield. In contrast, photobleaching is a long-term effect (time scale of days) and operates on the phycobilisome antenna. This finding is supported by experiments with sodium azide, a protective agent against photobleaching. Although sodium azide cannot fully prevent the formation of long-lived fluorescence components (detached phycobiliproteins), large negative amplitudes measured in the sodium-azide-protected samples indicate an efficient energy transfer to the reaction centre which is absent in the control. © 1990.