PHYSIOLOGICAL AND EVOLUTIONARY ASPECTS OF THE O-2/H2O2-CYCLE IN CYANOBACTERIA

Various cyanobacteria evolve oxygen upon illumination with the very first flash of a sequence and interact with the oxygen from the surrounding atmosphere. In the present paper I describe recent experiments allowing the discrimination between photosynthetic water splitting and the light induced peroxide decomposition, the latter showing a strong dependence on the oxygen partial pressure in the suspension. Under appropriate conditions a clear period-2 oscillation of a flash sequence is observed. This fits into the interpretation, as the decomposition of hydrogen peroxide requires only two light quanta and S-2 has been shown to be the most reactive redox state within the S-state system. Comparison of the relaxation kinetics of the first two flashes of a sequence with the steady state signals as well as the detailed analysis of the mass spectrometric signals revealed completely different time constants for both water splitting and peroxide decomposition. These observations can be correlated to the suggestion that a peroxide might be involved in the higher oxidation states (Renger, G. (1987) Photosynthetica 21, 203-224). Moreover, the phenomena can be specifically and independently influenced and stimulated e.g. by the addition of various salts which strongly increases the water splitting reaction. In the present discussion on the participation of hydrogen peroxide the reaction sequences described for Oscillatoria in this and in previous papers might be the linking condition between oxygenic photosynthesis and the type of anoxygenic photooxidations such as the photooxidation of ferrous or sulfide ions which prevailed earlier. From the evolutionary point of view these two mechanisms might have been linked by the involvement of a quasi semireduced component like hydrogen peroxide.