Klaus P. Bader and Susanne Höper Fakultät für Biologie, Lehrstuhl Zellphysiologie, Universität Bielefeld, Postfach 1001 31, D-33501 Bielefeld, Bundesrepublik Deutschland Z. Naturforsch. 49c, 87-94 (1994); received November 8, 1993 Photosynthesis, Electron Transport, Biocide, Quaternary Ammonium Salt Alkylbenzyldimethylammonium chloride (ABDAC, zephirol) has been shown to improve the functioning of the photosynthetic apparatus of the filamentous cyanobacterium Oscillatoria chalybea (Bader, K. P. (1989) Biochim. Biophys. Acta 975, 399-402). This bio cide exerts stimulatory effects on various electron transport reactions in Oscillatoria chaly bea and chloroplasts from higher plants. By means of oxygen evolution measurements and of repetitive flash-induced absorption spectroscopy we were able to demonstrate an impact of the drug on the major complexes of photosynthetic membranes, i.e. the water splitting complex, photosystem II and photosystem I. Both, P820and X320-absorption change signals were enhanced by the addition of ABDAC. Along with the quantitative analysis we investi gated the relaxation kinetics of the signals and observed a substantial stabilization of the oxi dized states of the respective redox components in the presence of the ammonium salt. Under appropriate conditions the relaxation kinetics of the absorption signals were significantly slowed down. ABDAC also affects photosystem I in Oscillatoria chalybea, but only under conditions, where a donor/acceptor system i.e. an isolated photosystem I reaction with photo system II being disconnected was measured. Electron transport through the whole chain i.e. with water as the electron donor yielded no effect of the quaternary ammonium salt. It is sug gested that this is due to an extremely bad linkage between the two photosystem, each of which, however, shows good reaction rates, when separately measured. The described inter actions of the biocide with photosynthetic membranes are not restricted to Oscillatoria chalybea but are also observed with higher plant chloroplasts. In these systems, ABDAC enhances X320and P700-signals to a comparable extent. In this case the P700-signal is stimulat ed in assays with electrons which are furnished from water which hints at good coupling be tween the two photosystems in our tobacco chloroplast preparations.