Acid-induced double hits in glutaraldehyde-treated Chlorella

Abstract

Glutaraldehyde-treated Chlorella cells show pH-dependent oxygen evolution characteristics. 1. 1. The oxygen yield on the second flash ( Y 2) of a sequence of 2 μs flashes is pH-dependent; the rest of the sequence is unchanged. 2. 2. This yield on the second flash increases as the dark-time between sequences is increased. 3. 3. The deactivation times for the S 2 and S 3 states are not significantly changed at different pH values. 4. 4. Using a short flash (0.5 μs) for the first flash, no oxygen was detected on the second flash, indicating that there is no S 2 remaining in the dark. 5. 5. When the intensity of the first flash is varied, a sigmoidal curve is obtained for Y 2 as a function of flash intensity at pH 6.0 but not at pH 7.5. This is indicative of a double hit process. 6. 6. At pH 6.0 the turnover of the Photosystem II centers after the first flash has a fast rise ( t 1 2 < 25 μ s ) followed by a slow phase. This behaviour is not seen at pH 7.5, nor after two flashes. 7. 7. We propose a scheme in which an auxiliary acceptor is in series with Q 1 and in competition with B. The redox potential of the auxiliary acceptor is pH dependent.