Acid-induced double hits in glutaraldehyde-treated Chlorella.

Glutaraldehyde-treated Chlorella cells show pH-dependent oxygen evolution characteristics. 1. The oxygen yield on the second flash (Y2) of a sequence of 2 mus flashes is pH-dependent, the rest of the sequence is unchanged. 2. This yield on the second flash increases as the dark-time between sequences is increased. 3. The deactivation times for the S2 and S3 states are not significantly changed at different pH values. 4. Using a short flash (0.5 mus) for the first flash, no oxygen was detected on the second flash, indicating that there is no S2 remaining in the dark. 5. When the intensity of the first flash is varied, a sigmoidal curve is obtained for Y2 as a function of flash intensity of the first flash is varied, a sigmoidal curve is obtained for Y2 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. At pH 6.0 the turnover of the Photosystem II centers after the first flash has a fast rise (t 1/2 < 25 mus) followed by a slow phase. This behaviour is not seen at pH 7.5, nor after two flashes. 7. We propose a scheme in which an auxiliary acceptor is in series with Q1 and in competition with B. The redox potential of the auxiliary acceptor is pH dependent.