The effects of photooxidative stress on photosystem I measured in vivo in Chlamydomonas

The effects of different photooxidative stresses on the function of photosystem I were measured in vivo in Chlamydomonas reinhardtii. Pholooxidative stresses included strong light, light combined with chilling to 0 °C, and light combined with several concentrations of methyl viologen. Photosystem I function was measured in vivo using the absorbance change at 820 nm associated with P₇₀₀ oxidation. Photosystem II function was measured in vivo using chlorophyll fluorescence. Strong light or light combined with chilling caused inhibition of photosystem II function earlier than inhibition of photosystem I function. When photosystem I was inhibited, however, it did not recover. Light combined with 5 mmol m^?3 methyl viologen caused inhibition of photosystem I function earlier than inhibition of photosystem II. If the methyl viologen concentration was reduced to 1 mmol m^?3, the damage to PSI was accelerated by addition of 90 mmol m^?3 chloramphenicol. This effect of chloroamphenicol suggests a role for chloroplast-encoded proteins in protecting photosystem I against photooxidative damage caused by methyl viologen.