Effect of the respiratory activity on photoinhibition of the cyanobacterium Synechocystis sp.

The influence of respiratory activity on photosynthesis in Synechocystis cells that had been exposed to high light intensity was studied using distinct conditions of nitrogen supply. The photoinhibitory rate of N-sufficient cells was not influenced by the presence of different nitrogen sources. In contrast, when N-starved cells were resupplied with ammonium, they were protected from photoinhibition. Although N-starved cells presented a higher rate of dark O(2) uptake than N-sufficient ones, the photoinhibitory rate increased in both cases after addition of sodium azide or sodium azide plus salicylhydroxamic acid in the photoinhibitory treatment. In the absence of the D(1) protein repair mechanism, photodamage to Photosystem II was faster in N-sufficient cells than in N-starved ones. Mitigation of photodamage disappeared when the respiratory activity of N-starved cells was partially suppressed by the addition of sodium azide or sodium azide and salicylhydroxamic acid. Our results suggest that electron flow through cyanobacterial terminal oxidases can assist Photosystem I in removing electrons from the reduced plastoquinone pool, thus contributing to both reopening of Photosystem II reaction centers and avoiding photogeneration of reactive oxygen species under photoinhibitory conditions.