Light dependence of quantum yields for PSII charge separation and oxygen evolution in eucaryotic algae

Quantum yields of photosystem II (PSII) charge separation (Phi(P)) and oxygen production (Phi(O2)) were determined by simultaneous measurements of oxygen production and variable fluorescence in four different aquatic microalgae representing three different taxonomic groups: the freshwater alga Scenedesmus protuberans (Chlorophyceae) and the marine algae Phaeocystis globosa (Prymnesiophyceae), Emiliania huxleyi (Prymnesiophyceae), and Phaeodactylum tricornutum (Bacillariophyceae). In S. protuberans, P. tricornutum, and E. huxleyi, light-dependent variability was observed in the ratio of Phi(O2) to Phi(P), i.e. in the number of oxygen molecules produced per electron generated by PSII. The ratio Phi(O2):Phi(P) was highly variable at low light intensities (E < 0.5E(k)), and at higher light intensities (E > 0.5E(k)) Phi(O2):Phi(P) showed a nonlinear decrease with increasing light intensity. In contrast, in P. globosa, a Vend in Phi(O2):Phi(P) could not be distinguished, and this species showed a decrease in Phi(O2):Phi(P) during the day, indicating a dependency of Phi(O2):Phi(P) on light history. Additionally, considerable interspecific quantitative differences in Phi(O2):Phi(P) were observed. Two possible interpretations to explain the variability in Phi(O2):Phi(P) are discussed. Assuming that Phi(P) is a reliable measure of the quantum yield for charge separation at PSII, one interpretation is that net oxygen production is influenced by processes that consume oxygen or affect linear electron transport (e.g. cyclic electron transport around PSII, pseudocyclic electron transport in the Mehler reaction, Rubisco oxygenase activity, and light-dependent mitochondrial respiration). A second interpretation, however, suggests that at saturating light, changes in photosynthesis turnover time occur, such that Phi(P) does not predict the steady-state O-2 yield.