LIGHT DEPENDENCE OF QUANTUM YIELDS FOR PSII CHARGE SEPARATION AND OXYGEN EVOLUTION IN EUKARYOTIC ALGAE

Quantum yields of photosystem II (PSII) charge separation (Phi(P)) and oxygen production (Phi(O2)) were determined by simultaneous measureme nts of oxygen production and variable fluorescence in four different a quatic microalgae representing three different taxonomic groups: the f reshwater alga Scenedesmus protuberans (Chlorophyceae) and the marine algae Phaeocystis globosa (Prymnesiophyceae), Emiliania huxleyi (Prymn esiophyceae), and Phaeodactylum tricornutum (Bacillariophyceae). In S. protuberans, P. tricornutum, and E. huxleyi, light-dependent variabil ity 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 rati o Phi(O2):Phi(P) was highly variable at low light intensities (E < 0.5 E(k)), and at higher light intensities (E > 0.5E(k)) Phi(O2):Phi(P) sh owed 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, i ndicating a dependency of Phi(O2):Phi(P) on light history. Additionall y, considerable interspecific quantitative differences in Phi(O2):Phi( P) were observed. Two possible interpretations to explain the variabil ity in Phi(O2):Phi(P) are discussed. Assuming that Phi(P) is a reliabl e measure of the quantum yield for charge separation at PSII, one inte rpretation is that net oxygen production is influenced by processes th at consume oxygen or affect linear electron transport (e.g. cyclic ele ctron transport around PSII, pseudocyclic electron transport in the Me hler reaction, Rubisco oxygenase activity, and light-dependent mitocho ndrial respiration). A second interpretation, however, suggests that a t saturating light, changes in photosynthesis turnover time occur, suc h that Phi(P) does not predict the steady-state O-2 yield.