Luminescence decay kinetics in relation to quenching and stimulation of dark fluorescence from high and low CO2 adapted cells of Scenedesmus obliquus and Chlamydomonas reinhardtii

Two green algal species, Chlamydomonas reinhardtii and Scenedesmus obliquus, exhibited a relative maximum during the decay of luminescence, when adapted to low CO₂ conditions that was not observed in high CO₂ adapted cells.From the kinetics of transient changes in the level of dark fluorescence, after illumination and parallel to the luminescence maxima, it was concluded that the maximum in Scenedesmus was mainly related to a decrease in nonphotochemical quenching, whereas in Chlamydomonas the maximum was mainly related to a dark reduction of the primary PS II acceptor Q_A.ATP/ADP ratios from low CO₂ adapted Scenedesmus showed transient high levels after a dark/light transition that was not observed in high CO₂ adapted cells. After 30 s of illumination the ATP/ADP ratios however stabilized at the same steady state level as in high CO₂ adapted cells.Dark addition of HCO₃^- to low CO₂ adapted cells of Chlamydomonas resulted in a rapid transient quenching of luminescence that was not observed in low CO₂ adapted cells of neither species.It is concluded that the luminescence maxima present in both low CO₂ adapted Scenedesmus and Chlamydomonas reflect adaptation of the cells to low CO₂ conditions. It is further suggested that the difference in mechanistic origin of luminescence maxima in the two species reflects differences in adaptation.Abbreviations ADP adenosine-diphosphate - ATP adenosine-triphosphate - C_i inorganic carbon - F_D dark fluorescence recorded under dark adapted conditions - F₀ fluorescence with all reaction centers open - FV variable fluorescence - PS I photosystem I - PS II photosystem II - Q_A the first quinone acceptor of PS II