Biophysical Journal 52: 717-728 (1987)
© 1987 the Biophysical Society

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State Transitions in the Green Alga Scenedesmus Obliquus Probed by Time-Resolved Chlorophyll Fluorescence Spectroscopy and Global Data Analysis

ABSTRACT

Decay-associated fluorescence spectra of the green alga Scenedesmus obliquus have been measured by single-photon timing with picosecond resolution in various states of light adaptation. The data have been analyzed by applying a global data analysis procedure. The amplitudes of the decay-associated spectra allow a determination of the relative antenna sizes of the photosystems. We arrive at the following conclusions: (a) The fluorescence kinetics of algal cells with open PS II centers (F₀ level) have to be described by a sum of three exponential components. These decay components are attributed to photosystem (PS) I ( 85 ps, _max^em 695-700 nm), open PS II -centers ( 300 ps, _max^em = 685 nm), and open PS II ß-centers ( 600 ps, _max^em = 685 nm). A fourth component of very low amplitude ( 2.2-2.3 ns, _max^em = 685 nm) derives from dead chlorophyll. (b) At the F_max level of fluorescence there are also three decay components. They originate from PS I with properties identical to those at the F₀ level, from closed PS II -centers ( 2.2 ns, _max^em = 685 nm) and from closed PS ß-centers ( 1.2 ns, _max^em = 685 nm). (c) The major effect of light-induced state transitions on the fluorescence kinetics involves a change in the relative antenna size of - and ß-units brought about by the reversible migration of light-harvesting complexes between -centers and ß-centers. (d) A transition to state II does not measurably increase the direct absorption cross-section (antenna size) of PS I. Our data can be rationalized in terms of a model of the antenna organization that relates the effects of state transitions and light-harvesting complex phosphorylation with the concepts of PS II ,ß-heterogeneity. We discuss why our results are in disagreement with those of a recent lifetime study of Chlorella by M. Hodges and I. Moya (1986, Biochim. Biophys. Acta., 849:193-202).

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