Functional organization of the photosynthetic apparatus of the primitive alga Mantoniella squamata

Abstract

The organization of photosystem I (PS I) and photosystem II (PS II) as well as their light harvesting complexes (LHC) in the thylakoid membrane of Mantoniella squamata was characterized by measurements of fluorescence induction and picosecond fluorescence decay kinetics. This microalga possesses a homogeneous thylakoid membrane system for which no state transitions are known and in which the two photosystems are somehow mingled. Here we addressed the questions whether PS I and PS II are in such a close contact that PS I drains excitation energy from PS II as suggested by Trissl and Wilhelm (Trissl, H.-W. and Wilhelm, C. (1993) Trends Biochem. Sci. 18, 415-419) and whether the PS II units themselves are excitonically coupled (connected units) or organized as separated units. By quantitative analysis of fluorescence induction curves we determined the antenna size of PS II to be N approximate to 510 (all chlorophylls). From the ratio of maximal F-m/F-0 = 3.1 we conclude that the excitonic contact between PS II and PS I is weak. However, there is significant contact between PS II units as obvious from the sigmoidicity of the fluorescence induction curve. The fluorescence induction kinetics are well described by the connected units model (Lavergne, J, and Trissl, H.-W. (1995) Biophys. J. 65, 2474-2492) using only PS IIalpha-centers. These results indicate that essentially one population of PS II units (PS IIalpha) exists in the thylakoid membrane of Mantoniella and that the PS II units are in closer contact to each other than to PS I. This closer excitonic contact of PS II units is consistent with the idea of a PS II dimer organization.