Multiple pathways of excitation energy flow in the photosynthetic pigment system of a cryptophyte, Cryptomonas sp. (CR‐1)*

Abstract

SUMMARYEnergy transfer pathways in a cryptophyte, Cryptomonas sp. (CR‐1 strain) were investigated mainly by the steady state fluorescence spectroscopy and the time‐resolved spectrum. Cryptomonas sp. (CR‐1) contains chlorophyll (Chi) a, Chi c2, carotenoids and cryptomonad phycoer‐ythrin (Cr‐PE565), the last of which is known to be located in the lumenal side of the thyiakoid membranes. The spectral heterogeneity cf pigments was resolved by fluorescence spectra; there were at least five emission bands of Chi a at ‐196°C. Chlorophyll C2 and carotenoids transferred independently to the common Chi a form (Chi a663), and Cr‐PE565, to the different form (Chi a682). Chlorophyll c2 was not an intermediary component of energy transfer from carotenoid to Ch a; this is a common phenomenon to green algae and brown algae. The Chi a663 and Chi a682 are postulated to be located in the light‐harvesting chlorophyll protein (LHC) II; thus, the energy is accumulated on Chi a682‘n LHC II. The energy transfer step in Cr‐PE;565, was short, which was shown by a small, time‐dependent red‐shift of the emission. In the photosystem (PS) II core, two fluorescence components were resolved at 688 and 696 nm. The former was the trap at cryogenic temperatures. A large red‐shift induced by the low temperature was explained by an equilibrium between Chi a682 in LHC II and Chi a688 in PS II core. The presence of Chi a682 emission at physiological temperature is a unique feature of this alga. This was also reported in dinophyceae, which contain peridinin‐ChI a‐protein in the lumenal side of the thyiakoid membrane. Thus, this modification might be common in systems where the antenna complexes bind to the LHC II on the lumenal side. Based on the spectral data, we proposed a model for the molecular organization of PS II and the energy transfer pathways in cryptophyceae.