Structurally flexible macro-organization of the pigment-protein complexes of the diatom Phaeodactylum tricornutum

By means of circular dichroism (CD) spectroscopy, we have characterized the organization of the photosynthetic complexes of the diatom Phaeodactylum tricornutum at different levels of structural complexity: in intact cells, isolated thylakoid membranes and purified fucoxanthin chlorophyll protein (FCP) complexes. We found that the CD spectrum of whole cells was dominated by a large band at (+)698 nm, accompanied by a long tail from differential scattering, features typical for psi-type (polymerization or salt-induced) CD. The CD spectrum additionally contained intense (-)679 nm, (+)445 nm and (-)470 nm bands, which were also present in isolated thylakoid membranes and FCPs. While the latter two bands were evidently produced by excitonic interactions, the nature of the (-)679 nm band remained unclear. Electrochromic absorbance changes also revealed the existence of a CD-silent long-wavelength (similar to 545 nm) absorbing fucoxanthin molecule with very high sensitivity to the transmembrane electrical field. In intact cells the main CD band at (+)698 nm appeared to be associated with the multilamellar organization of the thylakoid membranes. It was sensitive to the osmotic pressure and was selectively diminished at elevated temperatures and was capable of undergoing light-induced reversible changes. In isolated thylakoid membranes, the psi-type CD band, which was lost during the isolation procedure, could be partially restored by addition of Mg-ions, along with the maximum quantum yield and the non-photochemical quenching of singlet excited chlorophyll a, measured by fluorescence transients.