The first crystallographic structure of an FCP complex belongs to the diatom Phaeodactylum tricornutum and shows a dimeric unit whereas later studies on the organism Chaetoceros gracilis found different tetrameric as well as monomeric units near the PSII core of those diatoms . Although the polypeptide structures of these FCP complexes are quite similar, they differ in the chlorophyll (Chl) and carotenoid content. The monomeric FCP unit from P. tricornutum contains nine chlorophylls (seven Chl-a and two Chl-c), seven fucoxanthin (Fx) and one diadinoxanthin (Ddx) molecule. In the case of the diatom C. gracilis, the PSII core is surrounded by two homo-tetramers and three dissimilar FCP monomers. The homo-tetramers are named MT ("moderately" bound tetrameter) and ST ("strongly" bound tetrameter). Each monomer unit of these tetramers (FCP-A) contains seven Chl-a, three Chl-c, and seven Fx molecules. The monomeric units around PSII are called FCP-D, FCP-E and FCP-F. The FCP-D monomer is found at the interface between the MT-tetrameter and the CP43 complex of the PSII core. This FCP-D monomer binds nine Chl-a, one Chl-c and six Fx pigment molecules. Moreover, the structural location of the FCP-D monomer is analogous to that of the CP29 minor antenna on the periphery of the PSII complex of higher plants. The FCP-F contains seven Chl-a, two Chl-c and six Fx molecules, whereas FCP-E binds two additional Chl-a and one additional Fx molecules. Moreover, the latter two monomeric units contain one Ddx carotenoid molecule each. Interestingly, none of the FCP monomeric units from the organism C. gracilis shows the same Chl/carotenoid ratio as that of the structurally resolved FCP protein of the diatom P. tricornutum. At the same time, their stoichiometry does not match the FCP complex from Cyclotella meninghiniana whose structure has not yet been resolved but based on experimental studies it has been predicted to contain eight Chl-a, two Chl-c and eight Fx molecules . More details of the structural differences between the FCP complexes of the two diatom organisms P. tricornutum and C. gracilis are given in Ref. 5. Moreover, a structural model of the trimeric unit of the FCP complex from C. meninghiniana has been predicted in Ref. 5 based on two-dimensional spectroscopy .