Subunit Composition and Pigmentation of Fucoxanthin−Chlorophyll Proteins in Diatoms: Evidence for a Subunit Involved in Diadinoxanthin and Diatoxanthin Binding

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Abstract
Two different fucoxanthin−chlorophyll protein complexes (FCP) were purified from the centric diatom Cyclotella meneghiniana and characterized with regard to their polypeptide and pigment composition. Whereas the oligomeric FCPb complex is most probably composed of fcp5 gene products, the trimeric FCPa has subunits encoded by fcp1-3 and fcp6/7. The amount of the latter polypeptide is enhanced when FCPa is isolated from algae grown under HL conditions. This increase in Fcp6/7 polypeptides is accompanied by an increase in the pool of xanthophyll cycle pigments, diadinoxanthin and diatoxanthin, and a concomitant decrease in fucoxanthin content. In addition, the de-epoxidation ratio, i.e., the amount of diatoxanthin in relation to the pool of xanthophyll cycle pigments, is increased by a factor of 2. With regard to fluorescence yield, HL FCPa was quenched in comparison to LL FCPa. This is in accordance with the larger amount of diatoxanthin that is bound, which is supposed to act as a quencher like zeaxanthin in higher plants. Thus, we conclude that the enhanced content of diatoxanthin in FCPa plays a protective role, which is paralleled by a weakened light harvesting function due to a smaller amount of fucoxanthin.