Fucoxanthin-Chlorophyll-Proteins and Non-Photochemical Fluorescence Quenching of Diatoms

Abstract

Diatoms possess membrane-intrinsic light-harvesting proteins, called fucoxanthin-chlorophyll-proteins (FCPs) that, based on their protein sequences, are related to higher plant light-harvesting complexes (LHCs). FCPs differ from LHCs in overall organization around the photosystems, and in that fucoxanthin and chlorophyll c (Chl c) serve as the diatoms’ accessory pigments in a high carotenoid-to-chlorophyll ratio. Most FCPs assemble into trimeric complexes, but higher oligomers of specific polypeptide composition were also found. Polypeptides fall into three different groups, namely Lhcf (Lhcs named after the main carotenoid fucoxanthin, the main light harvesters), Lhcr (photosystem I-associated polypeptides closely related to red algal intrinsic Lhc), and Lhcx proteins related to LhcSR proteins (stress related Lhcs) identified in the green alga Chlamydomonas reinhardtii as functional homologues to PsbS (photosystem II protein S). In the genomes of diatoms published so far, sequences for four to five different Lhcx proteins can be found. All of the latter proteins are expressed and most of them are up-regulated under high light. On the other hand, no homologue of psbS is present in diatoms. The Lhcx content was shown to correlate with the ability for non-photochemical quenching (NPQ) of chlorophyll fluorescence (as a measure of thermal dissipation of singlet-excited chlorophyll) in pennate diatoms, e.g., Phaeodactylum tricornutum, as well as in centric diatoms, e.g., Thalassiosira pseudonana. Localization of Lhcx and the supra-molecular organization of FCP complexes, however, appear to differ between the latter two diatom groups. For several centric species, occurrence of Lhcx proteins within trimeric FCP complexes was shown, whereas no Lhcx proteins have been identified in FCP trimers from pennate species thus far. In centric diatoms, the content of Lhcx proteins in trimeric FCP complexes, furthermore, correlates with the level of diatoxanthin, a carotenoid of the xanthophyll cycle of diatoms synthesized under high light. Diatoxanthin content was inversely correlated with the fluorescence yield of trimeric FCP complexes, i.e., higher diatoxanthin levels were associated with a more highly quenched state. Another factor which influences NPQ in vivo is a low lumenal pH. Low pH values were also shown to exert effects on the fluorescence yield of isolated trimeric FCPs. Likewise, aggregation of FCP complexes in vitro affected their fluorescence yield as proposed for the case in vivo. A model for the involvement of FCPs in NPQ is presented.