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.

The fluorescence yield of isolated fucoxanthin chlorophyll proteins, serving as light harvesting proteins in diatoms, was compared to the amount of diatoxanthin bound. Diatoxanthin was earlier shown to be involved in the xanthophyll cycle in diatoms as a functional analogue of zeaxanthin in higher plants. By growing cells under different light conditions, the amount of diatoxanthin in both the trimeric FCPa as well as the oligomeric FCPb of the diatom Cyclotella meneghiniana was increased. In the trimeric FCPa, the fluorescence yield decreased with increasing diatoxanthin content, whereas in the oligomeric FCPb fluorescence was generally lower, albeit constant. No pH dependence of fluorescence yield could be demonstrated except for artificially aggregated FCPa. Thus, diatoxanthin is able to quench fluorescence in FCPa, but the yield is also influenced by pH when the protein becomes aggregated.

The ultrafast carotenoid to chlorophyll a energy transfer dynamics of the isolated fucoxanthin-chlorophyll proteins FCPa and FCPb from the diatom Cyclotella meneghiniana was investigated in a comprehensive study using transient absorption in the visible and near infrared spectral region as well as static fluorescence spectroscopy. The altered oligomerization state of both antenna systems results in a more efficient energy transfer for FCPa, which is also reflected in the different chlorophyll a fluorescence quantum yields. We therefore assume an increased quenching in the higher oligomers of FCPb. The influence of the carotenoid composition was investigated using FCPa and FCPb samples grown under different light conditions and excitation wavelengths at the blue (500nm) and red (550nm) wings of the carotenoid absorption. The different light conditions yield in altered amounts of the xanthophyll cycle pigments diadinoxanthin and diatoxanthin. Since no significant dynamic changes are observed for high light and low light samples, the contribution of the xanthophyll cycle pigments to the energy transfer is most likely negligible. On the contrary, the observed dynamics change drastically for the different excitation wavelengths. The analyses of the decay associated spectra of FCPb suggest an altered energy transfer pathway. For FCPa even an additional time constant was found after excitation at 500nm. It is assigned to the intrinsic lifetime of either the xanthophyll cycle carotenoids or more probable the blue absorbing fucoxanthins. Based on our studies we propose a detailed model explaining the different excitation energy transfer pathways in FCPa.

No abstract

Fucoxanthin-chlorophyll complexes (FCP) from the centric diatom Cyclotella meneghiniana were isolated and the trimeric FCPa complex was reconstituted into liposomes at different lipid to Chl a ratios. The fluorescence yield of the complexes in different environments was calculated from room temperature fluorescence emission spectra and compared to the aggregated state of FCPa. FCPa surrounded by high amounts of lipids resembled detergent solubilised complexes and with decreasing lipid levels, i.e. in a situation where protein contacts were increasingly favoured, the fluorescence yield of FCPa gradually decreased. In addition, the yield displayed a strong pH-dependency in case of lower lipid contents. The further reduction in fluorescence yield brought about by the conversion of diadinoxanthin to diatoxanthin was pH independent and only depended on the amount of diatoxanthin synthesised. The implications of these data for non-photochemical quenching in centric diatoms are discussed.