Factors determining the fluorescence yield of fucoxanthin-chlorophyll complexes (FCP) involved in non-photochemical quenching in diatoms.

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.