Carotenoid triplet state formation in the light-harvesting complexes of selected algae from the phylogenetic SAR group

Jakub Pšenčíka, Petro Khoroshyya, David Bínab,c, Zdenko Gardianb,c, Radek Litvínb,c
aDepartment of Chemical Physics and Optics, Faculty of Mathematics and Physics, Charles University in Prague, 121 16 Prague 2, Czech Republic; bBiological Centre, Czech Academy of Sciences, České Budějovice, CZ-37005, Czech Republic; cFaculty of Science, University of South Bohemia, České Budějovice, CZ-37005, Czech Republic

The SAR phylogenetic supergroup is composed of Stramenopiles, Alveolates and Rhizaria. Photosynthetic members of this group acquired a plastid via a secondary endosymbiotic event between a non-photosynthetic bicont-like protist and a red alga. We have studied light-harvesting complexes of Chromera velia, belonging to Alveolata, and Nannochloropsis oceanica and Xanthonema debile, both belonging to Stramenopiles. All three species contain only chlorophyll a and lack chlorophyll c. The latter pigment is characteristic of other photosynthetic algae from the group. In addition to chlorophyll a, the light-harvesting complexes contain various carotenoids.

We have used time-resolved absorption and fluorescence spectroscopy with a nanoseconds resolution to study a triplet-triplet energy transfer from chlorophylls to carotenoids in a protective process preventing formation of reactive singlet oxygen. The composition of the complexes allowed us to study quenching of the chlorophyll a triplet states by different carotenoids in a similar environment. No chlorophyll triplet states could be detected upon excitation of the carotenoids. Carotenoids embedded in the complexes transferred the absorbed singlet excitation energy rapidly to the chlorophylls where an intersystem crossing might occur. In this way formed chlorophyll triplets were quenched by carotenoids with efficiency close to 100%. Carotenoid triplet states observed in the studied complexes were spectrally and kinetically homogeneous, indicating a single chlorophyll triplet quenching site. Chlorophyll triplet states could only be observed after a direct chlorophyll excitation, which included also chlorophylls functionally not connected with the light-harvesting complex and thus not protected by carotenoids.