Survival of phototrophic organisms depends on their ability to collect and convert enough light energy to support their metabolism. Phototrophs can extend their absorption cross section by using diverse pigments and by tuning the properties of these pigments via pigment-pigment and pigment-protein interaction. It is well known that some cyanobacteria can grow in heavily shaded habitats by utilizing far-red light harvested with far-red-absorbing chlorophylls d and f. We describe a red-shifted light-harvesting system based on chlorophyll a from a freshwater eustigmatophyte alga Trachydiscus minutus (Eustigmatophyceae, Goniochloridales). A comprehensive characterization of the photosynthetic apparatus of T. minutus is presented. We show that thylakoid membranes of T. minutus contain light-harvesting complexes of several sizes differing in the relative amount of far-red chlorophyll a forms absorbing around 700 nm. The pigment arrangement of the major red-shifted light-harvesting complex is similar to that of the red-shifted antenna of a marine alveolate alga Chromera velia. Evolutionary aspects of the algal far-red light-harvesting complexes are discussed. The presence of these antennas in eustigmatophyte algae opens up new ways to modify organisms of this promising group for effective use of far-red light in mass cultures.

• Keywords
• Chromatic acclimation, Eustigmatophyta, Light-harvesting protein, Oligomeric LHC, Red-shifted LHC, Violaxanthin,
• MeSH
• Pigments, Biological metabolism   MeSH
• Diuron MeSH
• Spectrometry, Fluorescence MeSH
• Stramenopiles metabolism   radiation effects   MeSH
• Membrane Proteins metabolism   MeSH
• Fresh Water * MeSH
• Light * MeSH
• Light-Harvesting Protein Complexes metabolism   MeSH
• Temperature MeSH
• Thylakoids metabolism   MeSH
• Publication type
• Journal Article MeSH
• Names of Substances
• Pigments, Biological MeSH
• Diuron MeSH
• Membrane Proteins MeSH
• Light-Harvesting Protein Complexes MeSH

The soil chromophyte alga Xanthonema (X.) debile contains only non-carbonyl carotenoids and Chl-a. X. debile has an antenna system denoted Xanthophyte light-harvesting complex (XLH) that contains the carotenoids diadinoxanthin, heteroxanthin, and vaucheriaxanthin. The XLH pigment stoichiometry was calculated by chromatographic techniques and the pigment-binding structure studied by resonance Raman spectroscopy. The pigment ratio obtained by HPLC was found to be close to 8:1:2:1 Chl-a:heteroxanthin:diadinoxanthin:vaucheriaxanthin. The resonance Raman spectra suggest the presence of 8-10 Chl-a, all of which are 5-coordinated to the central Mg, with 1-3 Chl-a possessing a macrocycle distorted from the relaxed conformation. The three populations of carotenoids are in the all-trans configuration. Vaucheriaxanthin absorbs around 500-530 nm, diadinoxanthin at 494 nm and heteroxanthin at 487 nm at 4.5 K. The effective conjugation length of heteroxanthin and diadinoxanthin has been determined as 9.4 in both cases; the environment polarizability of the heteroxanthin and diadinoxanthin binding pockets is 0.270 and 0.305, respectively.

• Keywords
• Algae, Carotenoids, Chl-a, Diadinoxanthin, Heteroxanthin, Light-harvesting complex, Resonance Raman,
• MeSH
• Stramenopiles chemistry   MeSH
• Carotenoids chemistry   MeSH
• Protein Conformation MeSH
• Spectrum Analysis, Raman MeSH
• Light-Harvesting Protein Complexes chemistry   MeSH
• Chromatography, High Pressure Liquid MeSH
• Publication type
• Journal Article MeSH
• Names of Substances
• Carotenoids MeSH
• Light-Harvesting Protein Complexes MeSH

We have used time-resolved absorption and fluorescence spectroscopy with nanosecond resolution to study triplet energy transfer from chlorophylls to carotenoids in a protective process that prevents the formation of reactive singlet oxygen. The light-harvesting complexes studied were isolated from Chromera velia, belonging to a group Alveolata, and Xanthonema debile and Nannochloropsis oceanica, both from Stramenopiles. All three light-harvesting complexes are related to fucoxanthin-chlorophyll protein, but contain only chlorophyll a and no chlorophyll c. In addition, they differ in the carotenoid content. This composition of the complexes allowed us to study the quenching of chlorophyll a triplet states by different carotenoids in a comparable environment. The triplet states of chlorophylls bound to the light-harvesting complexes were quenched by carotenoids with an efficiency close to 100%. Carotenoid triplet states were observed to rise with a ~5 ns lifetime and were spectrally and kinetically homogeneous. The triplet states were formed predominantly on the red-most chlorophylls and were quenched by carotenoids which were further identified or at least spectrally characterized.

• Keywords
• Algae, Energy transfer, Light harvesting, Photoprotection, Photosynthesis, Transient spectroscopy,
• MeSH
• Anaerobiosis MeSH
• Time Factors MeSH
• Chlorophyll metabolism   MeSH
• Spectrometry, Fluorescence MeSH
• Photochemical Processes * MeSH
• Stramenopiles metabolism   MeSH
• Carotenoids metabolism   MeSH
• Kinetics MeSH
• Chlorophyll Binding Proteins metabolism   MeSH
• Light-Harvesting Protein Complexes metabolism   MeSH
• Publication type
• Journal Article MeSH
• Names of Substances
• Chlorophyll MeSH
• Carotenoids MeSH
• Chlorophyll Binding Proteins MeSH
• Light-Harvesting Protein Complexes MeSH