Far-red light-regulated efficient energy transfer from phycobilisomes to photosystem I in the red microalga Galdieria sulphuraria and photosystems-related heterogeneity of phycobilisome population

Cyanidiales as Polyextreme Eukaryotes

Cyanidiales were named enigmatic microalgae due to their unique polyextreme properties, considered for a very long time unattainable for eukaryotes. Cyanidiales mainly inhabit hot sulfuric springs with high acidity (pH 0-4), temperatures up to 56°C, and ability to survive in the presence of dissolved heavy metals. Owing to the minimal for eukaryotes genome size, Cyanidiales have become one of the most important research objects in plant cell physiology, biochemistry, molecular biology, phylogenomics, and evolutionary biology. They play an important role in studying many aspects of oxygenic photosynthesis and chloroplasts origin. The ability to survive in stressful habitats and the corresponding metabolic pathways were acquired by Cyanidiales from archaea and bacteria via horizontal gene transfer (HGT). Thus, the possibility of gene transfer from prokaryotes to eukaryotes was discovered, which was a new step in understanding of the origin of eukaryotic cell.

State 1 and State 2 in Photosynthetic Apparatus of Red Microalgae and Cyanobacteria

Imbalanced light absorption by photosystem I (PSI) and photosystem II (PSII) in oxygenic phototrophs leads to changes in interaction of photosystems altering the linear electron flow. In plants and green algae, this imbalance is mitigated by a partial migration of the chlorophyll a/b containing light-harvesting antenna between the two photosystem core complexes. This migration is registered as fluorescence changes of the pigment apparatus and is termed the reverse transitions between States 1 and 2. By contrast, the molecular mechanism of State 1/2 transitions in phycobilisome (PBS)-containing photosynthetics, cyanobacteria and red algae, is still insufficiently understood. The suggested hypotheses - PBS movement along the surface of thylakoid membrane between PSI and PSII complexes, reversible PBS detachment from the dimeric PSII complex, and spillover - have some limitations as they do not fully explain the accumulated data. Here, we have recorded changes in the stationary fluorescence emission spectra of red algae and cyanobacteria in States 1/2 at room temperature, which allowed us to offer an explanation of the existing contradictions. The change of room temperature fluorescence of chlorophyll belonged to PSII was revealed, while the fluorescence of PBS associated with the PSII complexes remained during States 1/2 transitions at the stable level. Only the reversible dissociation of PBS from the monomeric PSI was revealed earlier which implied different degree of surface contact of PBS with the two photosystems. The detachment of PBS from the PSI corresponds to ferredoxin oxidation as electron carrier and the increase of cyclic electron transport in the pigment apparatus in State I.

Coupled rows of PBS cores and PSII dimers in cyanobacteria: symmetry and structure

Phycobilisome (PBS) is a giant water-soluble photosynthetic antenna transferring the energy of absorbed light mainly to the photosystem II (PSII) in cyanobacteria. Under the low light conditions, PBSs and PSII dimers form coupled rows where each PBS is attached to the cytoplasmic surface of PSII dimer, and PBSs come into contact with their face surfaces (state 1). The model structure of the PBS core that we have developed earlier by comparison and combination of different fine allophycocyanin crystals, as reported in Zlenko et al. (Photosynth Res 130(1):347-356, 2016b), provides a natural way of the PBS core face-to-face stacking. According to our model, the structure of the protein-protein contact between the neighboring PBS cores in the rows is the same as the contact between the APC hexamers inside the PBS core. As a result, the rates of energy transfer between the cores can occur, and the row of PBS cores acts as an integral PBS "supercore" providing energy transfer between the individual PBS cores. The PBS cores row pitch in our elaborated model (12.4 nm) is very close to the PSII dimers row pitch obtained by the electron microscopy (12.2 nm) that allowed to unite a model of the PBS cores row with a model of the PSII dimers row. Analyzing the resulting model, we have determined the most probable locations of ApcD and ApcE terminal emitter subunits inside the bottom PBS core cylinders and also revealed the chlorophyll molecules of PSII gathering energy from the PBS.

Structural modeling of the phycobilisome core and its association with the photosystems

The phycobilisome (PBS) is a major light-harvesting complex in cyanobacteria and red algae. To obtain the detailed structure of the hemidiscoidal PBS core composed of allophycocyanin (APC) and minor polypeptide components, we analyzed all nine available 3D structures of APCs from different photosynthetic species and found several variants of crystal packing that potentially correspond to PBS core organization. Combination of face-to-face APC trimer crystal packing with back-to-back APC hexamer packing suggests two variants of the tricylindrical PBS core. To choose one of these structures, a computational model of the PBS core complex and photosystem II (PSII) dimer with minimized distance between the terminal PBS emitters and neighboring antenna chlorophylls was built. In the selected model, the distance between two types of pigments does not exceed 37 Å corresponding to the Förster mechanism of energy transfer. We also propose a model of PBS and photosystem I (PSI) monomer interaction showing a possibility of supercomplex formation and direct energy transfer from the PBS to PSI.

Electronic coupling of the phycobilisome with the orange carotenoid protein and fluorescence quenching

Using computational modeling and known 3D structure of proteins, we arrived at a rational spatial model of the orange carotenoid protein (OCP) and phycobilisome (PBS) interaction in the non-photochemical fluorescence quenching. The site of interaction is formed by the central cavity of the OCP monomer in the capacity of a keyhole to the characteristic external tip of the phycobilin-containing domain (PB) and folded loop of the core-membrane linker LCM within the PBS core. The same central protein cavity was shown to be also the site of the OCP and fluorescence recovery protein (FRP) interaction. The revealed geometry of the OCP to the PBLCM attachment is believed to be the most advantageous one as the LCM, being the major terminal PBS fluorescence emitter, gathers, before quenching by OCP, the energy from most other phycobilin chromophores of the PBS. The distance between centers of mass of the OCP carotenoid 3'-hydroxyechinenone (hECN) and the adjacent phycobilin chromophore of the PBLCM was determined to be 24.7 Å. Under the dipole-dipole approximation, from the point of view of the determined mutual orientation and the values of the transition dipole moments and spectral characteristics of interacting chromophores, the time of the direct energy transfer from the phycobilin of PBLCM to the S1 excited state of hECN was semiempirically calculated to be 36 ps, which corresponds to the known experimental data and implies the OCP is a very efficient energy quencher. The complete scheme of OCP and PBS interaction that includes participation of the FRP is proposed.

Light emitting diodes (LEDs) applied to microalgal production

Light-emitting diodes (LEDs) will become one of the world's most important light sources and their integration in microalgal production systems (photobioreactors) needs to be considered. LEDs can improve the quality and quantity of microalgal biomass when applied during specific growth phases. However, microalgae need a balanced mix of wavelengths for normal growth, and respond to light differently according to the pigments acquired or lost during their evolutionary history. This review highlights recently published results on the effect of LEDs on microalgal physiology and biochemistry and how this knowledge can be applied in selecting different LEDs with specific technical properties for regulating biomass production by microalgae belonging to diverse taxonomic groups.