Photosynthetic organisms harvest light for energy. Some eukaryotic algae have specialized in harvesting far-red light by tuning chlorophyll a absorption through a mechanism still to be elucidated. Here, we combined optically detected magnetic resonance and pulsed electron paramagnetic resonance measurements on red-adapted light-harvesting complexes, rVCP, isolated from the freshwater eustigmatophyte alga Trachydiscus minutus to identify the location of the pigments responsible for this remarkable adaptation. The pigments have been found to belong to an excitonic cluster of chlorophylls a at the core of the complex, close to the central carotenoids in L1/L2 sites. A pair of structural features of the Chl a403/a603 binding site, namely the histidine-to-asparagine substitution in the magnesium-ligation residue and the small size of the amino acid at the i-4 position, resulting in a [A/G]xxxN motif, are proposed to be the origin of this trait. Phylogenetic analysis of various eukaryotic red antennae identified several potential LHCs that could share this tuning mechanism. This knowledge of the red light acclimation mechanism in algae is a step towards rational design of algal strains in order to enhance light capture and efficiency in large-scale biotechnology applications.

• MeSH
• Chlorophyll A * metabolism   chemistry   MeSH
• Chlorophyll metabolism   MeSH
• Electron Spin Resonance Spectroscopy MeSH
• Phylogeny MeSH
• Light MeSH
• Light-Harvesting Protein Complexes * metabolism   genetics   chemistry   MeSH
• Publication type
• Journal Article MeSH
• Names of Substances
• Chlorophyll A * MeSH
• Chlorophyll MeSH
• Light-Harvesting Protein Complexes * MeSH

Most organisms on Earth are affected by periodic changes in their environment. The circadian clock is an endogenous device that synchronizes behavior, physiology, or biochemical processes to an approximately 24-hour cycle, allowing organisms to anticipate the periodic changes of day and night. Although circadian clocks are widespread in organisms, the actual molecular components differ remarkably among the clocks of plants, animals, fungi, and prokaryotes. Chromera velia is the closest known photosynthetic relative of apicomplexan parasites. Formation of its motile stage, zoospores, has been described as associated with the light part of the day. We examined the effects on the periodic release of the zoospores under different light conditions and investigated the influence of the spectral composition on zoosporogenesis. We performed a genomic search for homologs of known circadian clock genes. Our results demonstrate the presence of an almost 24-hour free-running cycle of zoosporogenesis. We also identified the blue light spectra as the essential compound for zoosporogenesis. Further, we developed a new and effective method for zoospore separation from the culture and estimated the average motility speed and lifespan of the C. velia zoospores. Our genomic search identified six cryptochrome-like genes, two genes possibly related to Arabidopsis thaliana CCA/LHY, whereas no homolog of an animal, cyanobacterial, or fungal circadian clock gene was found. Our results suggest that C. velia has a functional circadian clock, probably based mainly on a yet undefined mechanism.

• Keywords
• Chromera velia, apicomplexa, circadian clock, cryptochrome, zoospore formation,
• Publication type
• Journal Article MeSH

Antenna proteins play a major role in the regulation of light-harvesting in photosynthesis. However, less is known about a possible link between their sizes (oligomerization state) and fluorescence intensity (number of photons emitted). Here, we used a microscopy-based method, Fluorescence Correlation Spectroscopy (FCS), to analyze different antenna proteins at the particle level. The direct comparison indicated that Chromera Light Harvesting (CLH) antenna particles (isolated from Chromera velia) behaved as the monomeric Light Harvesting Complex II (LHCII) (from higher plants), in terms of their radius (based on the diffusion time) and fluorescence yields. FCS data thus indicated a monomeric oligomerization state of algal CLH antenna (at our experimental conditions) that was later confirmed also by biochemical experiments. Additionally, our data provide a proof of concept that the FCS method is well suited to measure proteins sizes (oligomerization state) and fluorescence intensities (photon counts) of antenna proteins per single particle (monomers and oligomers). We proved that antenna monomers (CLH and LHCIIm) are more "quenched" than the corresponding trimers. The FCS measurement thus represents a useful experimental approach that allows studying the role of antenna oligomerization in the mechanism of photoprotection.

• Keywords
• Chromera velia, antenna proteins, fluorescence correlation spectroscopy, light-harvesting, microscopy, photosynthesis, protein diffusion, protein oligomerization,
• MeSH
• Algal Proteins chemistry   metabolism   MeSH
• Fluorescence * MeSH
• Spectrometry, Fluorescence MeSH
• Photosynthesis * MeSH
• Kinetics MeSH
• Protein Multimerization MeSH
• Protein Transport MeSH
• Publication type
• Journal Article MeSH
• Names of Substances
• Algal Proteins MeSH

Oxygenic photosynthesis takes place in thylakoid membranes (TM) of cyanobacteria, algae, and higher plants. It begins with light absorption by pigments in large (modular) assemblies of pigment-binding proteins, which then transfer excitation energy to the photosynthetic reaction centers of photosystem (PS) I and PSII. In green algae and plants, these light-harvesting protein complexes contain chlorophylls (Chls) and carotenoids (Cars). However, cyanobacteria, red algae, and glaucophytes contain, in addition, phycobiliproteins in phycobilisomes that are attached to the stromal surface of TM, and transfer excitation energy to the reaction centers via the Chl a molecules in the inner antennas of PSI and PSII. The color and the intensity of the light to which these photosynthetic organisms are exposed in their environment have a great influence on the composition and the structure of the light-harvesting complexes (the antenna) as well as the rest of the photosynthetic apparatus, thus affecting the photosynthetic process and even the entire organism. We present here a perspective on 'Light Quality and Oxygenic Photosynthesis', in memory of George Christos Papageorgiou (9 May 1933-21 November 2020; see notes a and b). Our review includes (1) the influence of the solar spectrum on the antenna composition, and the special significance of Chl a; (2) the effects of light quality on photosynthesis, measured using Chl a fluorescence; and (3) the importance of light quality, intensity, and its duration for the optimal growth of photosynthetic organisms.

• Keywords
• Chl fluorescence induction, chromatic acclimation of cyanobacteria, photoreceptors, photosynthetic pigments, photosystems I and II, stomatal and chloroplast photoinduced movements,
• Publication type
• Journal Article MeSH
• Review MeSH

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

Heavy metal pollution is an increasing global concern. Among heavy metals, mercury (Hg) is especially dangerous because of its massive release into the environment and high toxicity, especially for aquatic organisms. The molecular response mechanisms of algae to Hg exposure are mostly unknown. Here, we combine physiological, biochemical, and transcriptomic analysis to provide, for the first time, a comprehensive view on the pathways activated in Chromera velia in response to toxic levels of Hg. Production of hydrogen peroxide and superoxide anion, two reactive oxygen species (ROS), showed opposite patterns in response to Hg2+ while reactive nitrogen species (RNS) levels did not change. A deep RNA sequencing analysis generated a total of 307,738,790 high-quality reads assembled in 122,874 transcripts, representing 89,853 unigenes successfully annotated in databases. Detailed analysis of the differently expressed genes corroborates the biochemical results observed in ROS production and suggests novel putative molecular mechanisms in the algal response to Hg2+. Moreover, we indicated that important transcription factor (TF) families associated with stress responses differentially expressed in C. velia cultures under Hg stress. Our study presents the first in-depth transcriptomic analysis of C. velia, focusing on the expression of genes involved in different detoxification defense systems in response to heavy metal stress.

• Keywords
• antioxidant enzymes, chromerids, heavy metal, phylogenies, reactive nitrogen species, reactive oxygen species, transcriptome, xenobiotics,
• MeSH
• Alveolata drug effects   genetics   growth & development   metabolism   MeSH
• Water Pollutants, Chemical toxicity   MeSH
• Chlorophyll metabolism   MeSH
• Hydrogen Peroxide metabolism   MeSH
• Reactive Nitrogen Species metabolism   MeSH
• Reactive Oxygen Species metabolism   MeSH
• Mercury toxicity   MeSH
• Transcriptome drug effects   MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Names of Substances
• Water Pollutants, Chemical MeSH
• Chlorophyll MeSH
• Hydrogen Peroxide MeSH
• Reactive Nitrogen Species MeSH
• Reactive Oxygen Species MeSH
• Mercury MeSH

Chromera velia is a marine photosynthetic relative of human apicomplexan parasites. It has been isolated from coral reefs and is indicted for being involved in symbioses with hermatypic corals. C. velia has been subject to intensive research, but still very little is known of its response to light quality and quantity. Here, we have studied the growth and compositional responses of C. velia to culture under monochromatic light (blue, green or red), at two photon flux densities (PFD, 20 and 100 μmol photons m-2 s-1). Our results show that C. velia growth rate is unaffected by the quality of light, whereas it responds to PFD. However, light quality influenced cell size, which was smaller for cells exposed to blue monochromatic light, regardless of PFD. PFD strongly influenced carbon allocation: at 20 μmol photons m-2 s-1, carbon was mainly allocated into proteins while at 100 μmol photons m-2 s-1, carbon was allocated mainly into carbohydrate and lipid pools. The blue light treatment caused a decrease in the lipids and carbohydrates to proteins and thus suggested to affect nitrogen metabolism in acclimated cells. Whole-cell absorption spectra revealed the existence of red-shifted chlorophyll a antenna not only under red light but in all low PFD treatments. These findings show the ability of C. velia to successfully adapt and thrive in spectrally very different environments of coral reefs.

• MeSH
• Alveolata growth & development   metabolism   radiation effects   MeSH
• Chlorophyll A metabolism   MeSH
• Photosynthesis radiation effects   MeSH
• Light MeSH
• Carbon metabolism   MeSH
• Publication type
• Journal Article MeSH
• Names of Substances
• Chlorophyll A MeSH
• Carbon MeSH

It has previously been shown that the long-term treatment of Arabidopsis thaliana with the chloroplast inhibitor lincomycin leads to photosynthetic membranes enriched in antennas, strongly reduced in photosystem II reaction centers (PSII) and with enhanced nonphotochemical quenching (NPQ) (Belgio et al. Biophys J 102:2761-2771, 2012). Here, a similar physiological response was found in the microalga Chromera velia grown under high light (HL). In comparison to cells acclimated to low light, HL cells displayed a severe re-organization of the photosynthetic membrane characterized by (1) a reduction of PSII but similar antenna content; (2) partial uncoupling of antennas from PSII; (3) enhanced NPQ. The decrease in the number of PSII represents a rather unusual acclimation response compared to other phototrophs, where a smaller PSII antenna size is more commonly found under high light. Despite the diminished PSII content, no net damage could be detected on the basis of the Photosynthesis versus irradiance curve and electron transport rates pointing at the excess capacity of PSII. We therefore concluded that the photoinhibition is minimized under high light by a lower PSII content and that cells are protected by NPQ in the antennas.

• Keywords
• Chromera velia alga, High light acclimation, Nonphotochemical quenching, Photoinhibition, Uncoupling of antennas from Photosystem II.,
• MeSH
• Acclimatization radiation effects   MeSH
• Alveolata cytology   physiology   radiation effects   MeSH
• Chlorophyll A MeSH
• Chlorophyll metabolism   MeSH
• Fluorescence MeSH
• Photochemical Processes radiation effects   MeSH
• Photosynthesis radiation effects   MeSH
• Photosystem II Protein Complex metabolism   MeSH
• Solubility MeSH
• Light * MeSH
• Light-Harvesting Protein Complexes metabolism   MeSH
• Publication type
• Journal Article MeSH
• Names of Substances
• Chlorophyll A MeSH
• Chlorophyll MeSH
• Photosystem II Protein Complex MeSH
• Light-Harvesting Protein Complexes MeSH

In oxygenic photosynthesis the initial photochemical processes are carried out by photosystem I (PSI) and II (PSII). Although subunit composition varies between cyanobacterial and plastid photosystems, the core structures of PSI and PSII are conserved throughout photosynthetic eukaryotes. So far, the photosynthetic complexes have been characterised in only a small number of organisms. We performed in silico and biochemical studies to explore the organization and evolution of the photosynthetic apparatus in the chromerids Chromera velia and Vitrella brassicaformis, autotrophic relatives of apicomplexans. We catalogued the presence and location of genes coding for conserved subunits of the photosystems as well as cytochrome b6f and ATP synthase in chromerids and other phototrophs and performed a phylogenetic analysis. We then characterised the photosynthetic complexes of Chromera and Vitrella using 2D gels combined with mass-spectrometry and further analysed the purified Chromera PSI. Our data suggest that the photosynthetic apparatus of chromerids underwent unique structural changes. Both photosystems (as well as cytochrome b6f and ATP synthase) lost several canonical subunits, while PSI gained one superoxide dismutase (Vitrella) or two superoxide dismutases and several unknown proteins (Chromera) as new regular subunits. We discuss these results in light of the extraordinarily efficient photosynthetic processes described in Chromera.

• MeSH
• Alveolata genetics   physiology   MeSH
• Gene Deletion MeSH
• Photosynthesis genetics   physiology   MeSH
• Photosystem I Protein Complex genetics   isolation & purification   physiology   MeSH
• Phylogeny MeSH
• Mass Spectrometry MeSH
• Evolution, Molecular MeSH
• Superoxide Dismutase metabolism   MeSH
• Thylakoids metabolism   MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Names of Substances
• Photosystem I Protein Complex MeSH
• Superoxide Dismutase MeSH

Diatoms greatly contribute to carbon fixation and thus strongly influence the global biogeochemical balance. Capable of chromatic acclimation (CA) to unfavourable light conditions, diatoms often dominate benthic ecosystems in addition to their planktonic lifestyle. Although CA has been studied at the molecular level, our understanding of this phenomenon remains incomplete. Here we provide new data to better explain the acclimation-associated changes under red-enhanced ambient light (RL) in diatom Phaeodactylum tricornutum, known to express a red-shifted antenna complex (F710). The complex was found to be an oligomer of a single polypeptide, Lhcf15. The steady-state spectroscopic properties of the oligomer were also studied. The oligomeric assembly of the Lhcf15 subunits is required for the complex to exhibit a red-shifted absorption. The presence of the red antenna in RL culture coincides with the development of a rounded phenotype of the diatom cell. A model summarizing the modulation of the photosynthetic apparatus during the acclimation response to light of different spectral quality is proposed. Our study suggests that toggling between alternative organizations of photosynthetic apparatus and distinct cell morphologies underlies the remarkable acclimation capacity of diatoms.

• MeSH
• Phenotype * MeSH
• Adaptation, Physiological MeSH
• Protein Multimerization MeSH
• Diatoms physiology   radiation effects   MeSH
• Spectrum Analysis MeSH
• Light-Harvesting Protein Complexes metabolism   MeSH
• Aquatic Organisms physiology   radiation effects   MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Names of Substances
• Light-Harvesting Protein Complexes MeSH