A novel type of light-harvesting antenna protein of red algal origin in algae with secondary plastids

The structure of PSI-LHCI from Cyanidium caldarium provides evolutionary insights into conservation and diversity of red-lineage LHCs

Light-harvesting complexes (LHCs) are diversified among photosynthetic organisms, and the structure of the photosystem I-LHC (PSI-LHCI) supercomplex has been shown to be variable depending on the species of organisms. However, the structural and evolutionary correlations of red-lineage LHCs are unknown. Here, we determined a 1.92-Å resolution cryoelectron microscopic structure of a PSI-LHCI supercomplex isolated from the red alga Cyanidium caldarium RK-1 (NIES-2137), which is an important taxon in the Cyanidiophyceae. We subsequently investigated the correlations of PSI-LHCIs from different organisms through structural comparisons and phylogenetic analysis. The PSI-LHCI structure obtained shows five LHCI subunits surrounding a PSI-monomer core. The five LHCIs are composed of two Lhcr1s, two Lhcr2s, and one Lhcr3. Phylogenetic analysis of LHCs bound to PSI in the red-lineage algae showed clear orthology of LHCs between C. caldarium and Cyanidioschyzon merolae, whereas no orthologous relationships were found between C. caldarium Lhcr1-3 and LHCs in other red-lineage PSI-LHCI structures. These findings provide evolutionary insights into conservation and diversity of red-lineage LHCs associated with PSI.

Consequences of light spectra for pigment composition and gene expression in the cryptophyte Rhodomonas salina

Algae with a more diverse suite of pigments can, in principle, exploit a broader swath of the light spectrum through chromatic acclimation, the ability to maximize light capture via plasticity of pigment composition. We grew Rhodomonas salina in wide-spectrum, red, green, and blue environments and measured how pigment composition differed. We also measured expression of key light-capture and photosynthesis-related genes and performed a transcriptome-wide expression analysis. We observed the highest concentration of phycoerythrin in green light, consistent with chromatic acclimation. Other pigments showed trends inconsistent with chromatic acclimation, possibly due to feedback loops among pigments or high-energy light acclimation. Expression of some photosynthesis-related genes was sensitive to spectrum, although expression of most was not. The phycoerythrin α-subunit was expressed two-orders of magnitude greater than the β-subunit even though the peptides are needed in an equimolar ratio. Expression of genes related to chlorophyll-binding and phycoerythrin concentration were correlated, indicating a potential synthesis relationship. Pigment concentrations and expression of related genes were generally uncorrelated, implying post-transcriptional regulation of pigments. Overall, most differentially expressed genes were not related to photosynthesis; thus, examining associations between light spectrum and other organismal functions, including sexual reproduction and glycolysis, may be important.

In situ structure of the red algal phycobilisome-PSII-PSI-LHC megacomplex

In oxygenic photosynthetic organisms, light energy is captured by antenna systems and transferred to photosystem II (PSII) and photosystem I (PSI) to drive photosynthesis1,2. The antenna systems of red algae consist of soluble phycobilisomes (PBSs) and transmembrane light-harvesting complexes (LHCs)3. Excitation energy transfer pathways from PBS to photosystems remain unclear owing to the lack of structural information. Here we present in situ structures of PBS-PSII-PSI-LHC megacomplexes from the red alga Porphyridium purpureum at near-atomic resolution using cryogenic electron tomography and in situ single-particle analysis4, providing interaction details between PBS, PSII and PSI. The structures reveal several unidentified and incomplete proteins and their roles in the assembly of the megacomplex, as well as a huge and sophisticated pigment network. This work provides a solid structural basis for unravelling the mechanisms of PBS-PSII-PSI-LHC megacomplex assembly, efficient energy transfer from PBS to the two photosystems, and regulation of energy distribution between PSII and PSI.

Molecular phylogeny of fucoxanthin-chlorophyll a/c proteins from Chaetoceros gracilis and Lhcq/Lhcf diversity

Diatoms adapt to various aquatic light environments and play major roles in the global carbon cycle using their unique light-harvesting system, i.e. fucoxanthin chlorophyll a/c binding proteins (FCPs). Structural analyses of photosystem II (PSII)-FCPII and photosystem I (PSI)-FCPI complexes from the diatom Chaetoceros gracilis have revealed the localization and interactions of many FCPs; however, the entire set of FCPs has not been characterized. Here, we identify 46 FCPs in the newly assembled genome and transcriptome of C. gracilis. Phylogenetic analyses suggest that these FCPs can be classified into five subfamilies: Lhcr, Lhcf, Lhcx, Lhcz, and the novel Lhcq, in addition to a distinct type of Lhcr, CgLhcr9. The FCPs in Lhcr, including CgLhcr9 and some Lhcqs, have orthologous proteins in other diatoms, particularly those found in the PSI-FCPI structure. By contrast, the Lhcf subfamily, some of which were found in the PSII-FCPII complex, seems to be diversified in each diatom species, and the number of Lhcqs differs among species, indicating that their diversification may contribute to species-specific adaptations to light. Further phylogenetic analyses of FCPs/light-harvesting complex (LHC) proteins using genome data and assembled transcriptomes of other diatoms and microalgae in public databases suggest that our proposed classification of FCPs is common among various red-lineage algae derived from secondary endosymbiosis of red algae, including Haptophyta. These results provide insights into the loss and gain of FCP/LHC subfamilies during the evolutionary history of the red algal lineage.

Structural Diversity of Photosystem I and Its Light-Harvesting System in Eukaryotic Algae and Plants

Photosystem I (PSI) is one of the most efficient photoelectric apparatus in nature, converting solar energy into condensed chemical energy with almost 100% quantum efficiency. The ability of PSI to attain such high conversion efficiency depends on the precise spatial arrangement of its protein subunits and binding cofactors. The PSI structures of oxygenic photosynthetic organisms, namely cyanobacteria, eukaryotic algae, and plants, have undergone great variation during their evolution, especially in eukaryotic algae and vascular plants for which light-harvesting complexes (LHCI) developed that surround the PSI core complex. A detailed understanding of the functional and structural properties of this PSI-LHCI is not only an important foundation for understanding the evolution of photosynthetic organisms but is also useful for designing future artificial photochemical devices. Recently, the structures of such PSI-LHCI supercomplexes from red alga, green alga, diatoms, and plants were determined by X-ray crystallography and single-particle cryo-electron microscopy (cryo-EM). These findings provide new insights into the various structural adjustments of PSI, especially with respect to the diversity of peripheral antenna systems arising via evolutionary processes. Here, we review the structural details of the PSI tetramer in cyanobacteria and the PSI-LHCI and PSI-LHCI-LHCII supercomplexes from different algae and plants, and then discuss the diversity of PSI-LHCI in oxygenic photosynthesis organisms.

Comparative Transcriptome Profiling of Kappaphycus alvarezii (Rhodophyta, Solieriaceae) in Response to Light of Different Wavelengths and Carbon Dioxide Enrichment

Rhodophyta (red algae) comprises over 6000 species, however, there have only been a few comparative transcriptomic studies due to their under-representation in genomic databases. Kappaphycus alvarezii, a Gigartinales algae, is a valuable source of carrageenan and is extensively cultivated in many countries. The majority of seaweed farming in Southeast Asia is done in intertidal zones under varying light (i.e., spectra and irradiance) and carbon dioxide (CO₂) conditions, which affects the rate of photosynthesis. This study conducted transcriptome profiling to investigate the photosynthetic mechanisms in K. alvarezii exposed to different wavelengths of light (i.e., blue, green, and red light, in comparison to white light) and CO₂ availability. We analyzed the responses of photosynthetic protein complexes to light and observed that light of different wavelengths regulates a similar set of photosynthetic apparatuses. Under CO₂ enrichment, genes encoding C₃ and C₄ enzymes were found to be actively transcribed, suggesting the likely shift in the carbon metabolism pathway or the involvement of these genes in adaptive physiological processes. This study contributes to the understanding of the regulatory mechanisms of photosynthetic carbon metabolism in red algae and has implications for the culture and commercial production of these economically valuable macroalgae.

Co-suppression in Pyropia yezoensis (Rhodophyta) Reveals the Role of PyLHCI in Light Harvesting and Generation Switch

The red macroalga Pyropia yezoensis is an economically important seaweed widely cultured in Asian countries and is a model organism for molecular biological and commercial research. This species is unique in that it utilizes both phycobilisomes and transmembrane light-harvesting proteins as its antenna system. Here, one of the genes of P. yezoensis (PyLHCI) was selected for introduction into its genome to overexpress PyLHCI. However, the co-suppression phenomenon occurred. This is the first documentation of co-suppression in algae, in which it exhibits a different mechanism from that in higher plants. The transformant (T1) was demonstrated to have higher phycobilisomes and lower LHC binding pigments, resulting in a redder color, higher sensitivity to salt stress, smaller in size, and slower growth rate than the wildtype (WT). The photosynthetic performances of T1 and WT showed similar characteristics; however, P700 reduction was slower in T1. Most importantly, T1 could release a high percentage of carpospores in young blades to switch generation during its life cycle, which was rarely seen in WT. The co-suppression of PyLHCI revealed its key roles in light harvesting, stress resistance, and generation alternation (generation switch from gametophytes to sporophytes, and reproduction from asexual to sexual).

Light harvesting complexes in chlorophyll c-containing algae

Besides the so-called 'green lineage' of eukaryotic photosynthetic organisms that include vascular plants, a huge variety of different algal groups exist that also harvest light by means of membrane intrinsic light harvesting proteins (Lhc). The main taxa of these algae are the Cryptophytes, Haptophytes, Dinophytes, Chromeridae and the Heterokonts, the latter including diatoms, brown algae, Xanthophyceae and Eustigmatophyceae amongst others. Despite the similarity in Lhc proteins between vascular plants and these algae, pigmentation is significantly different since no Chl b is bound, but often replaced by Chl c, and a large diversity in carotenoids functioning in light harvesting and/or photoprotection is present. Due to the presence of Chl c in most of the taxa the name 'Chl c-containing organisms' has become common, however, Chl b-less is more precise since some harbour Lhc proteins that only bind one type of Chl, Chl a. In recent years huge progress has been made about the occurrence and function of Lhc in diatoms, so-called fucoxanthin chlorophyll proteins (FCP), where also the first molecular structure became available recently. In addition, especially energy transfer amongst the unusual pigments bound was intensively studied in many of these groups. This review summarises the present knowledge about the molecular structure, the arrangement of the different Lhc in complexes, the excitation energy transfer abilities and the involvement in photoprotection of the different Lhc systems in the so-called Chl c-containing organisms. This article is part of a Special Issue entitled Light harvesting, edited by Dr. Roberta Croce.

Evidence of the supercomplex organization of photosystem II and light-harvesting complexes in Nannochloropsis granulata

Diverse light-harvesting complexes (LHCs) have been found in photosynthetic microalgae that originated from secondary endosymbiosis involving primary red algae. However, the associations between LHCs and photosystem I (PSI) and photosystem II (PSII) in these microalgae are not fully understood. Eustigmatophyta is a red algal lineage that appears to have a unique organization in its photosynthetic machinery, consisting of only chlorophyll a and carotenoids that are atypical compared with other closely related groups. In this study, the supramolecular organization of pigment-protein complexes in the eustigmatophyte alga, Nannochloropsis granulata was investigated using Clear Native (CN) PAGE coupled with two-dimensional (2D) SDS-PAGE. Our results showed two slowly migrating green bands that corresponded to PSII supercomplexes, which consisted of reaction centers and LHCs. These green bands were also characterized as PSII complexes by their low temperature fluorescence emission spectra. The protein subunits of the PSII-LHC resolved by 2D CN/SDS-PAGE were analyzed by mass spectrometry, and four different LHC proteins were identified. Phylogenetic analysis of the identified LHC protein sequences revealed that they belonged to four different Lhc groups; (1) stress-related Lhcx proteins, (2) fucoxanthin chlorophyll a/c-binding Lhcf proteins, (3) red-shifted Chromera light-harvesting proteins (Red-CLH), and (4) Lhcr proteins, which are commonly found in organisms possessing red algal plastids. This is the first report showing evidence of a pigment-protein supercomplex consisting of PSII and LHCs, and to identify PSII-associated LHC proteins in Nannochloropsis.

The Influence of a Cryptochrome on the Gene Expression Profile in the Diatom Phaeodactylum tricornutum under Blue Light and in Darkness

Diatoms, albeit being only distantly related with higher plants, harbor a plant-like cryptochrome (CryP) that was proposed to act as a photoreceptor required for the regulation of some photosynthetic proteins. Plant cryptochromes are involved in the regulation of developmental processes relevant only to multicellular organisms. Their role in the unicellular diatoms to date is mostly enigmatic. To elucidate the function of this plant-like cryptochrome in a unicellular species, we examined the role of CryP in the regulation of transcription in the diatom Phaeodactylum tricornutum by comparative RNA-seq of wild type and CryP knock-down mutants, under prolonged darkness and one hour after onset of blue light. In total, mRNAs of 12,298 genes were identified and more than 70% of the genes could be sorted into functional bins. CryP influenced groups of transcripts in three different ways: some transcripts displayed altered expression under blue light only, others independent of the light condition, and, surprisingly, some were influenced by CryP only in darkness. Genes regulated in any condition were distributed over almost all functional categories. CryP exerted an influence on two other photoreceptors: the genes encoding phytochrome and CPF1, another cryptochrome, which were down-regulated by CryP independent of the light condition. However, the regulatory responses of the affected photoreceptors on transcriptional output were independent. The influence of CryP on the expression of other photoreceptors hints to the existence of a regulatory signaling network in diatoms that includes several cryptochromes and phytochrome, whereby CryP acts as a regulator of transcript abundance under light as well as in darkness.

Insights into the red algae and eukaryotic evolution from the genome of Porphyra umbilicalis (Bangiophyceae, Rhodophyta)

Porphyra umbilicalis (laver) belongs to an ancient group of red algae (Bangiophyceae), is harvested for human food, and thrives in the harsh conditions of the upper intertidal zone. Here we present the 87.7-Mbp haploid Porphyra genome (65.8% G + C content, 13,125 gene loci) and elucidate traits that inform our understanding of the biology of red algae as one of the few multicellular eukaryotic lineages. Novel features of the Porphyra genome shared by other red algae relate to the cytoskeleton, calcium signaling, the cell cycle, and stress-tolerance mechanisms including photoprotection. Cytoskeletal motor proteins in Porphyra are restricted to a small set of kinesins that appear to be the only universal cytoskeletal motors within the red algae. Dynein motors are absent, and most red algae, including Porphyra, lack myosin. This surprisingly minimal cytoskeleton offers a potential explanation for why red algal cells and multicellular structures are more limited in size than in most multicellular lineages. Additional discoveries further relating to the stress tolerance of bangiophytes include ancestral enzymes for sulfation of the hydrophilic galactan-rich cell wall, evidence for mannan synthesis that originated before the divergence of green and red algae, and a high capacity for nutrient uptake. Our analyses provide a comprehensive understanding of the red algae, which are both commercially important and have played a major role in the evolution of other algal groups through secondary endosymbioses.

Architecture of the light-harvesting apparatus of the eustigmatophyte alga Nannochloropsis oceanica

We present proteomic, spectroscopic, and phylogenetic analysis of light-harvesting protein (Lhc) function in oleaginous Nannochloropsis oceanica (Eustigmatophyta, Stramenopila). N. oceanica utilizes Lhcs of multiple classes: Lhcr-type proteins (related to red algae LHCI), Lhcv (VCP) proteins (violaxanthin-containing Lhcs related to Lhcf/FCP proteins of diatoms), Lhcx proteins (related to Lhcx/LhcSR of diatoms and green algae), and Lhc proteins related to Red-CLH of Chromera velia. Altogether, 17 Lhc-type proteins of the 21 known from genomic data were found in our proteomic analyses. Besides Lhcr-type antennas, a RedCAP protein and a member of the Lhcx protein subfamily were found in association with Photosystem I. The free antenna fraction is formed by trimers of a mixture of Lhcs of varied origins (Lhcv, Lhcr, Lhcx, and relatives of Red-CLH). Despite possessing several proteins of the Red-CLH-type Lhc clade, N. oceanica is not capable of chromatic adaptation under the same conditions as the diatom Phaeodactylum tricornutum or C. velia. In addition, a naming scheme of Nannochloropsis Lhcs is proposed to facilitate further work.

Photosynthetic adaptation strategy of Ulva prolifera floating on the sea surface to environmental changes

For 8 consecutive years, a green tide has originated in the southern Yellow Sea and spread to the Qingdao offshore area. The causative species, Ulva prolifera, always forms a very thick thallus mat that is capable of drifting long distances over long periods. During this process, although the thalli face disturbance by complex environmental factors, they maintain high biomass and proliferation. We hypothesized that some form of photosynthetic adaptation strategy must exist to protect the thalli. Therefore, we studied the different photosynthetic response characteristics of the surface and lower layers of the floating thallus mats, and investigated the physiological and molecular-level adaptation mechanisms. The results showed that: (1) U. prolifera has strong photosynthetic capability that ensures it can gain sufficient energy to increase its biomass and adapt to long-distance migration. (2) Surface layer thalli adapt to the complex environment by dissipating excess energy via photosynthetic quantum control (energy quenching and energy redistribution between PSII/PSI) to avoid irreversible damage to the photosynthetic system. (3) Lower layer thalli increase their contents of Chlorophyll a (Chl a) and Chlorophyll b (Chl b) and decrease their Chl a/Chl b ratio to improve their ability to use light energy. (4) U. prolifera has strong photosynthetic plasticity and can adapt to frequent exchange between the surface and lower layer environments because of wave disturbance. Pigment component changes, energy quenching, and energy redistribution between PSII/PSI contribute to this photosynthetic plasticity.

Rapid induction of GFP expression by the nitrate reductase promoter in the diatom Phaeodactylum tricornutum

An essential prerequisite for a controlled transgene expression is the choice of a suitable promoter. In the model diatom Phaeodactylum tricornutum, the most commonly used promoters for trans-gene expression are the light dependent lhcf1 promoters (derived from two endogenous genes encoding fucoxanthin chlorophyll a/c binding proteins) and the nitrate dependent nr promoter (derived from the endogenous nitrate reductase gene). In this study, we investigated the time dependent expression of the green fluorescent protein (GFP) reporter under control of the nitrate reductase promoter in independently genetically transformed P. tricornutum cell lines following induction of expression by change of the nitrogen source in the medium via flow cytometry, microscopy and western blotting. In all investigated cell lines, GFP fluorescence started to increase 1 h after change of the medium, the fastest increase rates were observed between 2 and 3 h. Fluorescence continued to increase slightly for up to 7 h even after transfer of the cells to ammonium medium. The subsequent decrease of GFP fluorescence was much slower than the increase, probably due to the stability of GFP. The investigation of several cell lines transformed with nr based constructs revealed that, also in the absence of nitrate, the promoter may show residual activity. Furthermore, we observed a strong variation of gene expression between independent cell lines, emphasising the importance of a thorough characterisation of genetically modified cell lines and their individual expression patterns.

Allosteric communication between DNA-binding and light-responsive domains of diatom class I aureochromes

The modular architecture of aureochrome blue light receptors, found in several algal groups including diatoms, is unique by having the LOV-type photoreceptor domain fused to the C-terminus of its putative effector, an N-terminal DNA-binding bZIP module. The structural and functional understanding of aureochromes’ light-dependent signaling mechanism is limited, despite their promise as an optogenetic tool. We show that class I aureochromes 1a and 1c from the diatom Phaeodactylum tricornutum are regulated in a light-independent circadian rhythm. These aureochromes are capable to form functional homo- and heterodimers, which recognize the ACGT core sequence within the canonical ‘aureo box’, TGACGT, in a light-independent manner. The bZIP domain holds a more folded and less flexible but extended conformation in the duplex DNA-bound state. FT-IR spectroscopy in the absence and the presence of DNA shows light-dependent helix unfolding in the LOV domain, which leads to conformational changes in the bZIP region. The solution structure of DNA bound to aureochrome points to a tilted orientation that was further validated by molecular dynamics simulations. We propose that aureochrome signaling relies on an allosteric pathway from LOV to bZIP that results in conformational changes near the bZIP-DNA interface without major effects on the binding affinity.

The upstream regulatory sequence of the light harvesting complex Lhcf2 gene of the marine diatom Phaeodactylum tricornutum enhances transcription in an orientation- and distance-independent fashion

Diatoms are a key phytoplankton group in the contemporary ocean, showing extraordinary adaptation capacities to rapidly changing environments. The recent availability of whole genome sequences from representative species has revealed distinct features in their genomes, like novel combinations of genes encoding distinct metabolisms and a significant number of diatom-specific genes. However, the regulatory mechanisms driving diatom gene expression are still largely uncharacterized. Considering the wide variety of fields of study orbiting diatoms, ranging from ecology, evolutionary biology to biotechnology, it is thus essential to increase our understanding of fundamental gene regulatory processes such as transcriptional regulation. To this aim, we explored the functional properties of the 5'-flanking region of the Phaeodatylum tricornutum Lhcf2 gene, encoding a member of the Light Harvesting Complex superfamily and we showed that this region enhances transcription of a GUS reporter gene in an orientation- and distance-independent fashion. This represents the first example of a cis-regulatory sequence with enhancer-like features discovered in diatoms and it is instrumental for the generation of novel genetic tools and diatom exploitation in different areas of study.

Diversification of the light-harvesting complex gene family via intra- and intergenic duplications in the coral symbiotic alga Symbiodinium

The light-harvesting complex (LHC) is an essential component in light energy capture and transduction to facilitate downstream photosynthetic reactions in plant and algal chloroplasts. The unicellular dinoflagellate alga Symbiodinium is an endosymbiont of cnidarian animals, including corals and sea anemones, and provides carbohydrates generated through photosynthesis to host animals. Although Symbiodinium possesses a unique LHC gene family, called chlorophyll a-chlorophyll c₂-peridinin protein complex (acpPC), its genome-level diversity and evolutionary trajectories have not been investigated. Here, we describe a phylogenetic analysis revealing that many of the LHCs are encoded by highly duplicated genes with multi-subunit polyprotein structures in the nuclear genome of Symbiodinium minutum. This analysis provides an extended list of the LHC gene family in a single organism, including 80 loci encoding polyproteins composed of 145 LHC subunits recovered in the phylogenetic tree. In S. minutum, 5 phylogenetic groups of the Lhcf-type gene family, which is exclusively conserved in algae harboring secondary plastids of red algal origin, were identified. Moreover, 5 groups of the Lhcr-type gene family, of which members are known to be associated with PSI in red algal plastids and secondary plastids of red algal origin, were identified. Notably, members classified within a phylogenetic group of the Lhcf-type (group F1) are highly duplicated, which may explain the presence of an unusually large number of LHC genes in this species. Some gene units were homologous to other units within single loci of the polyprotein genes, whereas intergenic homologies between separate loci were conspicuous in other cases, implying that gene unit ‘shuffling’ by gene conversion and/or genome rearrangement might have been a driving force for diversification. These results suggest that vigorous intra- and intergenic gene duplication events have resulted in the genomic framework of photosynthesis in coral symbiont dinoflagellate algae.

Plastid proteome prediction for diatoms and other algae with secondary plastids of the red lineage

The plastids of ecologically and economically important algae from phyla such as stramenopiles, dinoflagellates and cryptophytes were acquired via a secondary endosymbiosis and are surrounded by three or four membranes. Nuclear-encoded plastid-localized proteins contain N-terminal bipartite targeting peptides with the conserved amino acid sequence motif 'ASAFAP'. Here we identify the plastid proteomes of two diatoms, Thalassiosira pseudonana and Phaeodactylum tricornutum, using a customized prediction tool (ASAFind) that identifies nuclear-encoded plastid proteins in algae with secondary plastids of the red lineage based on the output of SignalP and the identification of conserved 'ASAFAP' motifs and transit peptides. We tested ASAFind against a large reference dataset of diatom proteins with experimentally confirmed subcellular localization and found that the tool accurately identified plastid-localized proteins with both high sensitivity and high specificity. To identify nucleus-encoded plastid proteins of T. pseudonana and P. tricornutum we generated optimized sets of gene models for both whole genomes, to increase the percentage of full-length proteins compared with previous assembly model sets. ASAFind applied to these optimized sets revealed that about 8% of the proteins encoded in their nuclear genomes were predicted to be plastid localized and therefore represent the putative plastid proteomes of these algae.

Aureochrome 1a is involved in the photoacclimation of the diatom Phaeodactylum tricornutum

Aureochromes constitute a family of blue light (BL) receptors which are found exclusively in heterokont algae such as diatoms (Bacillariophyceae) and yellow-green algae (Xanthophyceae). Previous studies on the diatom Phaeodactylum tricornutum indicate that the formation of a high light acclimated phenotype is mediated by the absorption of BL and that aureochromes might play an important role in this process. P. tricornutum possesses four genes encoding aureochromes. In this study we confirm the nuclear localisation of the PtAUREO1a, 1b and 2 proteins. Furthermore we studied the physiology of light quality acclimation in genetically transformed P. tricornutum cell lines with reduced expression of the aureochrome 1a gene. The results demonstrate that the AUREO1a protein has a distinct function in light acclimation. However, rather unexpectedly AUREO1a seems to repress high light acclimation which resulted in a state of ‘hyper’ high light acclimation in aureo1a silenced strains. This was indicated by characteristic changes of several photosynthetic parameters, including increased maximum photosynthesis rates, decreased chlorophyll a contents per cell and increased values of non-photochemical quenching in AUREO1a silenced strains compared to wild type cultures. Strikingly, AUREO1a silenced strains exhibited phenotypic differences compared to wild type cells during cultivation under BL as well as under red light (RL) conditions. Therefore, AUREO1a might influence the RL signalling process, suggesting an interaction of AUREO1a with RL perception pathways.