A reaction center-dependent photoprotection mechanism in a highly robust photosystem II from an extremophilic red alga, Cyanidioschyzon merolae

Genomic imprints of unparalleled growth

Chlorella ohadii was isolated from desert biological soil crusts, one of the harshest habitats on Earth, and is emerging as an exciting new green model for studying growth, photosynthesis and metabolism under a wide range of conditions. Here, we compared the genome of C. ohadii, the fastest growing alga on record, to that of other green algae, to reveal the genomic imprints empowering its unparalleled growth rate and resistance to various stressors, including extreme illumination. This included the genome of its close relative, but slower growing and photodamage sensitive, C. sorokiniana UTEX 1663. A larger number of ribosome-encoding genes, high intron abundance, increased codon bias and unique genes potentially involved in metabolic flexibility and resistance to photodamage are all consistent with the faster growth of C. ohadii. Some of these characteristics highlight general trends in Chlorophyta and Chlorella spp. evolution, and others open new broad avenues for mechanistic exploration of their relationship with growth. This work entails a unique case study for the genomic adaptations and costs of exceptionally fast growth and sheds light on the genomic signatures of fast growth in photosynthetic cells. It also provides an important resource for future studies leveraging the unique properties of C. ohadii for photosynthesis and stress response research alongside their utilization for synthetic biology and biotechnology aims.

Dynamic adaptation of the extremophilic red microalga Cyanidioschyzon merolae to high nickel stress

The order of Cyanidiales comprises seven acido-thermophilic red microalgal species thriving in hot springs of volcanic origin characterized by extremely low pH, moderately high temperatures and the presence of high concentrations of sulphites and heavy metals that are prohibitive for most other organisms. Little is known about the physiological processes underlying the long-term adaptation of these extremophiles to such hostile environments. Here, we investigated the long-term adaptive responses of a red microalga Cyanidioschyzon merolae, a representative of Cyanidiales, to extremely high nickel concentrations. By the comprehensive physiological, microscopic and elemental analyses we dissected the key physiological processes underlying the long-term adaptation of this model extremophile to high Ni exposure. These include: (i) prevention of significant Ni accumulation inside the cells; (ii) activation of the photoprotective response of non-photochemical quenching; (iii) significant changes of the chloroplast ultrastructure associated with the formation of prolamellar bodies and plastoglobuli together with loosening of the thylakoid membranes; (iv) activation of ROS amelioration machinery; and (v) maintaining the efficient respiratory chain functionality. The dynamically regulated processes identified in this study are discussed in the context of the mechanisms driving the remarkable adaptability of C. merolae to extremely high Ni levels exceeding by several orders of magnitude those found in the natural environment of the microalga. The processes identified in this study provide a solid basis for the future investigation of the specific molecular components and pathways involved in the adaptation of Cyanidiales to the extremely high Ni concentrations.

Factors affecting light harvesting in the red alga Cyanidioschyzon merolae

The phycobilisome antennas, which contain phycobilin pigments instead of chlorophyll, are crucial for the photosynthetic activity of Cyanidioschyzon merolae cells, which thrive in an acidic and hot water environment. The accessible light intensity and quality, temperature, acidity, and other factors in this environment are quite different from those in the air available for terrestrial plants. Under these conditions, adaptation to the intensity and quality of light, as well as temperature, which are key factors in photosynthesis of higher plants, also affects this process in Cyanidioschyzon merolae cells. Adaptation to varying light conditions requires fast remodeling and re-tuning of their light-harvesting antennas (phycobilisomes) at multiple levels, from regulation of gene expression to structural reorganization of protein-pigment complexes. This review presents selected data on the structure of phycobilisomes, the genetic engineering of the constituent proteins, and the latest results and opinions on the adaptation of phycobilisomes to light intensity and quality, and temperature to photosynthetic activities. We pay special attention to the latest results of the C. merolae research.

How Light Modulates the Growth of Cyanidioschyzon merolae Cells by Changing the Function of Phycobilisomes

The aim of this study was to examine how light intensity and quality affect the photosynthetic apparatus of Cyanidioschyzon merolae cells by modulating the structure and function of phycobilisomes. Cells were grown in equal amounts of white, blue, red, and yellow light of low (LL) and high (HL) intensity. Biochemical characterization, fluorescence emission, and oxygen exchange were used to investigate selected cellular physiological parameters. It was found that the allophycocyanin content was sensitive only to light intensity, whereas the phycocynin content was also sensitive to light quality. Furthermore, the concentration of the PSI core protein was not affected by the intensity or quality of the growth light, but the concentration of the PSII core D1 protein was. Finally, the amount of ATP and ADP was lower in HL than LL. In our opinion, both light intensity and quality are main factors that play an important regulatory role in acclimatization/adaptation of C. merolae to environmental changes, and this is achieved by balancing the amounts of thylakoid membrane and phycobilisome proteins, the energy level, and the photosynthetic and respiratory activity. This understanding contributes to the development of a mix of cultivation techniques and genetic changes for a future large-scale synthesis of desirable biomolecules.

Identification of multiple nonphotochemical quenching processes in the extremophilic red alga Cyanidioschyzon merolae

Nonphotochemical quenching acts as a frontline response to prevent excitation energy from reaching the photochemical reaction center of photosystem II before photodamage occurs. Strong fluorescence quenching after merely one multi-turnover saturating light pulse characterizes a unique feature of nonphotochemical quenching in red algae. Several mechanisms underlying red algal nonphotochemical quenching have been proposed, yet which process(es) dominantly account for the strong fluorescence quenching is still under discussion. Here we assessed multiple nonphotochemical quenching processes in the extremophilic red alga Cyanidioschyzon merolae under light pulse and continuous illumination conditions. To assess the nonphotochemical quenching processes that might display different kinetics, fluorescence emission spectra at 77 K were measured after different periods of light treatments, and external fluorophores were added for normalization of the fluorescence level. The phycobilisome- and photosystem II-related nonphotochemical quenching processes were distinguished by light preferentially absorbed by phycobilisomes and photosystems, respectively. Multiple nonphotochemical quenching processes, including the energetic decoupling of phycobilisomes from photosystem II, the energy spillover from phycobilisomes to photosystem I and from photosystem II to photosystem I, were identified along with the previously identified intrinsic quenching within photosystem II. The ability to use multiple nonphotochemical quenching processes appears to maximize the light harvesting efficiency for photochemistry and to provide the flexibility of the energy redistribution between photosystem II and photosystem I. The effect of the various ionophores on the nonphotochemical quenching level suggests that nonphotochemical quenching is modulated by transmembrane gradients of protons and other cations.

Photoprotective energy quenching in the red alga Porphyridium purpureum occurs at the core antenna of the photosystem II but not at its reaction center

Photosynthetic organisms have evolved light-harvesting antennae over time. In cyanobacteria, external phycobilisomes (PBSs) are the dominant antennae, whereas in green algae and higher plants, PBSs have been replaced by proteins of the Lhc family that are integrated in the membrane. Red algae represent an evolutionary intermediate between these two systems, as they employ both PBSs and membrane LHCR proteins as light-harvesting units. Understanding how red algae cope with light is not only interesting for biotechnological applications, but is also of evolutionary interest. For example, energy-dependent quenching (qE) is an essential photoprotective mechanism widely used by species from cyanobacteria to higher plants to avoid light damage; however, the quenching mechanism in red algae remains largely unexplored. Here, we used both pulse amplitude-modulated (PAM) and time-resolved chlorophyll fluorescence to characterize qE kinetics in the red alga Porphyridium purpureum. PAM traces confirmed that qE in P. purpureum is activated by a decrease in the thylakoid lumen pH, whereas time-resolved fluorescence results further revealed the quenching site and ultrafast quenching kinetics. We found that quenching exclusively takes place in the photosystem II (PSII) complexes and preferentially occurs at PSII’s core antenna rather than at its reaction center, with an overall quenching rate of 17.6 ± 3.0 ns⁻¹. In conclusion, we propose that qE in red algae is not a reaction center type of quenching, and that there might be a membrane-bound protein that resembles PsbS of higher plants or LHCSR of green algae that senses low luminal pH and triggers qE in red algae.

A cotransformation system of the unicellular red alga Cyanidioschyzon merolae with blasticidin S deaminase and chloramphenicol acetyltransferase selectable markers

BACKGROUND

The unicellular red alga Cyanidioschyzon merolae exhibits a very simple cellular and genomic architecture. In addition, procedures for genetic modifications, such as gene targeting by homologous recombination and inducible/repressible gene expression, have been developed. However, only two markers for selecting transformants, uracil synthase (URA) and chloramphenicol acetyltransferase (CAT), are available in this alga. Therefore, manipulation of two or more different chromosomal loci in the same strain in C. merolae is limited.

RESULTS

This study developed a nuclear targeting and transformant selection system using an antibiotics blasticidin S (BS) and the BS deaminase (BSD) selectable marker by homologous recombination in C. merolae. In addition, this study has succeeded in simultaneously modifying two different chromosomal loci by a single-step cotransformation based on the combination of BSD and CAT selectable markers. A C. merolae strain that expresses mitochondrion-targeted mSCARLET (with the BSD marker) and mVENUS (with the CAT marker) from different chromosomal loci was generated with this procedure.

CONCLUSIONS

The newly developed BSD selectable marker enables an additional genetic modification to the already generated C. merolae transformants based on the URA or CAT system. Furthermore, the cotransformation system facilitates multiple genetic modifications. These methods and the simple nature of the C. merolae cellular and genomic architecture will facilitate studies on several phenomena common to photosynthetic eukaryotes.

Acclimation of photosynthetic apparatus in the mesophilic red alga Dixoniella giordanoi

Eukaryotic algae are photosynthetic organisms capable of exploiting sunlight to fix carbon dioxide into biomass with highly variable genetic and metabolic features. Information on algae metabolism from different species is inhomogeneous and, while green algae are, in general, more characterized, information on red algae is relatively scarce despite their relevant position in eukaryotic algae diversity. Within red algae, the best-known species are extremophiles or multicellular, while information on mesophilic unicellular organisms is still lacunose. Here, we investigate the photosynthetic properties of a recently isolated seawater unicellular mesophilic red alga, Dixoniella giordanoi. Upon exposure to different illuminations, D. giordanoi shows the ability to acclimate, modulate chlorophyll content, and re-organize thylakoid membranes. Phycobilisome content is also largely regulated, leading to almost complete disassembly of this antenna system in cells grown under intense illumination. Despite the absence of a light-induced xanthophyll cycle, cells accumulate zeaxanthin upon prolonged exposure to strong light, likely contributing to photoprotection. D. giordanoi cells show the ability to perform cyclic electron transport that is enhanced under strong illumination, likely contributing to the protection of Photosystem I from over-reduction and enabling cells to survive PSII photoinhibition without negative impact on growth.

The Unicellular Red Alga Cyanidioschyzon merolae-The Simplest Model of a Photosynthetic Eukaryote

Several species of unicellular eukaryotic algae exhibit relatively simple genomic and cellular architecture. Laboratory cultures of these algae grow faster than plants and often provide homogeneous cellular populations exposed to an almost equal environment. These characteristics are ideal for conducting experiments at the cellular and subcellular levels. Many microalgal lineages have recently become genetically tractable, which have started to evoke new streams of studies. Among such algae, the unicellular red alga Cyanidioschyzon merolae is the simplest organism; it possesses the minimum number of membranous organelles, only 4,775 protein-coding genes in the nucleus, and its cell cycle progression can be highly synchronized with the diel cycle. These properties facilitate diverse omics analyses of cellular proliferation and structural analyses of the intracellular relationship among organelles. C. merolae cells lack a rigid cell wall and are thus relatively easily disrupted, facilitating biochemical analyses. Multiple chromosomal loci can be edited by highly efficient homologous recombination. The procedures for the inducible/repressive expression of a transgene or an endogenous gene in the nucleus and for chloroplast genome modification have also been developed. Here, we summarize the features and experimental techniques of C. merolae and provide examples of studies using this alga. From these studies, it is clear that C. merolae-either alone or in comparative and combinatory studies with other photosynthetic organisms-can provide significant insights into the biology of photosynthetic eukaryotes.

Cryo-EM photosystem I structure reveals adaptation mechanisms to extreme high light in Chlorella ohadii

Photosynthesis in deserts is challenging since it requires fast adaptation to rapid night-to-day changes, that is, from dawn's low light (LL) to extreme high light (HL) intensities during the daytime. To understand these adaptation mechanisms, we purified photosystem I (PSI) from Chlorella ohadii, a green alga that was isolated from a desert soil crust, and identified the essential functional and structural changes that enable the photosystem to perform photosynthesis under extreme high light conditions. The cryo-electron microscopy structures of PSI from cells grown under low light (PSI_LL) and high light (PSI_HL), obtained at 2.70 and 2.71 Å, respectively, show that part of light-harvesting antenna complex I (LHCI) and the core complex subunit (PsaO) are eliminated from PSI_HL to minimize the photodamage. An additional change is in the pigment composition and their number in LHCI_HL; about 50% of chlorophyll b is replaced by chlorophyll a. This leads to higher electron transfer rates in PSI_HL and might enable C. ohadii PSI to act as a natural photosynthesiser in photobiocatalytic systems. PSI_HL or PSI_LL were attached to an electrode and their induced photocurrent was determined. To obtain photocurrents comparable with PSI_HL, 25 times the amount of PSI_LL was required, demonstrating the high efficiency of PSI_HL. Hence, we suggest that C. ohadii PSI_HL is an ideal candidate for the design of desert artificial photobiocatalytic systems.

Development of a Novel Nanoarchitecture of the Robust Photosystem I from a Volcanic Microalga Cyanidioschyzon merolae on Single Layer Graphene for Improved Photocurrent Generation

Here, we report the development of a novel photoactive biomolecular nanoarchitecture based on the genetically engineered extremophilic photosystem I (PSI) biophotocatalyst interfaced with a single layer graphene via pyrene-nitrilotriacetic acid self-assembled monolayer (SAM). For the oriented and stable immobilization of the PSI biophotocatalyst, an His6-tag was genetically engineered at the N-terminus of the stromal PsaD subunit of PSI, allowing for the preferential binding of this photoactive complex with its reducing side towards the graphene monolayer. This approach yielded a novel robust and ordered nanoarchitecture designed to generate an efficient direct electron transfer pathway between graphene, the metal redox center in the organic SAM and the photo-oxidized PSI biocatalyst. The nanosystem yielded an overall current output of 16.5 µA·cm-2 for the nickel- and 17.3 µA·cm-2 for the cobalt-based nanoassemblies, and was stable for at least 1 h of continuous standard illumination. The novel green nanosystem described in this work carries the high potential for future applications due to its robustness, highly ordered and simple architecture characterized by the high biophotocatalyst loading as well as simplicity of manufacturing.

Reading and surviving the harsh conditions in desert biological soil crust: The cyanobacterial viewpoint

Biological soil crusts (BSCs) are found in drylands, cover ∼12% of the Earth's surface in arid and semi-arid lands and their destruction is considered an important promoter of desertification. These crusts are formed by the adhesion of soil particles to polysaccharides excreted mostly by filamentous cyanobacteria, which are the pioneers and main primary producers in BSCs. Desert BSCs survive in one of the harshest environments on Earth, and are exposed to daily fluctuations of extreme conditions. The cyanobacteria inhabiting these habitats must precisely read the changing conditions and predict, for example, the forthcoming desiccation. Moreover, they evolved a comprehensive regulation of multiple adaptation strategies to enhance their stress tolerance. Here we focus on what distinguishes cyanobacteria able to revive after dehydration from those that cannot. While important progress has been made in our understanding of physiological, biochemical and omics aspects, clarification of the sensing, signal transduction and responses enabling desiccation tolerance are just emerging. We plot the trajectory of current research and open questions ranging from general strategies and regulatory adaptations in the hydration/desiccation cycle, to recent advances in our understanding of photosynthetic adaptation. The acquired knowledge provides new insights to mitigate desertification and improve plant productivity under drought conditions.

Juggling Lightning: How Chlorella ohadii handles extreme energy inputs without damage

The green alga Chlorella ohadii was isolated from a desert biological soil crust, one of the harshest environments on Earth. When grown under optimal laboratory settings it shows the fastest growth rate ever reported for a photosynthetic eukaryote and a complete resistance to photodamage even under unnaturally high light intensities. Here we examined the energy distribution along the photosynthetic pathway under four light and carbon regimes. This was performed using various methodologies such as membrane inlet mass spectrometer with stable O₂ isotopes, variable fluorescence, electrochromic shift and fluorescence assessment of NADPH level, as well as the use of specific inhibitors. We show that the preceding illumination and CO₂ level during growth strongly affect the energy dissipation strategies employed by the cell. For example, plastid terminal oxidase (PTOX) plays an important role in energy dissipation, particularly in high light- and low-CO₂-grown cells. Of particular note is the reliance on PSII cyclic electron flow as an effective and flexible dissipation mechanism in all conditions tested. The energy management observed here may be unique to C. ohadii, as it is the only known organism to cope with such conditions. However, the strategies demonstrated may provide an insight into the processes necessary for photosynthesis under high-light conditions.

Photosynthesis of the Cyanidioschyzon merolae cells in blue, red, and white light

Photosynthesis and respiration rates, pigment contents, CO2 compensation point, and carbonic anhydrase activity in Cyanidioschizon merolae cultivated in blue, red, and white light were measured. At the same light quality as during the growth, the photosynthesis of cells in blue light was significantly lowered, while under red light only slightly decreased as compared with white control. In white light, the quality of light during growth had no effect on the rate of photosynthesis at low O2 and high CO2 concentration, whereas their atmospheric level caused only slight decrease. Blue light reduced markedly photosynthesis rate of cells grown in white and red light, whereas the effect of red light was not so great. Only cells grown in the blue light showed increased respiration rate following the period of both the darkness and illumination. Cells grown in red light had the greatest amount of chlorophyll a, zeaxanthin, and β-carotene, while those in blue light had more phycocyanin. The dependence on O2 concentration of the CO2 compensation point and the rate of photosynthesis indicate that this alga possessed photorespiration. Differences in the rate of photosynthesis at different light qualities are discussed in relation to the content of pigments and transferred light energy together with the possible influence of related processes. Our data showed that blue and red light regulate photosynthesis in C. merolae for adjusting its metabolism to unfavorable for photosynthesis light conditions.

Regulation of photosystem I-light-harvesting complex I from a red alga Cyanidioschyzon merolae in response to light intensities

Photosynthetic organisms use different means to regulate their photosynthetic activity in respond to different light conditions under which they grow. In this study, we analyzed changes in the photosystem I (PSI) light-harvesting complex I (LHCI) supercomplex from a red alga Cyanidioschyzon merolae, upon growing under three different light intensities, low light (LL), medium light (ML), and high light (HL). The results showed that the red algal PSI-LHCI is separated into two bands on blue-native PAGE, which are designated PSI-LHCI-A and PSI-LHCI-B, respectively, from cells grown under LL and ML. The former has a higher molecular weight and binds more Lhcr subunits than the latter. They are considered to correspond to the two types of PSI-LHCI identified by cryo-electron microscopic analysis recently, namely, the former with five Lhcrs and the latter with three Lhcrs. The amount of PSI-LHCI-A is higher in the LL-grown cells than that in the ML-grown cells. In the HL-grown cells, PSI-LHCI-A completely disappeared and only PSI-LHCI-B was observed. Furthermore, PSI core complexes without Lhcr attached also appeared in the HL cells. Fluorescence decay kinetics measurement showed that Lhcrs are functionally connected with the PSI core in both PSI-LHCI-A and PSI-LHCI-B obtained from LL and ML cells; however, Lhcrs in the PSI-LHCI-B fraction from the HL cells are not coupled with the PSI core. These results indicate that the red algal PSI not only regulates its antenna size but also adjusts the functional connection of Lhcrs with the PSI core in response to different light intensities.

Bioengineering of Microalgae: Recent Advances, Perspectives, and Regulatory Challenges for Industrial Application

Microalgae, due to their complex metabolic capacity, are being continuously explored for nutraceuticals, pharmaceuticals, and other industrially important bioactives. However, suboptimal yield and productivity of the bioactive of interest in local and robust wild-type strains are of perennial concerns for their industrial applications. To overcome such limitations, strain improvement through genetic engineering could play a decisive role. Though the advanced tools for genetic engineering have emerged at a greater pace, they still remain underused for microalgae as compared to other microorganisms. Pertaining to this, we reviewed the progress made so far in the development of molecular tools and techniques, and their deployment for microalgae strain improvement through genetic engineering. The recent availability of genome sequences and other omics datasets form diverse microalgae species have remarkable potential to guide strategic momentum in microalgae strain improvement program. This review focuses on the recent and significant improvements in the omics resources, mutant libraries, and high throughput screening methodologies helpful to augment research in the model and non-model microalgae. Authors have also summarized the case studies on genetically engineered microalgae and highlight the opportunities and challenges that are emerging from the current progress in the application of genome-editing to facilitate microalgal strain improvement. Toward the end, the regulatory and biosafety issues in the use of genetically engineered microalgae in commercial applications are described.

On the nature of uncoupled chlorophylls in the extremophilic photosystem I-light harvesting I supercomplex

Photosystem I core-light-harvesting antenna supercomplexes (PSI-LHCI) were isolated from the extremophilic red alga Cyanidioschyzon merolae and studied by three fluorescence techniques in order to characterize chlorophylls (Chls) energetically uncoupled from the PSI reaction center (RC). Such Chls are observed in virtually all optical experiments of any PSI core and PSI-LHCI supercomplex preparations across various species and may influence the operation of PSI-based solar cells and other biohybrid systems. However, the nature of the uncoupled Chls (uChls) has never been explored deeply before. In this work, the amount of uChls was controlled by stirring the solution of C. merolae PSI-LHCI supercomplex samples at elevated temperature (~303 K) and was found to increase from <2% in control samples up to 47% in solutions stirred for 3.5 h. The fluorescence spectrum of uChls was found to be blue-shifted by ~20 nm (to ~680 nm) relative to the fluorescence band from Chls that are well coupled to PSI RC. This effect indicates that mechanical stirring leads to disappearance of some red Chls (emitting at above ~700 nm) that are present in the intact LHCI antenna associated with the PSI core. Comparative diffusion studies of control and stirred samples by fluorescence correlation spectroscopy together with biochemical analysis by SDS-PAGE and BN-PAGE indicate that energetically uncoupled Lhcr subunits are likely to be still physically attached to the PSI core, albeit with altered three-dimensional organization due to the mechanical stress.

Characterization of structure and protein of vitelline membranes of precocial (ring-necked pheasant, gray partridge) and superaltricial (cockatiel parrot, domestic pigeon) birds

Of all the known oviparous taxa, female birds lay the most diverse types of eggs that differ in terms of shape, shell pigmentation, and shell structure. The pigmentation of the shell, the weight of the egg, and the composition of the yolk correlate with environmental conditions and the needs of the developing embryos. In this study, we analyzed the structure and protein composition of the vitelline membrane (VM) of ring-necked pheasant, gray partridge, cockatiel parrot, and domestic pigeon eggs. We found that the VM structure is characteristic of each species and varies depending on whether the species is precocial (ring-necked pheasant and gray partridge) or superaltrical (cockatiel parrot and domestic pigeon). We hypothesize that a multilayer structure of VM is necessary to counteract the aging process of the egg. The multilayer structure of VM is only found in species with a large number of eggs in one clutch and is characterized by a long incubation period. An interesting discovery of this study is the three-layered VM of pheasant and partridge eggs. This shows that the formation of individual layers of VM in specific sections of the hen's reproductive system is not confirmed in other species. The number of protein fractions varied between 19 and 23, with a molecular weight ranging from 15 to 250 kDa, depending on the species. The number of proteins identified in the VM of the study birds' eggs is as follows: chicken-14, ring-necked pheasant-7, gray partridge-10, cockatiel parrot-6, and domestic pigeon-23. The highest number of species-specific proteins (21) was detected in the VM of domestic pigeon. This study is the first to present the structure and protein composition in the VM of ring-necked pheasant, gray partridge, cockatiel parrot, and domestic pigeon eggs. In addition, we analyzed the relationship between the hatching specification of birds and the structure of the VM.

Photoprotective strategies in the motile cryptophyte alga Rhodomonas salina-role of non-photochemical quenching, ions, photoinhibition, and cell motility

We explored photoprotective strategies in a cryptophyte alga Rhodomonas salina. This cryptophytic alga represents phototrophs where chlorophyll a/c antennas in thylakoids are combined with additional light-harvesting system formed by phycobiliproteins in the chloroplast lumen. The fastest response to excessive irradiation is induction of non-photochemical quenching (NPQ). The maximal NPQ appears already after 20 s of excessive irradiation. This initial phase of NPQ is sensitive to Ca²⁺ channel inhibitor (diltiazem) and disappears, also, in the presence of non-actin, an ionophore for monovalent cations. The prolonged exposure to high light of R. salina cells causes photoinhibition of photosystem II (PSII) that can be further enhanced when Ca²⁺ fluxes are inhibited by diltiazem. The light-induced reduction in PSII photochemical activity is smaller when compared with immotile diatom Phaeodactylum tricornutum. We explain this as a result of their different photoprotective strategies. Besides the protective role of NPQ, the motile R. salina also minimizes high light exposure by increased cell velocity by almost 25% percent (25% from 82 to 104 μm/s). We suggest that motility of algal cells might have a photoprotective role at high light because algal cell rotation around longitudinal axes changes continual irradiation to periodically fluctuating light.

Different roles for ApcD and ApcF in Synechococcus elongatus and Synechocystis sp. PCC 6803 phycobilisomes

The phycobilisome, the cyanobacterial light harvesting complex, is a huge phycobiliprotein containing extramembrane complex, formed by a core from which rods radiate. The phycobilisome has evolved to efficiently absorb sun energy and transfer it to the photosystems via the last energy acceptors of the phycobilisome, ApcD and ApcE. ApcF also affects energy transfer by interacting with ApcE. In this work we studied the role of ApcD and ApcF in energy transfer and state transitions in Synechococcus elongatus and Synechocystis PCC6803. Our results demonstrate that these proteins have different roles in both processes in the two strains. The lack of ApcD and ApcF inhibits state transitions in Synechocystis but not in S. elongatus. In addition, lack of ApcF decreases energy transfer to both photosystems only in Synechocystis, while the lack of ApcD alters energy transfer to photosystem I only in S. elongatus. Thus, conclusions based on results obtained in one cyanobacterial strain cannot be systematically transferred to other strains and the putative role(s) of phycobilisomes in state transitions need to be reconsidered.

Integration of a Galdieria plasma membrane sugar transporter enables heterotrophic growth of the obligate photoautotrophic red alga Cynanidioschyzon merolae

The unicellular thermoacidophilic red alga Cyanidioschyzon merolae is an emerging model organism of photosynthetic eukaryotes. Its relatively simple genome (16.5 Mbp) with very low-genetic redundancy and its cellular structure possessing one chloroplast, mitochondrion, peroxisome, and other organelles have facilitated studies. In addition, this alga is genetically tractable, and the nuclear and chloroplast genomes can be modified by integration of transgenes via homologous recombination. Recent studies have attempted to clarify the structure and function of the photosystems of this alga. However, it is difficult to obtain photosynthesis-defective mutants for molecular genetic studies because this organism is an obligate autotroph. To overcome this issue in C. merolae, we expressed a plasma membrane sugar transporter, GsSPT1, from Galdieria sulphuraria, which is an evolutionary relative of C. merolae and capable of heterotrophic growth. The heterologously expressed GsSPT1 localized at the plasma membrane. GsSPT1 enabled C. merolae to grow mixotrophically and heterotrophically, in which cells grew in the dark with glucose or in the light with a photosynthetic inhibitor 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) and glucose. When the GsSPT1 transgene multiplied on the C. merolae chromosome via the URA Cm-Gs selection marker, which can multiply itself and its flanking transgene, GsSPT1 protein level increased and the heterotrophic and mixotrophic growth of the transformant accelerated. We also found that GsSPT1 overexpressing C. merolae efficiently formed colonies on solidified medium under light with glucose and DCMU. Thus, GsSPT1 overexpresser will facilitate single colony isolation and analyses of photosynthesis-deficient mutants produced either by random or site-directed mutagenesis. In addition, our results yielded evidence supporting that the presence or absence of plasma membrane sugar transporters is a major cause of difference in trophic properties between C. merolae and G. sulphuraria.

The Cytochrome b 6 f Complex Is Not Involved in Cyanobacterial State Transitions

Photosynthetic organisms must sense and respond to fluctuating environmental conditions in order to perform efficient photosynthesis and to avoid the formation of dangerous reactive oxygen species. The excitation energy arriving at each photosystem permanently changes due to variations in the intensity and spectral properties of the absorbed light. Cyanobacteria, like plants and algae, have developed a mechanism, named "state transitions," that balances photosystem activities. Here, we characterize the role of the cytochrome b ₆ f complex and phosphorylation reactions in cyanobacterial state transitions using Synechococcus elongatus PCC 7942 and Synechocystis PCC 6803 as model organisms. First, large photosystem II (PSII) fluorescence quenching was observed in State II, a result that does not appear to be related to energy transfer from PSII to PSI (spillover). This membrane-associated process was inhibited by betaine, Suc, and high concentrations of phosphate. Then, using different chemicals affecting the plastoquinone pool redox state and cytochrome b ₆ f activity, we demonstrate that this complex is not involved in state transitions in S. elongatus or Synechocystis PCC6803. Finally, by constructing and characterizing 21 protein kinase and phosphatase mutants and using chemical inhibitors, we demonstrate that phosphorylation reactions are not essential for cyanobacterial state transitions. Thus, signal transduction is completely different in cyanobacterial and plant (green alga) state transitions.

Unequal misses during the flash-induced advancement of photosystem II: effects of the S state and acceptor side cycles

Photosynthetic water oxidation is catalyzed by the oxygen-evolving complex (OEC) in photosystem II (PSII). This process is energetically driven by light-induced charge separation in the reaction center of PSII, which leads to a stepwise accumulation of oxidizing equivalents in the OEC (S_i states, i = 0-4) resulting in O₂ evolution after each fourth flash, and to the reduction of plastoquinone to plastoquinol on the acceptor side of PSII. However, the S_i-state advancement is not perfect, which according to the Kok model is described by miss-hits (misses). These may be caused by redox equilibria or kinetic limitations on the donor (OEC) or the acceptor side. In this study, we investigate the effects of individual S state transitions and of the quinone acceptor side on the miss parameter by analyzing the flash-induced oxygen evolution patterns and the S₂, S₃ and S₀ state lifetimes in thylakoid samples of the extremophilic red alga Cyanidioschyzon merolae. The data are analyzed employing a global fit analysis and the results are compared to the data obtained previously for spinach thylakoids. These two organisms were selected, because the redox potential of Q_A/Q_A^- in PSII is significantly less negative in C. merolae (E_m = - 104 mV) than in spinach (E_m = - 163 mV). This significant difference in redox potential was expected to allow the disentanglement of acceptor and donor side effects on the miss parameter. Our data indicate that, at slightly acidic and neutral pH values, the E_m of Q_A^-/Q_A plays only a minor role for the miss parameter. By contrast, the increased energy gap for the backward electron transfer from Q_A^- to Pheo slows down the charge recombination reaction with the S₃ and S₂ states considerably. In addition, our data support the concept that the S₂ → S₃ transition is the least efficient step during the oxidation of water to molecular oxygen in the Kok cycle of PSII.

The evolution of the photoprotective antenna proteins in oxygenic photosynthetic eukaryotes

Photosynthetic organisms require rapid and reversible down-regulation of light harvesting to avoid photodamage. Response to unpredictable light fluctuations is achieved by inducing energy-dependent quenching, qE, which is the major component of the process known as non-photochemical quenching (NPQ) of chlorophyll fluorescence. qE is controlled by the operation of the xanthophyll cycle and accumulation of specific types of proteins, upon thylakoid lumen acidification. The protein cofactors so far identified to modulate qE in photosynthetic eukaryotes are the photosystem II subunit S (PsbS) and light-harvesting complex stress-related (LHCSR/LHCX) proteins. A transition from LHCSR- to PsbS-dependent qE took place during the evolution of the Viridiplantae (also known as ‘green lineage’ organisms), such as green algae, mosses and vascular plants. Multiple studies showed that LHCSR and PsbS proteins have distinct functions in the mechanism of qE. LHCX(-like) proteins are closely related to LHCSR proteins and found in ‘red lineage’ organisms that contain secondary red plastids, such as diatoms. Although LHCX proteins appear to control qE in diatoms, their role in the mechanism remains poorly understood. Here, we present the current knowledge on the functions and evolution of these crucial proteins, which evolved in photosynthetic eukaryotes to optimise light harvesting.

On the origin of the slow M-T chlorophyll a fluorescence decline in cyanobacteria: interplay of short-term light-responses

The slow kinetic phases of the chlorophyll a fluorescence transient (induction) are valuable tools in studying dynamic regulation of light harvesting, light energy distribution between photosystems, and heat dissipation in photosynthetic organisms. However, the origin of these phases are not yet fully understood. This is especially true in the case of prokaryotic oxygenic photoautotrophs, the cyanobacteria. To understand the origin of the slowest (tens of minutes) kinetic phase, the M-T fluorescence decline, in the context of light acclimation of these globally important microorganisms, we have compared spectrally resolved fluorescence induction data from the wild type Synechocystis sp. PCC 6803 cells, using orange (λ = 593 nm) actinic light, with those of mutants, ΔapcD and ΔOCP, that are unable to perform either state transition or fluorescence quenching by orange carotenoid protein (OCP), respectively. Our results suggest a multiple origin of the M-T decline and reveal a complex interplay of various known regulatory processes in maintaining the redox homeostasis of a cyanobacterial cell. In addition, they lead us to suggest that a new type of regulatory process, operating on the timescale of minutes to hours, is involved in dissipating excess light energy in cyanobacteria.

Molecular Mechanisms of Photoadaptation of Photosystem I Supercomplex from an Evolutionary Cyanobacterial/Algal Intermediate

The monomeric photosystem I-light-harvesting antenna complex I (PSI-LHCI) supercomplex from the extremophilic red alga Cyanidioschyzon merolae represents an intermediate evolutionary link between the cyanobacterial PSI reaction center and its green algal/higher plant counterpart. We show that the C. merolae PSI-LHCI supercomplex is characterized by robustness in various extreme conditions. By a combination of biochemical, spectroscopic, mass spectrometry, and electron microscopy/single particle analyses, we dissected three molecular mechanisms underlying the inherent robustness of the C. merolae PSI-LHCI supercomplex: (1) the accumulation of photoprotective zeaxanthin in the LHCI antenna and the PSI reaction center; (2) structural remodeling of the LHCI antenna and adjustment of the effective absorption cross section; and (3) dynamic readjustment of the stoichiometry of the two PSI-LHCI isomers and changes in the oligomeric state of the PSI-LHCI supercomplex, accompanied by dissociation of the PsaK core subunit. We show that the largest low light-treated C. merolae PSI-LHCI supercomplex can bind up to eight Lhcr antenna subunits, which are organized as two rows on the PsaF/PsaJ side of the core complex. Under our experimental conditions, we found no evidence of functional coupling of the phycobilisomes with the PSI-LHCI supercomplex purified from various light conditions, suggesting that the putative association of this antenna with the PSI supercomplex is absent or may be lost during the purification procedure.

Photoprotective, excited-state quenching mechanisms in diverse photosynthetic organisms

Light-harvesting complexes (LHCs) serve a dual role in photosynthesis, depending on the prevailing light conditions. In low light, they ensure photosynthetic efficiency by maximizing the light absorption cross-section and subsequent energy storage. Under excess light conditions, LHCs perform photoprotective quenching functions to prevent harmful chemical species such as triplet chlorophyll and singlet oxygen from forming and damaging the photosynthetic apparatus. In this Minireview, various photoprotective quenching mechanisms that have been identified in different photosynthetic organisms are surveyed and summarized, and implications for improving photosynthetic productivity are briefly discussed.

Deletion of psbQ' gene in Cyanidioschyzon merolae reveals the function of extrinsic PsbQ' in PSII

We have successfully produced single-cell colonies of C. merolae mutants, lacking the PsbQ' subunit in its PSII complex by application of DTA-aided mutant selection. We have investigated the physiological changes in PSII function and structure and proposed a tentative explanation of the function of PsbQ' subunit in the PSII complex. We have improved the selectivity of the Cyanidioschyzon merolae nuclear transformation method by the introduction of diphtheria toxin genes into the transformation vector as an auxiliary selectable marker. The revised method allowed us to obtained single-cell colonies of C. merolae, lacking the gene of the PsbQ' extrinsic protein. The efficiency of gene replacement was extraordinarily high, allowing for a complete deletion of the gene of interest, without undesirable illegitimate integration events. We have confirmed the absence of PsbQ' protein at genetic and protein level. We have characterized the physiology of mutant cells and isolated PSII protein complex and concluded that PsbQ' is involved in nuclear regulation of PSII activity, by influencing several parameters of PSII function. Among these: oxygen evolving activity, partial dissociation of PsbV, regulation of dimerization, downsizing of phycobilisomes rods and regulation of zeaxanthin abundance. The adaptation of cellular physiology appeared to favorite upregulation of PSII and concurrent downregulation of PSI, resulting in an imbalance of energy distribution, decrease of photosynthesis and inhibition of cell proliferation.

Isolation and characterization of PSI-LHCI super-complex and their sub-complexes from a red alga Cyanidioschyzon merolae

Photosystem I (PSI)-light-harvesting complex I (LHCI) super-complex and its sub-complexes PSI core and LHCI, were purified from a unicellular red alga Cyanidioschyzon merolae and characterized. PSI-LHCI of C. merolae existed as a monomer with a molecular mass of 580 kDa. Mass spectrometry analysis identified 11 subunits (PsaA, B, C, D, E, F, I, J, K, L, O) in the core complex and three LHCI subunits, CMQ142C, CMN234C, and CMN235C in LHCI, indicating that at least three Lhcr subunits associate with the red algal PSI core. PsaG was not found in the red algae PSI-LHCI, and we suggest that the position corresponding to Lhca1 in higher plant PSI-LHCI is empty in the red algal PSI-LHCI. The PSI-LHCI complex was separated into two bands on native PAGE, suggesting that two different complexes may be present with slightly different protein compositions probably with respective to the numbers of Lhcr subunits. Based on the results obtained, a structural model was proposed for the red algal PSI-LHCI. Furthermore, pigment analysis revealed that the C. merolae PSI-LHCI contained a large amount of zeaxanthin, which is mainly associated with the LHCI complex whereas little zeaxanthin was found in the PSI core. This indicates a unique feature of the carotenoid composition of the Lhcr proteins and may suggest an important role of Zea in the light-harvesting and photoprotection of the red algal PSI-LHCI complex.

Temperature-Induced Remodeling of the Photosynthetic Machinery Tunes Photosynthesis in the Thermophilic Alga Cyanidioschyzon merolae

The thermophilic alga C. merolae thrives in extreme environments (low pH and temperature between 40°C and 56°C). In this study, we investigated the acclimation process of the alga to a colder temperature (25°C). A long-term cell growth experiment revealed an extensive remodeling of the photosynthetic apparatus in the first 250 h of acclimation, which was followed by cell growth to an even higher density than the control (grown at 42°C) cell density. Once the cells were shifted to the lower temperature, the proteins of the light-harvesting antenna were greatly down-regulated and the phycobilisome composition was altered. The amount of PSI and PSII subunits was also decreased, but the chlorophyll to photosystems ratio remained unchanged. The 25°C cells possessed a less efficient photon-to-oxygen conversion rate and require a 2.5 times higher light intensity to reach maximum photosynthetic efficiency. With respect to chlorophyll, however, the photosynthetic oxygen evolution rate of the 25°C culture was 2 times higher than the control. Quantitative proteomics revealed that acclimation requires, besides remodeling of the photosynthetic apparatus, also adjustment of the machinery for protein folding, degradation, and homeostasis. In summary, these remodeling processes tuned photosynthesis according to the demands placed on the system and revealed the capability of C. merolae to grow under a broad range of temperatures.

Characterization of the influence of chlororespiration on the regulation of photosynthesis in the glaucophyte Cyanophora paradoxa

Glaucophytes are primary symbiotic algae with unique plastids called cyanelles, whose structure is most similar to ancestral cyanobacteria among plastids in photosynthetic organisms. Here we compare the regulation of photosynthesis in glaucophyte with that in cyanobacteria in the aim of elucidating the changes caused by the symbiosis in the interaction between photosynthetic electron transfer and other metabolic pathways. Chlorophyll fluorescence measurements of the glaucophyte Cyanophora paradoxa NIES-547 indicated that plastoquinone (PQ) pool in photosynthetic electron transfer was reduced in the dark by chlororespiration. The levels of nonphotochemical quenching of chlorophyll fluorescence was high in the dark but decreased under low light, and increased again under high light. This type of concave light dependence was quite similar to that observed in cyanobacteria. Moreover, the addition of ionophore hardly affected nonphotochemical quenching, suggesting state transition as a main component of the regulatory system in C. paradoxa. These results suggest that cyanelles of C. paradoxa retain many of the characteristics observed in their ancestral cyanobacteria. From the viewpoint of metabolic interactions, C. paradoxa is the primary symbiotic algae most similar to cyanobacteria than other lineages of photosynthetic organisms.

Differences in photosynthetic responses of NADP-ME type C4 species to high light

MAIN CONCLUSION

Three species chosen as representatives of NADP-ME C4 subtype exhibit different sensitivity toward photoinhibition, and great photochemical differences were found to exist between the species. These characteristics might be due to the imbalance in the excitation energy between the photosystems present in M and BS cells, and also due to that between species caused by the penetration of light inside the leaves. Such regulation in the distribution of light intensity between M and BS cells shows that co-operation between both the metabolic systems determines effective photosynthesis and reduces the harmful effects of high light on the degradation of PSII through the production of reactive oxygen species (ROS). We have investigated several physiological parameters of NADP-ME-type C4 species (e.g., Zea mays, Echinochloa crus-galli, and Digitaria sanguinalis) grown under moderate light intensity (200 µmol photons m-2 s-1) and, subsequently, exposed to excess light intensity (HL, 1600 µmol photons m-2 s-1). Our main interest was to understand why these species, grown under identical conditions, differ in their responses toward high light, and what is the physiological significance of these differences. Among the investigated species, Echinochloa crus-galli is best adapted to HL treatment. High resistance of the photosynthetic apparatus of E. crus-galli to HL was accompanied by an elevated level of phosphorylation of PSII proteins, and higher values of photochemical quenching, ATP/ADP ratio, activity of PSI and PSII complexes, as well as integrity of the thylakoid membranes. It was also shown that the non-radiative dissipation of energy in the studied plants was not dependent on carotenoid contents and, thus, other photoprotective mechanisms might have been engaged under HL stress conditions. The activity of the enzymes superoxide dismutase and ascorbate peroxidase as well as the content of malondialdehyde and H2O2 suggests that antioxidant defense is not responsible for the differences observed in the tolerance of NADP-ME species toward HL stress. We concluded that the chloroplasts of the examined NADP-ME species showed different sensitivity to short-term high light irradiance, suggesting a role of other factors excluding light factors, thus influencing the response of thylakoid proteins. We also observed that HL affects the mesophyll chloroplasts first hand and, subsequently, the bundle sheath chloroplasts.

Biofunctionalisation of p-doped silicon with cytochrome c553minimises charge recombination and enhances photovoltaic performance of the all-solid-state photosystem I-based biophotoelectrode

Passivation of p-doped silicon substrate was achieved by its biofunctionalisation with hexahistidine-tagged cytochrome c₅₅₃, a soluble electroactive photosynthetic protein responsible for electron donation to photooxidised photosystem I.

Transformation of the Cyanidioschyzon merolae chloroplast genome: prospects for understanding chloroplast function in extreme environments

We have successfully transformed an exthemophilic red alga with the chloramphenicol acetyltransferase gene, rendering this organism insensitive to its toxicity. Our work paves the way to further work with this new modelorganism. Here we report the first successful attempt to achieve a stable, under selectable pressure, chloroplast transformation in Cyanidioschizon merolae-an extremophilic red alga of increasing importance as a new model organism. The following protocol takes advantage of a double homologous recombination phenomenon in the chloroplast, allowing to introduce an exogenous, selectable gene. For that purpose, we decided to use chloramphenicol acetyltransferase (CAT), as chloroplasts are particularly vulnerable to chloramphenicol lethal effects (Zienkiewicz et al. in Protoplasma, 2015, doi: 10.1007/s00709-015-0936-9 ). We adjusted two methods of DNA delivery: the PEG-mediated delivery and the biolistic bombardment based delivery, either of these methods work sufficiently with noticeable preference to the former. Application of a codon-optimized sequence of the cat gene and a single colony selection yielded C. merolae strains, capable of resisting up to 400 µg/mL of chloramphenicol. Our method opens new possibilities in production of site-directed mutants, recombinant proteins and exogenous protein overexpression in C. merolae-a new model organism.

Role of Ions in the Regulation of Light-Harvesting

Regulation of photosynthetic light harvesting in the thylakoids is one of the major key factors affecting the efficiency of photosynthesis. Thylakoid membrane is negatively charged and influences both the structure and the function of the primarily photosynthetic reactions through its electrical double layer (EDL). Further, there is a heterogeneous organization of soluble ions (K+, Mg2+, Cl-) attached to the thylakoid membrane that, together with fixed charges (negatively charged amino acids, lipids), provides an electrical field. The EDL is affected by the valence of the ions and interferes with the regulation of "state transitions," protein interactions, and excitation energy "spillover" from Photosystem II to Photosystem I. These effects are reflected in changes in the intensity of chlorophyll a fluorescence, which is also a measure of photoprotective non-photochemical quenching (NPQ) of the excited state of chlorophyll a. A triggering of NPQ proceeds via lumen acidification that is coupled to the export of positive counter-ions (Mg2+, K+) to the stroma or/and negative ions (e.g., Cl-) into the lumen. The effect of protons and anions in the lumen and of the cations (Mg2+, K+) in the stroma are, thus, functionally tightly interconnected. In this review, we discuss the consequences of the model of EDL, proposed by Barber (1980b) Biochim Biophys Acta 594:253-308) in light of light-harvesting regulation. Further, we explain differences between electrostatic screening and neutralization, and we emphasize the opposite effect of monovalent (K+) and divalent (Mg2+) ions on light-harvesting and on "screening" of the negative charges on the thylakoid membrane; this effect needs to be incorporated in all future models of photosynthetic regulation by ion channels and transporters.

Cyanobacterial photoprotection by the orange carotenoid protein

In photosynthetic organisms, the production of dangerous oxygen species is stimulated under high irradiance. To cope with this stress, these organisms have evolved photoprotective mechanisms. One type of mechanism functions to decrease the energy arriving at the photochemical centres by increasing thermal dissipation at the level of antennae. In cyanobacteria, the trigger for this mechanism is the photoactivation of a soluble carotenoid protein, the orange carotenoid protein (OCP), which is a structurally and functionally modular protein. The inactive orange form (OCP^o) is compact and globular, with the carotenoid spanning the effector and the regulatory domains. In the active red form (OCP^r), the two domains are completely separated and the carotenoid has translocated entirely into the effector domain. The activated OCP^r interacts with the phycobilisome (PBS), the cyanobacterial antenna, and induces excitation-energy quenching. A second protein, the fluorescence recovery protein (FRP), dislodges the active OCP^r from the PBSs and accelerates its conversion to the inactive OCP.

The mechanisms whereby the green alga Chlorella ohadii, isolated from desert soil crust, exhibits unparalleled photodamage resistance

Excess illumination damages the photosynthetic apparatus with severe implications with regard to plant productivity. Unlike model organisms, the growth of Chlorella ohadii, isolated from desert soil crust, remains unchanged and photosynthetic O2 evolution increases, even when exposed to irradiation twice that of maximal sunlight. Spectroscopic, biochemical and molecular approaches were applied to uncover the mechanisms involved. D1 protein in photosystem II (PSII) is barely degraded, even when exposed to antibiotics that prevent its replenishment. Measurements of various PSII parameters indicate that this complex functions differently from that in model organisms and suggest that C. ohadii activates a nonradiative electron recombination route which minimizes singlet oxygen formation and the resulting photoinhibition. The light-harvesting antenna is very small and carotene composition is hardly affected by excess illumination. Instead of succumbing to photodamage, C. ohadii activates additional means to dissipate excess light energy. It undergoes major structural, compositional and physiological changes, leading to a large rise in photosynthetic rate, lipids and carbohydrate content and inorganic carbon cycling. The ability of C. ohadii to avoid photodamage relies on a modified function of PSII and the dissipation of excess reductants downstream of the photosynthetic reaction centers. The biotechnological potential as a gene source for crop plant improvement is self-evident.

Violaxanthin inhibits nonphotochemical quenching in light-harvesting antenna of Chromera velia

Non-photochemical quenching (NPQ) is a photoprotective mechanism in light-harvesting antennae. NPQ is triggered by chloroplast thylakoid lumen acidification and is accompanied by violaxanthin de-epoxidation to zeaxanthin, which further stimulates NPQ. In the present study, we show that violaxanthin can act in the opposite direction to zeaxanthin because an increase in the concentration of violaxanthin reduced NPQ in the light-harvesting antennae of Chromera velia. The correlation overlapped with a similar relationship between violaxanthin and NPQ as observed in isolated higher plant light-harvesting complex II. The data suggest that violaxanthin in C. velia can act as an inhibitor of NPQ, indicating that violaxanthin has to be removed from the vicinity of the protein to reach maximal NPQ.

Novel Features of Eukaryotic Photosystem II Revealed by Its Crystal Structure Analysis from a Red Alga

Photosystem II (PSII) catalyzes light-induced water splitting, leading to the evolution of molecular oxygen indispensible for life on the earth. The crystal structure of PSII from cyanobacteria has been solved at an atomic level, but the structure of eukaryotic PSII has not been analyzed. Because eukaryotic PSII possesses additional subunits not found in cyanobacterial PSII, it is important to solve the structure of eukaryotic PSII to elucidate their detailed functions, as well as evolutionary relationships. Here we report the structure of PSII from a red alga Cyanidium caldarium at 2.76 Å resolution, which revealed the structure and interaction sites of PsbQ', a unique, fourth extrinsic protein required for stabilizing the oxygen-evolving complex in the lumenal surface of PSII. The PsbQ' subunit was found to be located underneath CP43 in the vicinity of PsbV, and its structure is characterized by a bundle of four up-down helices arranged in a similar way to those of cyanobacterial and higher plant PsbQ, although helices I and II of PsbQ' were kinked relative to its higher plant counterpart because of its interactions with CP43. Furthermore, two novel transmembrane helices were found in the red algal PSII that are not present in cyanobacterial PSII; one of these helices may correspond to PsbW found only in eukaryotic PSII. The present results represent the first crystal structure of PSII from eukaryotic oxygenic organisms, which were discussed in comparison with the structure of cyanobacterial PSII.

Multiple copies of the PsbQ protein in a cyanobacterial photosystem II assembly intermediate complex

Photosystem II (PSII) undergoes frequent damage owing to the demanding electron transfer chemistry it performs. To sustain photosynthetic activity, damaged PSII undergoes a complex repair cycle consisting of many transient intermediate complexes. By purifying PSII from the cyanobacterium Synechocystis sp. PCC 6803 using a histidine-tag on the PsbQ protein, a lumenal extrinsic subunit, a novel PSII assembly intermediate was isolated in addition to the mature PSII complex. This new complex, which we refer to as PSII-Q4, contained four copies of the PsbQ protein per PSII monomer, instead of the expected one copy. In addition, PSII-Q4 lacked two other lumenal extrinsic proteins, PsbU and PsbV, which are present in the mature PSII complex. We suggest that PSII-Q4 is a late PSII assembly intermediate that is formed just before the binding of PsbU and PsbV, and we incorporate these results into an updated model of PSII assembly.

Crystal structure of CyanoQ from the thermophilic cyanobacterium Thermosynechococcus elongatus and detection in isolated photosystem II complexes

The PsbQ-like protein, termed CyanoQ, found in the cyanobacterium Synechocystis sp. PCC 6803 is thought to bind to the lumenal surface of photosystem II (PSII), helping to shield the Mn4CaO5 oxygen-evolving cluster. CyanoQ is, however, absent from the crystal structures of PSII isolated from thermophilic cyanobacteria raising the possibility that the association of CyanoQ with PSII might not be a conserved feature. Here, we show that CyanoQ (encoded by tll2057) is indeed expressed in the thermophilic cyanobacterium Thermosynechococcus elongatus and provide evidence in support of its assignment as a lipoprotein. Using an immunochemical approach, we show that CyanoQ co-purifies with PSII and is actually present in highly pure PSII samples used to generate PSII crystals. The absence of CyanoQ in the final crystal structure is possibly due to detachment of CyanoQ during crystallisation or its presence in sub-stoichiometric amounts. In contrast, the PsbP homologue, CyanoP, is severely depleted in isolated PSII complexes. We have also determined the crystal structure of CyanoQ from T. elongatus to a resolution of 1.6 Å. It lacks bound metal ions and contains a four-helix up-down bundle similar to the ones found in Synechocystis CyanoQ and spinach PsbQ. However, the N-terminal region and extensive lysine patch that are thought to be important for binding of PsbQ to PSII are not conserved in T. elongatus CyanoQ.

Structure and function of the hydrophilic Photosystem II assembly proteins: Psb27, Psb28 and Ycf48

Photosystem II (PS II) is a macromolecular complex responsible for light-driven oxidation of water and reduction of plastoquinone as part of the photosynthetic electron transport chain found in thylakoid membranes. Each PS II complex is composed of at least 20 protein subunits and over 80 cofactors. The biogenesis of PS II requires further hydrophilic and membrane-spanning proteins which are not part of the active holoenzyme. Many of these biogenesis proteins make transient interactions with specific PS II assembly intermediates: sometimes these are essential for biogenesis while in other examples they are required for optimizing assembly of the mature complex. In this review the function and structure of the Psb27, Psb28 and Ycf48 hydrophilic assembly factors is discussed by combining structural, biochemical and physiological information. Each of these assembly factors has homologues in all oxygenic photosynthetic organisms. We provide a simple overview for the roles of these protein factors in cyanobacterial PS II assembly emphasizing their participation in both photosystem biogenesis and recovery from photodamage.

Phycobilisome Mobility and Its Role in the Regulation of Light Harvesting in Red Algae

Red algae represent an evolutionarily important group that gave rise to the whole red clade of photosynthetic organisms. They contain a unique combination of light-harvesting systems represented by a membrane-bound antenna and by phycobilisomes situated on thylakoid membrane surfaces. So far, very little has been revealed about the mobility of their phycobilisomes and the regulation of their light-harvesting system in general. Therefore, we carried out a detailed analysis of phycobilisome dynamics in several red alga strains and compared these results with the presence (or absence) of photoprotective mechanisms. Our data conclusively prove phycobilisome mobility in two model mesophilic red alga strains, Porphyridium cruentum and Rhodella violacea. In contrast, there was almost no phycobilisome mobility in the thermophilic red alga Cyanidium caldarium that was not caused by a decrease in lipid desaturation in this extremophile. Experimental data attributed this immobility to the strong phycobilisome-photosystem interaction that highly restricted phycobilisome movement. Variations in phycobilisome mobility reflect the different ways in which light-harvesting antennae can be regulated in mesophilic and thermophilic red algae. Fluorescence changes attributed in cyanobacteria to state transitions were observed only in mesophilic P. cruentum with mobile phycobilisomes, and they were absent in the extremophilic C. caldarium with immobile phycobilisomes. We suggest that state transitions have an important regulatory function in mesophilic red algae; however, in thermophilic red algae, this process is replaced by nonphotochemical quenching.

Mobility of photosynthetic proteins

The mobility of photosynthetic proteins represents an important factor that affects light-energy conversion in photosynthesis. The specific feature of photosynthetic proteins mobility can be currently measured in vivo using advanced microscopic methods, such as fluorescence recovery after photobleaching which allows the direct observation of photosynthetic proteins mobility on a single cell level. The heterogeneous organization of thylakoid membrane proteins results in heterogeneity in protein mobility. The thylakoid membrane contains both, protein-crowded compartments with immobile proteins and fluid areas (less crowded by proteins), allowing restricted diffusion of proteins. This heterogeneity represents an optimal balance as protein crowding is necessary for efficient light-energy conversion, and protein mobility plays an important role in the regulation of photosynthesis. The mobility is required for an optimal light-harvesting process (e.g., during state transitions), and also for transport of proteins during their synthesis or repair. Protein crowding is then a key limiting factor of thylakoid membrane protein mobility; the less thylakoid membranes are crowded by proteins, the higher protein mobility is observed. Mobility of photosynthetic proteins outside the thylakoid membrane (lumen and stroma/cytosol) is less understood. Cyanobacterial phycobilisomes attached to the stromal side of the thylakoid can move relatively fast. Therefore, it seems that stroma with their active enzymes of the Calvin-Benson cycle, are a more fluid compartment in comparison to the rather rigid thylakoid lumen. In conclusion, photosynthetic protein diffusion is generally slower in comparison to similarly sized proteins from other eukaryotic membranes or organelles. Mobility of photosynthetic proteins resembles restricted protein diffusion in bacteria, and has been rationalized by high protein crowding similar to that of thylakoids.