Structural basis for energy transfer in a huge diatom PSI-FCPI supercomplex

Dimeric and high-resolution structures of Chlamydomonas Photosystem I from a temperature-sensitive Photosystem II mutant

Water molecules play a pivotal functional role in photosynthesis, primarily as the substrate for Photosystem II (PSII). However, their importance and contribution to Photosystem I (PSI) activity remains obscure. Using a high-resolution cryogenic electron microscopy (cryo-EM) PSI structure from a Chlamydomonas reinhardtii temperature-sensitive photoautotrophic PSII mutant (TSP4), a conserved network of water molecules - dating back to cyanobacteria - was uncovered, mainly in the vicinity of the electron transport chain (ETC). The high-resolution structure illustrated that the water molecules served as a ligand in every chlorophyll that was missing a fifth magnesium coordination in the PSI core and in the light-harvesting complexes (LHC). The asymmetric distribution of the water molecules near the ETC branches modulated their electrostatic landscape, distinctly in the space between the quinones and FX. The data also disclosed the first observation of eukaryotic PSI oligomerisation through a low-resolution PSI dimer that was comprised of PSI-10LHC and PSI-8LHC.

Caspy et al. report the structure of PSI from a temperature-sensitive photoautotrophic PSII mutant of Chlamydomonas reinhardtii (TSP4), and report the distribution of conserved water molecules in the structure from cyanobacterial to higher plant PSI. They suggest that the asymmetric distribution of water molecules near the electron transfer chain modulates the electron transfer from quinones to FX.

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.

Characterization of chloroplast genome of the marine diatom Chaetoceros gracilis

In the present study, the chloroplast genome of Chaetoceros gracilis was sequenced using the PacBio sequencing platform and phylogenetic analysis was conducted using 38 other complete chloroplast genomes of the Bacillariophyta. The chloroplast genome of C. gracilis was 116,421 bp in length with the typical quadripartite structure, including a large single copy (LSC) region of 61,904 bp, a small single copy (SSC) region of 39,367 bp, and a pair of inverted repeats (IR) regions of 7575 bp. The overall GC content of C. gracilis chloroplast genome was 30.79%. This genome encoded 131 genes incuding 93 protein-coding genes, 30 transfer RNA (tRNA) genes and 8 ribosomal RNA (rRNA) genes. Phylogenetic results exhibited that three Chaetoceros species were clustered together. Chaetoceros gracilis was closely related with Chaetoceros muelleri, and then formed a clade with Chaetoceros simplex with 100% bootstrap value This study will facilitate species identification and study of evolutionary in the family Chaetoceroceae.

The Critical Studies of Fucoxanthin Research Trends from 1928 to June 2021: A Bibliometric Review

Fucoxanthin is a major carotenoid in brown macroalgae and diatoms that possesses a broad spectrum of health benefits. This review evaluated the research trends of the fucoxanthin field from 1928 to June 2021 using the bibliometric method. The present findings unraveled that the fucoxanthin field has grown quickly in recent years with a total of 2080 publications. Japan was the most active country in producing fucoxanthin publications. Three Japan institutes were listed in the top ten productive institutions, with Hokkaido University being the most prominent institutional contributor in publishing fucoxanthin articles. The most relevant subject area on fucoxanthin was the agricultural and biological sciences category, while most fucoxanthin articles were published in Marine Drugs. A total of four research concepts emerged based on the bibliometric keywords analysis: “bioactivities”, “photosynthesis”, “optimization of process’’, and “environment”. The “bioactivities” of fucoxanthin was identified as the priority in future research. The current analysis highlighted the importance of collaboration and suggested that global collaboration could be the key to valorizing and efficiently boosting the consumer acceptability of fucoxanthin. The present bibliometric analysis offers valuable insights into the research trends of fucoxanthin to construct a better future development of this treasurable carotenoid.

A unique photosystem I reaction center from a chlorophyll d-containing cyanobacterium Acaryochloris marina

Photosystem I (PSI) is a large protein supercomplex that catalyzes the light-dependent oxidation of plastocyanin (or cytochrome c₆ ) and the reduction of ferredoxin. This catalytic reaction is realized by a transmembrane electron transfer chain consisting of primary electron donor (a special chlorophyll (Chl) pair) and electron acceptors A₀ , A₁ , and three Fe₄ S₄ clusters, F_X , F_A , and F_B . Here we report the PSI structure from a Chl d-dominated cyanobacterium Acaryochloris marina at 3.3 Å resolution obtained by single-particle cryo-electron microscopy. The A. marina PSI exists as a trimer with three identical monomers. Surprisingly, the structure reveals a unique composition of electron transfer chain in which the primary electron acceptor A₀ is composed of two pheophytin a rather than Chl a found in any other well-known PSI structures. A novel subunit Psa27 is observed in the A. marina PSI structure. In addition, 77 Chls, 13 α-carotenes, two phylloquinones, three Fe-S clusters, two phosphatidyl glycerols, and one monogalactosyl-diglyceride were identified in each PSI monomer. Our results provide a structural basis for deciphering the mechanism of photosynthesis in a PSI complex with Chl d as the dominating pigments and absorbing far-red light.

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.

Revealing the architecture of the photosynthetic apparatus in the diatom Thalassiosira pseudonana

Diatoms are a large group of marine algae that are responsible for about one-quarter of global carbon fixation. Light-harvesting complexes of diatoms are formed by the fucoxanthin chlorophyll a/c proteins and their overall organization around core complexes of photosystems (PSs) I and II is unique in the plant kingdom. Using cryo-electron tomography, we have elucidated the structural organization of PSII and PSI supercomplexes and their spatial segregation in the thylakoid membrane of the model diatom species Thalassiosira pseudonana. 3D sub-volume averaging revealed that the PSII supercomplex of T. pseudonana incorporates a trimeric form of light-harvesting antenna, which differs from the tetrameric antenna observed previously in another diatom, Chaetoceros gracilis. Surprisingly, the organization of the PSI supercomplex is conserved in both diatom species. These results strongly suggest that different diatom classes have various architectures of PSII as an adaptation strategy, whilst a convergent evolution occurred concerning PSI and the overall plastid structure.

Structure of plant photosystem I-light harvesting complex I supercomplex at 2.4 Å resolution

Photosystem I (PSI) is one of the two photosystems in photosynthesis, and performs a series of electron transfer reactions leading to the reduction of ferredoxin. In higher plants, PSI is surrounded by four light-harvesting complex I (LHCI) subunits, which harvest and transfer energy efficiently to the PSI core. The crystal structure of PSI-LHCI supercomplex has been analyzed up to 2.6 Å resolution, providing much information on the arrangement of proteins and cofactors in this complicated supercomplex. Here we have optimized crystallization conditions, and analyzed the crystal structure of PSI-LHCI at 2.4 Å resolution. Our structure showed some shift of the LHCI, especially the Lhca4 subunit, away from the PSI core, suggesting the indirect connection and inefficiency of energy transfer from this Lhca subunit to the PSI core. We identified five new lipids in the structure, most of them are located in the gap region between the Lhca subunits and the PSI core. These lipid molecules may play important roles in binding of the Lhca subunits to the core, as well as in the assembly of the supercomplex. The present results thus provide novel information for the elucidation of the mechanisms for the light-energy harvesting, transfer and assembly of this supercomplex.

Rapid Sorting of Fucoxanthin-Producing Phaeodactylum tricornutum Mutants by Flow Cytometry

Fucoxanthin, which is widely found in seaweeds and diatoms, has many benefits to human health, such as anti-diabetes, anti-obesity, and anti-inflammatory physiological activities. However, the low content of fucoxanthin in brown algae and diatoms limits the commercialization of this product. In this study, we introduced an excitation light at 488 nm to analyze the emitted fluorescence of Phaeodactylum tricornutum, a diatom model organism rich in fucoxanthin. We observed a unique spectrum peak at 710 nm and found a linear correlation between fucoxanthin content and the mean fluorescence intensity. We subsequently used flow cytometry to screen high-fucoxanthin-content mutants created by heavy ion irradiation. After 20 days of cultivation, the fucoxanthin content of sorted cells was 25.5% higher than in the wild type. This method provides an efficient, rapid, and high-throughput approach to screen fucoxanthin-overproducing mutants.

Antenna arrangement and energy-transfer pathways of PSI-LHCI from the moss Physcomitrella patens

Plants harvest light energy utilized for photosynthesis by light-harvesting complex I and II (LHCI and LHCII) surrounding photosystem I and II (PSI and PSII), respectively. During the evolution of green plants, moss is at an evolutionarily intermediate position from aquatic photosynthetic organisms to land plants, being the first photosynthetic organisms that landed. Here, we report the structure of the PSI–LHCI supercomplex from the moss Physcomitrella patens (Pp) at 3.23 Å resolution solved by cryo-electron microscopy. Our structure revealed that four Lhca subunits are associated with the PSI core in an order of Lhca1–Lhca5–Lhca2–Lhca3. This number is much decreased from 8 to 10, the number of subunits in most green algal PSI–LHCI, but the same as those of land plants. Although Pp PSI–LHCI has a similar structure as PSI–LHCI of land plants, it has Lhca5, instead of Lhca4, in the second position of Lhca, and several differences were found in the arrangement of chlorophylls among green algal, moss, and land plant PSI–LHCI. One chlorophyll, PsaF–Chl 305, which is found in the moss PSI–LHCI, is located at the gap region between the two middle Lhca subunits and the PSI core, and therefore may make the excitation energy transfer from LHCI to the core more efficient than that of land plants. On the other hand, energy-transfer paths at the two side Lhca subunits are relatively conserved. These results provide a structural basis for unravelling the mechanisms of light-energy harvesting and transfer in the moss PSI–LHCI, as well as important clues on the changes of PSI–LHCI after landing.

Enhancement of excitation-energy quenching in fucoxanthin chlorophyll a/c-binding proteins isolated from a diatom Phaeodactylum tricornutum upon excess-light illumination

Photosynthetic organisms regulate pigment composition and molecular oligomerization of light-harvesting complexes in response to solar light intensities, in order to improve light-harvesting efficiency. Here we report excitation-energy dynamics and relaxation of fucoxanthin chlorophyll a/c-binding protein (FCP) complexes isolated from a diatom Phaeodactylum tricornutum grown under high-light (HL) illumination. Two types of FCP complexes were prepared from this diatom under the HL condition, whereas one FCP complex was isolated from the cells grown under a low-light (LL) condition. The subunit composition and oligomeric states of FCP complexes under the HL condition are different from those under the LL condition. Absorption and fluorescence spectra at 77 K of the FCP complexes also vary between the two conditions, indicating modifications of the pigment composition and arrangement upon the HL illumination. Time-resolved fluorescence curves at 77 K of the FCP complexes under the HL condition showed shorter lifetime components compared with the LL condition. Fluorescence decay-associated spectra at 77 K showed distinct excitation-energy-quenching components and alterations of energy-transfer pathways in the FCP complexes under the HL condition. These findings provide insights into molecular and functional mechanisms of the dynamic regulation of FCPs in this diatom under excess-light conditions.