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

Structure of the red-shifted Fittonia albivenis photosystem I

Photosystem I (PSI) from Fittonia albivenis, an Acanthaceae ornamental plant, is notable among green plants for its red-shifted emission spectrum. Here, we solved the structure of a PSI-light harvesting complex I (LHCI) supercomplex from F. albivenis at 2.46-Å resolution using cryo-electron microscopy. The supercomplex contains a core complex of 14 subunits and an LHCI belt with four antenna subunits (Lhca1-4) similar to previously reported angiosperm PSI-LHCI structures; however, Lhca3 differs in three regions surrounding a dimer of low-energy chlorophylls (Chls) termed red Chls, which absorb far-red beyond visible light. The unique amino acid sequences within these regions are exclusively shared by plants with strongly red-shifted fluorescence emission, suggesting candidate structural elements for regulating the energy state of red Chls. These results provide a structural basis for unraveling the mechanisms of light harvest and transfer in PSI-LHCI of under canopy plants and for designing Lhc to harness longer-wavelength light in the far-red spectral range.

Structure of plant photosystem I in a native assembly state defines PsaF as a regulatory checkpoint

Plant photosystem I (PSI) consists of at least 13 nuclear-encoded and 4 chloroplast-encoded subunits that together act as a sunlight-driven oxidoreductase. Here we report the structure of a PSI assembly intermediate that we isolated from greening oat seedlings. The assembly intermediate shows an absence of at least eight subunits, including PsaF and LHCI, and lacks photoreduction activity. The data show that PsaF is a regulatory checkpoint that promotes the assembly of LHCI, effectively coupling biogenesis to function.

Impact of Peripheral Hydrogen Bond on Electronic Properties of the Primary Acceptor Chlorophyll in the Reaction Center of Photosystem I

Photosystem I (PS I) is a photosynthetic pigment-protein complex that absorbs light and uses the absorbed energy to initiate electron transfer. Electron transfer has been shown to occur concurrently along two (A- and B-) branches of reaction center (RC) cofactors. The electron transfer chain originates from a special pair of chlorophyll a molecules (P700), followed by two chlorophylls and one phylloquinone in each branch (denoted as A-1, A0, A1, respectively), converging in a single iron-sulfur complex Fx. While there is a consensus that the ultimate electron donor-acceptor pair is P700+A0-, the involvement of A-1 in electron transfer, as well as the mechanism of the very first step in the charge separation sequence, has been under debate. To resolve this question, multiple groups have targeted electron transfer cofactors by site-directed mutations. In this work, the peripheral hydrogen bonds to keto groups of A0 chlorophylls have been disrupted by mutagenesis. Four mutants were generated: PsaA-Y692F; PsaB-Y667F; PsaB-Y667A; and a double mutant PsaA-Y692F/PsaB-Y667F. Contrary to expectations, but in agreement with density functional theory modeling, the removal of the hydrogen bond by Tyr → Phe substitution was found to have a negligible effect on redox potentials and optical absorption spectra of respective chlorophylls. In contrast, Tyr → Ala substitution was shown to have a fatal effect on the PS I function. It is thus inferred that PsaA-Y692 and PsaB-Y667 residues have primarily structural significance, and their ability to coordinate respective chlorophylls in electron transfer via hydrogen bond plays a minor role.

Uncovering the photosystem I assembly pathway in land plants

Photosystem I (PSI) is one of two large pigment-protein complexes responsible for converting solar energy into chemical energy in all oxygenic photosynthetic organisms. The PSI supercomplex consists of the PSI core complex and peripheral light-harvesting complex I (LHCI) in eukaryotic photosynthetic organisms. However, how the PSI complex assembles in land plants is unknown. Here we describe PHOTOSYSTEM I BIOGENESIS FACTOR 8 (PBF8), a thylakoid-anchored protein in Arabidopsis thaliana that is required for PSI assembly. PBF8 regulates two key consecutive steps in this process, the building of two assembly intermediates comprising eight or nine subunits, by interacting with PSI core subunits. We identified putative PBF8 orthologues in charophytic algae and land plants but not in Cyanobacteria or Chlorophyta. Our data reveal the major PSI assembly pathway in land plants. Our findings suggest that novel assembly mechanisms evolved during plant terrestrialization to regulate PSI assembly, perhaps as a means to cope with terrestrial environments.

Structures and organizations of PSI-AcpPCI supercomplexes from red tidal and coral symbiotic photosynthetic dinoflagellates

Marine photosynthetic dinoflagellates are a group of successful phytoplankton that can form red tides in the ocean and also symbiosis with corals. These features are closely related to the photosynthetic properties of dinoflagellates. We report here three structures of photosystem I (PSI)-chlorophylls (Chls) a/c-peridinin protein complex (PSI-AcpPCI) from two species of dinoflagellates by single-particle cryoelectron microscopy. The crucial PsaA/B subunits of a red tidal dinoflagellate Amphidinium carterae are remarkably smaller and hence losing over 20 pigment-binding sites, whereas its PsaD/F/I/J/L/M/R subunits are larger and coordinate some additional pigment sites compared to other eukaryotic photosynthetic organisms, which may compensate for the smaller PsaA/B subunits. Similar modifications are observed in a coral symbiotic dinoflagellate Symbiodinium species, where two additional core proteins and fewer AcpPCIs are identified in the PSI-AcpPCI supercomplex. The antenna proteins AcpPCIs in dinoflagellates developed some loops and pigment sites as a result to accommodate the changed PSI core, therefore the structures of PSI-AcpPCI supercomplex of dinoflagellates reveal an unusual protein assembly pattern. A huge pigment network comprising Chls a and c and various carotenoids is revealed from the structural analysis, which provides the basis for our deeper understanding of the energy transfer and dissipation within the PSI-AcpPCI supercomplex, as well as the evolution of photosynthetic organisms.

Structural insights into photosystem II supercomplex and trimeric FCP antennae of a centric diatom Cyclotella meneghiniana

Diatoms are dominant marine algae and contribute around a quarter of global primary productivity, the success of which is largely attributed to their photosynthetic capacity aided by specific fucoxanthin chlorophyll-binding proteins (FCPs) to enhance the blue-green light absorption under water. We purified a photosystem II (PSII)-FCPII supercomplex and a trimeric FCP from Cyclotella meneghiniana (Cm) and solved their structures by cryo-electron microscopy (cryo-EM). The structures reveal detailed organizations of monomeric, dimeric and trimeric FCP antennae, as well as distinct assemblies of Lhcx6_1 and dimeric FCPII-H in PSII core. Each Cm-PSII-FCPII monomer contains an Lhcx6_1, an FCP heterodimer and other three FCP monomers, which form an efficient pigment network for harvesting energy. More diadinoxanthins and diatoxanthins are found in FCPs, which may function to quench excess energy. The trimeric FCP contains more chlorophylls c and fucoxanthins. These diversified FCPs and PSII-FCPII provide a structural basis for efficient light energy harvesting, transfer, and dissipation in C. meneghiniana.

Regulatory dynamics of the higher-plant PSI-LHCI supercomplex during state transitions

State transition is a fundamental light acclimation mechanism of photosynthetic organisms in response to the environmental light conditions. This process rebalances the excitation energy between photosystem I (PSI) and photosystem II through regulated reversible binding of the light-harvesting complex II (LHCII) to PSI. However, the structural reorganization of PSI-LHCI, the dynamic binding of LHCII, and the regulatory mechanisms underlying state transitions are less understood in higher plants. In this study, using cryoelectron microscopy we resolved the structures of PSI-LHCI in both state 1 (PSI-LHCI-ST1) and state 2 (PSI-LHCI-LHCII-ST2) from Arabidopsis thaliana. Combined genetic and functional analyses revealed novel contacts between Lhcb1 and PsaK that further enhanced the binding of the LHCII trimer to the PSI core with the known interactions between phosphorylated Lhcb2 and the PsaL/PsaH/PsaO subunits. Specifically, PsaO was absent in the PSI-LHCI-ST1 supercomplex but present in the PSI-LHCI-LHCII-ST2 supercomplex, in which the PsaL/PsaK/PsaA subunits undergo several conformational changes to strengthen the binding of PsaO in ST2. Furthermore, the PSI-LHCI module adopts a more compact configuration with shorter Mg-to-Mg distances between the chlorophylls, which may enhance the energy transfer efficiency from the peripheral antenna to the PSI core in ST2. Collectively, our work provides novel structural and functional insights into the mechanisms of light acclimation during state transitions in higher plants.

Evolutionary implications from lipids in membrane bilayers and photosynthetic complexes in cyanobacteria and chloroplasts

In biomembranes, lipids form bilayer structures that serve as the fluid matrix for membrane proteins and other hydrophobic compounds. Additionally, lipid molecules associate with membrane proteins and impact their structures and functions. In both cyanobacteria and the chloroplasts of plants and algae, the lipid bilayer of the thylakoid membrane consists of four distinct glycerolipid classes: monogalactosyldiacylglycerol, digalactosyldiacylglycerol, sulfoquinovosyldiacylglycerol, and phosphatidylglycerol. These lipids are also integral components of photosynthetic complexes such as photosystem II and photosystem I. The lipid-binding sites within the photosystems, as well as the lipid composition in the thylakoid membrane, are highly conserved between cyanobacteria and photosynthetic eukaryotes, and each lipid class has specific roles in oxygenic photosynthesis. This review aims to shed light on the potential evolutionary implications of lipid utilization in membrane lipid bilayers and photosynthetic complexes in oxygenic photosynthetic organisms.

Stimuli-Induced Subconformation Transformation of the PSI-LHCI Protein at Single-Molecule Resolution

Photosynthesis is a very important process for the current biosphere which can maintain such a subtle and stable circulatory ecosystem on earth through the transformation of energy and substance. Even though been widely studied in various aspects, the physiological activities, such as intrinsic structural vibration and self-regulation process to stress of photosynthetic proteins, are still not in-depth resolved in real-time. Herein, utilizing silicon nanowire biosensors with ultrasensitive temporal and spatial resolution, real-time responses of a single photosystem I-light harvesting complex I (PSI-LHCI) supercomplex of Pisum sativum to various conditions, including gradient variations in temperature, illumination, and electric field, are recorded. Under different temperatures, there is a bi-state switch process associated with the intrinsic thermal vibration behavior. When the variations of illumination and the bias voltage are applied, two additional shoulder states, probably derived from the self-conformational adjustment, are observed. Based on real-time monitoring of the dynamic processes of the PSI-LHCI supercomplex under various conditions, it is successively testified to promising nanotechnology for protein profiling and biological functional integration in photosynthesis studies.

Structural insights into a unique PSI-LHCI-LHCII-Lhcb9 supercomplex from moss Physcomitrium patens

Photosystem I (PSI) possesses a variable supramolecular organization among different photosynthetic organisms to adapt to different light environments. Mosses are evolutionary intermediates that diverged from aquatic green algae and evolved into land plants. The moss Physcomitrium patens (P. patens) has a light-harvesting complex (LHC) superfamily more diverse than those of green algae and higher plants. Here, we solved the structure of a PSI-LHCI-LHCII-Lhcb9 supercomplex from P. patens at 2.68 Å resolution using cryo-electron microscopy. This supercomplex contains one PSI-LHCI, one phosphorylated LHCII trimer, one moss-specific LHC protein, Lhcb9, and one additional LHCI belt with four Lhca subunits. The complete structure of PsaO was observed in the PSI core. One Lhcbm2 in the LHCII trimer interacts with PSI core through its phosphorylated N terminus, and Lhcb9 mediates assembly of the whole supercomplex. The complicated pigment arrangement provided important information for possible energy-transfer pathways from the peripheral antennae to the PSI core.

Mechanism of Absorption Wavelength Shift Depending on the Protonation State of the Acrylate Group in Chlorophyll c

Diatoms can use light in the blue-green region because they have chlorophyll c (Chlc) in light-harvesting antenna proteins, fucoxanthin and chlorophyll a/c-binding protein (FCP). Chlc has a protonatable acrylate group (-CH═CH-COOH/COO-) conjugated to the porphyrin ring. As the absorption wavelength of Chlc changes upon the protonation of the acrylate group, Chlc is a candidate component that is responsible for photoprotection in diatoms, which switches the FCP function between light-harvesting and energy-dissipation modes depending on the light intensity. Here, we investigate the mechanism by which the absorption wavelength of Chlc changes owing to the change in the protonation state of the acrylate group, using a quantum mechanical/molecular mechanical approach. The calculated absorption wavelength of the Soret band of protonated Chlc is ∼25 nm longer than that of deprotonated Chlc, which is due to the delocalization of the lowest (LUMO) and second lowest (LUMO+1) unoccupied molecular orbitals toward the acrylate group. These results suggest that in FCP, the decrease in pH on the lumenal side under high-light conditions leads to protonation of Chlc and thereby a red shift in the absorption wavelength.

LHCA4 residues surrounding red chlorophylls allow for fine-tuning of the spectral region for photosynthesis in Arabidopsis thaliana

Improving far-red light utilization could be an approach to increasing crop production under suboptimal conditions. In land plants, only a small part of far-red light can be used for photosynthesis, which is captured by the antenna proteins LHCAs of photosystem I (PSI) through the chlorophyll (Chl) pair a603 and a609. However, it is unknown how the energy level of Chls a603-a609 is fine-tuned by the local protein environment in vivo. In this study, we investigated how changing the amino acid ligand for Chl a603 in LHCA4, the most red-shifted LHCA in Arabidopsis thaliana, or one amino acid near Chl a609, affected the energy level of the resulting PSI-LHCI complexes in situ and in vitro. Substitutions of the Chl a603 ligand N99 caused a blue shift in fluorescence emission, whereas the E146Q substitution near Chl a609 expanded the emission range to the red. Purified PSI-LHCI complexes with N99 substitutions exhibited the same fluorescence emission maxima as their respective transgenic lines, while the extent of red shift in purified PSI-LHCI with the E146Q substitution was weaker than in the corresponding transgenic lines. We propose that substituting amino acids surrounding red Chls can tune their energy level higher or lower in vivo, while shifting the absorption spectrum more to the red could prove more difficult than shifting to the blue end of the spectrum. Here, we report the first in vivo exploration of changing the local protein environment on the energy level of the red Chls, providing new clues for engineering red/blue-shifted crops.

Spectral diversity of photosystem I from flowering plants

Photosystem I and II (PSI and PSII) work together to convert solar energy into chemical energy. Whilst a lot of research has been done to unravel variability of PSII fluorescence in response to biotic and abiotic factors, the contribution of PSI to in vivo fluorescence measurements has often been neglected or considered to be constant. Furthermore, little is known about how the absorption and emission properties of PSI from different plant species differ. In this study, we have isolated PSI from five plant species and compared their characteristics using a combination of optical and biochemical techniques. Differences have been identified in the fluorescence emission spectra and at the protein level, whereas the absorption spectra were virtually the same in all cases. In addition, the emission spectrum of PSI depends on temperature over a physiologically relevant range from 280 to 298 K. Combined, our data show a critical comparison of the absorption and emission properties of PSI from various plant species.

Algal photosystem I dimer and high-resolution model of PSI-plastocyanin complex

Photosystem I (PSI) enables photo-electron transfer and regulates photosynthesis in the bioenergetic membranes of cyanobacteria and chloroplasts. Being a multi-subunit complex, its macromolecular organization affects the dynamics of photosynthetic membranes. Here we reveal a chloroplast PSI from the green alga Chlamydomonas reinhardtii that is organized as a homodimer, comprising 40 protein subunits with 118 transmembrane helices that provide scaffold for 568 pigments. Cryogenic electron microscopy identified that the absence of PsaH and Lhca2 gives rise to a head-to-head relative orientation of the PSI-light-harvesting complex I monomers in a way that is essentially different from the oligomer formation in cyanobacteria. The light-harvesting protein Lhca9 is the key element for mediating this dimerization. The interface between the monomers is lacking PsaH and thus partially overlaps with the surface area that would bind one of the light-harvesting complex II complexes in state transitions. We also define the most accurate available PSI-light-harvesting complex I model at 2.3 Å resolution, including a flexibly bound electron donor plastocyanin, and assign correct identities and orientations to all the pigments, as well as 621 water molecules that affect energy transfer pathways.

Structure of cyanobacterial photosystem I complexed with ferredoxin at 1.97 Å resolution

Photosystem I (PSI) is a light driven electron pump transferring electrons from Cytochrome c₆ (Cyt c₆) to Ferredoxin (Fd). An understanding of this electron transfer process is hampered by a paucity of structural detail concerning PSI:Fd interface and the possible binding sites of Cyt c₆. Here we describe the high resolution cryo-EM structure of Thermosynechococcus elongatus BP-1 PSI in complex with Fd and a loosely bound Cyt c₆. Side chain interactions at the PSI:Fd interface including bridging water molecules are visualized in detail. The structure explains the properties of mutants of PsaE and PsaC that affect kinetics of Fd binding and suggests a molecular switch for the dissociation of Fd upon reduction. Calorimetry-based thermodynamic analyses confirms a single binding site for Fd and demonstrates that PSI:Fd complexation is purely driven by entropy. A possible reaction cycle for the efficient transfer of electrons from Cyt c₆ to Fd via PSI is proposed.

In order to aid the understanding of the electron transfer process within the cyanobacterial photosystem I, its structure - when complexed with Ferredoxin - is determined at 1.97 Å resolution.

Lipids in photosynthetic protein complexes in the thylakoid membrane of plants, algae, and cyanobacteria

In the thylakoid membrane of cyanobacteria and chloroplasts, many proteins involved in photosynthesis are associated with or integrated into the fluid bilayer matrix formed by four unique glycerolipid classes, monogalactosyldiacylglycerol, digalactosyldiacylglycerol, sulfoquinovosyldiacylglycerol, and phosphatidylglycerol. Biochemical and molecular genetic studies have revealed that these glycerolipids play essential roles not only in the formation of thylakoid lipid bilayers but also in the assembly and functions of photosynthetic complexes. Moreover, considerable advances in structural biology have identified a number of lipid molecules within the photosynthetic complexes such as PSI and PSII. These data have provided important insights into the association of lipids with protein subunits in photosynthetic complexes and the distribution of lipids in the thylakoid membrane. Here, we summarize recent high-resolution observations of lipid molecules in the structures of photosynthetic complexes from plants, algae, and cyanobacteria, and evaluate the distribution of lipids among photosynthetic protein complexes and thylakoid lipid bilayers. By integrating the structural information into the findings from biochemical and molecular genetic studies, we highlight the conserved and differentiated roles of lipids in the assembly and functions of photosynthetic complexes among plants, algae, and cyanobacteria.

Structural elucidation of vascular plant photosystem I and its functional implications

In vascular plants, bryophytes and algae, the photosynthetic light reaction takes place in the thylakoid membrane where two transmembrane supercomplexes PSII and PSI work together with cytochrome b 6 f and ATP synthase to harvest the light energy and produce ATP and NADPH. Vascular plant PSI is a 600-kDa protein-pigment supercomplex, the core complex of which is partly surrounded by peripheral light-harvesting complex I (LHCI) that captures sunlight and transfers the excitation energy to the core to be used for charge separation. PSI is unique mainly in absorption of longer-wavelengths than PSII, fast excitation energy transfer including uphill energy transfer, and an extremely high quantum efficiency. From the early 1980s, a lot of effort has been dedicated to structural and functional studies of PSI-LHCI, leading to the current understanding of how more than 200 cofactors are kept at the correct distance and geometry to facilitate fast energy transfer in this supercomplex at an atomic level. In this review, we review the history of studies on vascular plant PSI-LHCI, summarise the present research progress on its structure, and present some new and further questions to be answered in future studies.

Structure of a tetrameric photosystem I from a glaucophyte alga Cyanophora paradoxa

Photosystem I (PSI) is one of the two photosystems functioning in light-energy harvesting, transfer, and electron transfer in photosynthesis. However, the oligomerization state of PSI is variable among photosynthetic organisms. We present a 3.8-Å resolution cryo-electron microscopic structure of tetrameric PSI isolated from the glaucophyte alga Cyanophora paradoxa, which reveals differences with PSI from other organisms in subunit composition and organization. The PSI tetramer is organized in a dimer of dimers with a C2 symmetry. Unlike cyanobacterial PSI tetramers, two of the four monomers are rotated around 90°, resulting in a completely different pattern of monomer-monomer interactions. Excitation-energy transfer among chlorophylls differs significantly between Cyanophora and cyanobacterial PSI tetramers. These structural and spectroscopic features reveal characteristic interactions and excitation-energy transfer in the Cyanophora PSI tetramer, suggesting that the Cyanophora PSI could represent a turning point in the evolution of PSI from prokaryotes to eukaryotes.

Cryo-EM structure of a tetrameric photosystem I from Chroococcidiopsis TS-821, a thermophilic, unicellular, non-heterocyst-forming cyanobacterium

Photosystem I (PSI) is one of two photosystems involved in oxygenic photosynthesis. PSI of cyanobacteria exists in monomeric, trimeric, and tetrameric forms, in contrast to the strictly monomeric form of PSI in plants and algae. The tetrameric organization raises questions about its structural, physiological, and evolutionary significance. Here we report the ∼3.72 Å resolution cryo-electron microscopy structure of tetrameric PSI from the thermophilic, unicellular cyanobacterium Chroococcidiopsis sp. TS-821. The structure resolves 44 subunits and 448 cofactor molecules. We conclude that the tetramer is arranged via two different interfaces resulting from a dimer-of-dimers organization. The localization of chlorophyll molecules permits an excitation energy pathway within and between adjacent monomers. Bioinformatics analysis reveals conserved regions in the PsaL subunit that correlate with the oligomeric state. Tetrameric PSI may function as a key evolutionary step between the trimeric and monomeric forms of PSI organization in photosynthetic organisms.

Structure, Function, and Variations of the Photosystem I-Antenna Supercomplex from Different Photosynthetic Organisms

Photosystem I (PSI) is a protein complex functioning in light-induced charge separation, electron transfer, and reduction reactions of ferredoxin in photosynthesis, which finally results in the reduction of NAD(P)^- to NAD(P)H required for the fixation of carbon dioxide. In eukaryotic algae, PSI is associated with light-harvesting complex I (LHCI) subunits, forming a PSI-LHCI supercomplex. LHCI harvests and transfers light energy to the PSI core, where charge separation and electron transfer reactions occur. During the course of evolution, the number and sequences of protein subunits and the pigments they bind in LHCI change dramatically depending on the species of organisms, which is a result of adaptation of organisms to various light environments. In this chapter, I will describe the structure of various PSI-LHCI supercomplexes from different organisms solved so far either by X-ray crystallography or by cryo-electron microscopy, with emphasis on the differences in the number, structures, and association patterns of LHCI subunits associated with the PSI core found in different organisms.

Architecture of the chloroplast PSI-NDH supercomplex in Hordeum vulgare

The chloroplast NADH dehydrogenase-like (NDH) complex is composed of at least 29 subunits and has an important role in mediating photosystem I (PSI) cyclic electron transport (CET)^1-3. The NDH complex associates with PSI to form the PSI-NDH supercomplex and fulfil its function. Here, we report cryo-electron microscopy structures of a PSI-NDH supercomplex from barley (Hordeum vulgare). The structures reveal that PSI-NDH is composed of two copies of the PSI-light-harvesting complex I (LHCI) subcomplex and one NDH complex. Two monomeric LHCI proteins, Lhca5 and Lhca6, mediate the binding of two PSI complexes to NDH. Ten plant chloroplast-specific NDH subunits are presented and their exact positions as well as their interactions with other subunits in NDH are elucidated. In all, this study provides a structural basis for further investigations on the functions and regulation of PSI-NDH-dependent CET.

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.

Key Targets for Improving Algal Biofuel Production

A number of technological challenges need to be overcome if algae are to be utilized for commercial fuel production. Current economic assessment is largely based on laboratory scale up or commercial systems geared to the production of high value products, since no industrial scale plant exits that are dedicated to algal biofuel. For macroalgae (‘seaweeds’), the most promising processes are anaerobic digestion for biomethane production and fermentation for bioethanol, the latter with levels exceeding those from sugar cane. Currently, both processes could be enhanced by increasing the rate of degradation of the complex polysaccharide cell walls to generate fermentable sugars using specifically tailored hydrolytic enzymes. For microalgal biofuel production, open raceway ponds are more cost-effective than photobioreactors, with CO2 and harvesting/dewatering costs estimated to be ~50% and up to 15% of total costs, respectively. These costs need to be reduced by an order of magnitude if algal biodiesel is to compete with petroleum. Improved economics could be achieved by using a low-cost water supply supplemented with high glucose and nutrients from food grade industrial wastewater and using more efficient flocculation methods and CO2 from power plants. Solar radiation of not <3000 h·yr−1 favours production sites 30° north or south of the equator and should use marginal land with flat topography near oceans. Possible geographical sites are discussed. In terms of biomass conversion, advances in wet technologies such as hydrothermal liquefaction, anaerobic digestion, and transesterification for algal biodiesel are presented and how these can be integrated into a biorefinery are discussed.