Proteases are associated with a minor fucoxanthin chlorophyll a/c-binding protein from the diatom, Chaetoceros gracilis

Structural and spectroscopic insights into fucoxanthin chlorophyll a/c-binding proteins of diatoms in diverse oligomeric states

Diatoms, a group of prevalent marine algae, contribute significantly to global primary productivity. Their substantial biomass is linked to enhanced absorption of blue-green light underwater, facilitated by fucoxanthin chlorophyll (Chl) a/c-binding proteins (FCPs), which exhibit oligomeric diversity across diatom species. Using mild clear native PAGE analysis of solubilized thylakoid membranes, we displayed monomeric, dimeric, trimeric, tetrameric, and pentameric FCPs in diatoms. Mass spectrometry analysis revealed that each oligomeric FCP has a specific protein composition, and together they constitute a large Lhcf family of FCP antennas. In addition, we resolved the structures of the Thalassiosira pseudonana FCP (Tp-FCP) homotrimer and the Chaetoceros gracilis FCP (Cg-FCP) pentamer by cryoelectron microscopy at 2.73-Å and 2.65-Å resolution, respectively. The distinct pigment compositions and organizations of various oligomeric FCPs affect their blue-green light-harvesting, excitation energy transfer pathways. Compared with dimeric and trimeric FCPs, the Cg-FCP tetramer and Cg-FCP pentamer exhibit stronger absorption by Chl c, redshifted and broader Chl a fluorescence emission, and more robust circular dichroism signals originating from Chl a-carotenoid dimers. These spectroscopic characteristics indicate that Chl a molecules in the Cg-FCP tetramer and Cg-FCP pentamer are more heterogeneous than in both dimers and the Tp-FCP trimer. The structural and spectroscopic insights provided by this study contribute to a better understanding of the mechanisms that empower diatoms to adapt to fluctuating light environments.

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.

Expression of nuclear-encoded, haptophyte-derived ftsH genes support extremely rapid PSII repair and high-light photoacclimation in Karenia brevis (Dinophyceae)

Karenia brevis, a neurotoxic dinoflagellate that produces brevetoxins, is endemic to the Gulf of Mexico and can grow at high irradiances typical of surface waters found there. To build upon a growing number of studies addressing high-light tolerance in K. brevis, specific photobiology and molecular mechanisms underlying this capacity were evaluated in culture. Since photosystem II (PSII) repair cycle activity can be crucial to high light tolerance in plants and algae, the present study assessed this capacity in K. brevis and characterized the ftsH-like genes which are fundamental to this process. Compared with cultures grown in low-light, cultures grown in high-light showed a 65-fold increase in PSII photoinactivation, a ∼50-fold increase in PSII repair, enhanced nonphotochemical quenching (NPQ), and depressed F_v/F_m. Repair rates were among the fastest reported in phytoplankton. Publicly available K. brevis transcriptomes (MMETSP) were queried for ftsH-like sequences and refined with additional sequencing from two K. brevis strains. The genes were phylogenetically related to haptophyte orthologs, implicating acquisition during tertiary endosymbiosis. RT-qPCR of three of the four ftsH-like homologs revealed that poly-A tails predominated in all homologs, and that the most highly expressed homolog had a 5' splice leader and amino-acid motifs characteristic of chloroplast targeting, indicating nuclear encoding for this plastid-targeted gene. High-light cultures showed a ∼1.5-fold upregulation in mRNA expression of the thylakoid-associated genes. Overall, in conjunction with NPQ mechanisms, rapid PSII repair mediated by a haptophyte-derived ftsH prevents chronic photoinhibition in K. brevis. Our findings continue to build the case that high-light photobiology-supported by the acquisition and maintenance of tertiary endosymbiotic genes-is critical to the success of K. brevis in the Gulf of Mexico.

One-Step Preparative Separation of Fucoxanthin from Three Edible Brown Algae by Elution-Extrusion Countercurrent Chromatography

A method for batch preparation of fucoxanthin from brown algae was established, which possessed the advantages of high yield and high purity. The ultrasonic-assisted extraction method was used to obtain a crude extract from Sargassum fusiforme as the separation sample. Then the crude extract was separated by elution-extrusion countercurrent chromatography. The optimum preparation conditions of fucoxanthin were determined as follows: n-hexane-ethanol-water (20:9:11, v:v:v) as a two-phase solvent system, the mobile phase flow rate was 5 mL min-1, the revolution speed was 800 r min-1, the loading capacity was 60 mg 10 mL-1 and the temperature was 25 °C. By this method, 12.8 mg fucoxanthin with a purity of 94.72% was obtained from the crude extract of Sargassum fusiforme. In addition, when the loading capacity was 50 mg 10 mL-1, the purity of fucoxanthin reached 96.01%. Two types of by-products, chlorophyll and pheophytin, could also be obtained during the process of separation. This optimal method was further applied to separate fucoxanthin from Laminaria japonica and Undaria pinnatifida, and 6.0 mg and 9.7 mg fucoxanthin with a purity of 96.24% and 92.62% were acquired, respectively. Therefore, it was demonstrated that the preparation method of fucoxanthin established in this study had an applicability to brown algae, which improved the utilization value of raw materials.

Structural basis for different types of hetero-tetrameric light-harvesting complexes in a diatom PSII-FCPII supercomplex

Fucoxanthin chlorophyll (Chl) a/c-binding proteins (FCPs) function as light harvesters in diatoms. The structure of a diatom photosystem II-FCPII (PSII-FCPII) supercomplex have been solved by cryo-electron microscopy (cryo-EM) previously; however, the FCPII subunits that constitute the FCPII tetramers and monomers are not identified individually due to their low resolutions. Here, we report a 2.5 Å resolution structure of the PSII-FCPII supercomplex using cryo-EM. Two types of tetrameric FCPs, S-tetramer, and M-tetramer, are identified as different types of hetero-tetrameric complexes. In addition, three FCP monomers, m1, m2, and m3, are assigned to different gene products of FCP. The present structure also identifies the positions of most Chls c and diadinoxanthins, which form a complicated pigment network. Excitation-energy transfer from FCPII to PSII is revealed by time-resolved fluorescence spectroscopy. These structural and spectroscopic findings provide insights into an assembly model of FCPII and its excitation-energy transfer and quenching processes.

Fucoxanthin chlorophyll a/c-binding proteins (FCPs) harvest light energy in diatoms. The authors analyzed a structure of PSII-FCPII supercomplex at high resolution by cryo-EM, which identified each FCP subunit and pigment network in the supercomplex.

The genome of a nonphotosynthetic diatom provides insights into the metabolic shift to heterotrophy and constraints on the loss of photosynthesis

Although most of the tens of thousands of diatom species are photoautotrophs, a small number of heterotrophic species no longer photosynthesize. We sequenced the genome of a nonphotosynthetic diatom, Nitzschia Nitz4, to determine how carbon metabolism was altered in the wake of this trophic shift. Nitzschia Nitz4 has retained its plastid and plastid genome, but changes associated with the transition to heterotrophy were cellular-wide and included losses of photosynthesis-related genes from the nuclear and plastid genomes, elimination of isoprenoid biosynthesis in the plastid, and remodeling of mitochondrial glycolysis to maximize adenosine triphosphte (ATP) yield. The genome contains a β-ketoadipate pathway that may allow Nitzschia Nitz4 to metabolize lignin-derived compounds. Diatom plastids lack an oxidative pentose phosphate pathway (oPPP), leaving photosynthesis as the primary source of NADPH to support essential biosynthetic pathways in the plastid and, by extension, limiting available sources of NADPH in nonphotosynthetic plastids. The genome revealed similarities between nonphotosynthetic diatoms and apicomplexan parasites for provisioning NADPH in their plastids and highlighted the ancestral absence of a plastid oPPP as a potentially important constraint on loss of photosynthesis, a hypothesis supported by the higher frequency of transitions to parasitism or heterotrophy in lineages that have a plastid oPPP.

Effects of CO2 and temperature on photosynthetic performance in the diatom Chaetoceros gracilis

CO₂ concentration and temperature for growth of photosynthetic organisms are two important factors to ensure better photosynthetic performance. In this study, we investigated the effects of CO₂ concentration and temperature on the photosynthetic performance in a marine centric diatom Chaetoceros gracilis. Cells were grown under four different conditions, namely, at 25 °C with air bubbling, at 25 °C with a supplementation of 3% CO₂, at 30 °C with air bubbling, and at 30 °C with the CO₂ supplementation. It was found that the growth rate of cells at 30 °C with the CO₂ supplementation is faster than those at other three conditions. The pigment compositions of cells grown under the different conditions are altered, and fluorescence spectra measured at 77 K also showed different peak positions. A novel fucoxanthin chlorophyll a/c-binding protein complex is observed in the cells grown at 30 °C with the CO₂ supplementation but not in the other three types of cells. Since oxygen-evolving activities of the four types of cells are almost unchanged, it is suggested that the CO₂ supplementation and growth temperature are involved in the regulation of photosynthetic light-harvesting apparatus in C. gracilis at different degrees. Based on these observations, we discuss the favorable growth conditions for C. gracilis.

Basic pH-induced modification of excitation-energy dynamics in fucoxanthin chlorophyll a/c-binding proteins isolated from a pinguiophyte, Glossomastix chrysoplasta

Photosynthetic organisms have diversified light-harvesting complexes (LHCs) to collect solar energy efficiently, leading to an acquisition of their ecological niches. Herein we report on biochemical and spectroscopic characterizations of fucoxanthin chlorophyll a/c-binding protein (FCP) complexes isolated from a marine pinguiophyte Glossomastix chrysoplasta. The pinguiophyte FCP showed one subunit band in SDS-PAGE and one protein-complex band with a molecular weight at around 66 kDa in clear-native PAGE. By HPLC analysis, the FCP possesses chlorophylls a and c, fucoxanthin, and violaxanthin. To clarify excitation-energy-relaxation processes in the FCP, we measured time-resolved fluorescence spectra at 77 K of the FCP adapted to pH 5.0, 6.5, and 8.0. Fluorescence curves measured at pH 5.0 and 8.0 showed shorter lifetime components compared with those at pH 6.5. The rapid decay components at pH 5.0 and 8.0 are unveiled by fluorescence decay-associated (FDA) spectra; fluorescence decays occur in the 270 and 160-ps FDA spectra only at pH 5.0 and 8.0, respectively. In addition, energy-transfer pathways with time constants of tens of picoseconds are altered under the basic pH condition but not the acidic pH condition. These findings provide novel insights into pH-dependent energy-transfer and energy-quenching machinery in not only FCP family but also photosynthetic LHCs.

The pigment-protein network of a diatom photosystem II-light-harvesting antenna supercomplex

Diatoms play important roles in global primary productivity and biogeochemical cycling of carbon, in part owing to the ability of their photosynthetic apparatus to adapt to rapidly changing light intensity. We report a cryo-electron microscopy structure of the photosystem II (PSII)-fucoxanthin (Fx) chlorophyll (Chl) a/c binding protein (FCPII) supercomplex from the centric diatom Chaetoceros gracilis The supercomplex comprises two protomers, each with two tetrameric and three monomeric FCPIIs around a PSII core that contains five extrinsic oxygen-evolving proteins at the lumenal surface. The structure reveals the arrangement of a huge pigment network that contributes to efficient light energy harvesting, transfer, and dissipation processes in the diatoms.

Spectral Properties and Excitation Relaxation of Novel Fucoxanthin Chlorophyll a/c-Binding Protein Complexes

Fucoxanthin chlorophyll a/c-binding proteins (FCPs) are unique light harvesters for some photosynthetic organisms. There were several reports for the alterations of FCPs in response to light conditions. Here, we present the spectral profiles and excitation dynamics of novel FCP complexes isolated from the diatom Chaetoceros gracilis. Under a red-light condition, C. gracilis cells expressed three types of FCP complexes, two of which are very similar to FCP complexes found in the white-light grown cells, and one of which is the novel FCP complex. The combination of steady-state absorption and fluorescence spectra and time-resolved fluorescence spectra revealed that, compared to other types of FCP complexes, the novel FCP complexes exhibited red-shifted absorption and fluorescence spectra and fast decay of excitation. This finding will provide new insights into not only the light-harvesting strategies of diatoms but also the diversity of light adaptation machinery for photosynthetic organisms.

Light harvesting complexes in chlorophyll c-containing algae

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

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

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

Combined effects of ocean acidification and warming on physiological response of the diatom Thalassiosira pseudonana to light challenges

Diatoms are one of the most important groups of phytoplankton in terms of abundance and ecological functionality in the ocean. They usually dominate the phytoplankton communities in coastal waters and experience frequent and large fluctuations in light. In order to evaluate the combined effects of ocean warming and acidification on the diatom's exploitation of variable light environments, we grew a globally abundant diatom Thalassiosira pseudonana under two levels of temperature (18, 24 °C) and pCO₂ (400, 1000 μatm) to examine its physiological performance after light challenge. It showed that the higher temperature increased the photoinactivation rate in T. pseudonana at 400 μatm pCO₂, while the higher pCO₂ alleviated the negative effect of the higher temperature on PSII photoinactivation. Higher pCO₂ stimulated much faster PsbA removal, but it still lagged behind the photoinactivation of PSII under high light. Although the sustained phase of nonphotochemical quenching (NPQs) and activity of superoxide dismutase (SOD) were provoked during the high light exposure in T. pseudonana under the combined pCO₂ and temperature conditions, it could not offset the damage caused by these multiple environmental changes, leading to decreased maximum photochemical yield.

The photosynthetic cytochrome c 550 from the diatom Phaeodactylum tricornutum

The photosynthetic cytochrome c ₅₅₀ from the marine diatom Phaeodactylum tricornutum has been purified and characterized. Cytochrome c ₅₅₀ is mostly obtained from the soluble cell extract in relatively large amounts. In addition, the protein appeared to be truncated in the last hydrophobic residues of the C-terminus, both in the soluble cytochrome c ₅₅₀ and in the protein extracted from the membrane fraction, as deduced by mass spectrometry analysis and the comparison with the gene sequence. Interestingly, it has been described that the C-terminus of cytochrome c ₅₅₀ forms a hydrophobic finger involved in the interaction with photosystem II in cyanobacteria. Cytochrome c ₅₅₀ was almost absent in solubilized photosystem II complex samples, in contrast with the PsbO and Psb31 extrinsic subunits, thus suggesting a lower affinity of cytochrome c ₅₅₀ for the photosystem II complex. Under iron-limiting conditions the amount of cytochrome c ₅₅₀ decreases up to about 45% as compared to iron-replete cells, pointing to an iron-regulated synthesis. Oxidized cytochrome c ₅₅₀ has been characterized using continuous wave EPR and pulse techniques, including HYSCORE, and the obtained results have been interpreted in terms of the electrostatic charge distribution in the surroundings of the heme centre.

Conversion of photosystem II dimer to monomers during photoinhibition is tightly coupled with decrease in oxygen-evolving activity in the diatom Chaetoceros gracilis

The rapid turnover of photosystem II (PSII) in diatoms is thought to be at an exceptionally high rate compared with other oxyphototrophs; however, its molecular mechanisms are largely unknown. In this study, we examined the photodamage and repair processes of PSII in the marine centric diatom Chaetoceros gracilis incubated at 30 or 300 μmol photons m^-2 s^-1 in the presence of a de novo protein-synthesis inhibitor. When de novo protein synthesis was blocked by chloramphenicol (Cm), oxygen-evolving activity gradually decreased even at 30 μmol photons m^-2 s^-1 and could not be detected at 12 h. PSII inactivation was enhanced by higher illumination. Using Cm-treated cells, the conversion of PSII dimer to monomers was observed by blue native PAGE. The rate of PSII monomerization was very similar to that of the decrease in oxygen-evolving activity under both light conditions. Immunological detection of D1 protein in the Cm-treated cells showed that the rate of D1 degradation was slower than that of the former two events, although it was more rapid than that observed in other oxyphototrophs. Thus, the three accelerated events, especially PSII monomerization, appear to cause the unusually high rate of PSII turnover in diatoms.

Utilization of light by fucoxanthin-chlorophyll-binding protein in a marine centric diatom, Chaetoceros gracilis

The major light-harvesting pigment protein complex (fucoxanthin-chlorophyll-binding protein complex; FCP) was purified from a marine centric diatom, Chaetoceros gracilis, by mild solubilization followed by sucrose density gradient centrifugation, and then characterized. The dynamic light scattering measurement showed unimodality, indicating that the complex was highly purified. The amount of chlorophyll a (Chl a) bound to the purified FCP accounted for more than 60 % of total cellular Chl a. The complex was composed of three abundant polypeptides, although there are nearly 30 FCP-related genes. The two major components were identified as Fcp3 (Lhcf3)- and Fcp4 (Lhcf4)-equivalent proteins based on their internal amino acid sequences and a two-dimensional isoelectric focusing electrophoresis analysis developed in this work. Compared with the thylakoids, the FCP complex showed higher contents of fucoxanthin and chlorophyll c but lower contents of the xanthophyll cycle pigments diadinoxanthin and diatoxanthin. Fluorescence excitation spectra analyses indicated that light harvesting, rather than photosystem protection, is the major function of the purified FCP complex, which is associated with more than 60 % of total cellular Chl a. These findings suggest that the huge amount of Chl bound to the FCP complex composed of Lhcf3, Lhcf4, and an unidentified minor protein has a light-harvesting function to allow efficient photosynthesis under the dim-light conditions in the ocean.

The nitrogen costs of photosynthesis in a diatom under current and future pCO2

With each cellular generation, oxygenic photoautotrophs must accumulate abundant protein complexes that mediate light capture, photosynthetic electron transport and carbon fixation. In addition to this net synthesis, oxygenic photoautotrophs must counter the light-dependent photoinactivation of Photosystem II (PSII), using metabolically expensive proteolysis, disassembly, resynthesis and re-assembly of protein subunits. We used growth rates, elemental analyses and protein quantitations to estimate the nitrogen (N) metabolism costs to both accumulate the photosynthetic system and to maintain PSII function in the diatom Thalassiosira pseudonana, growing at two pCO2 levels across a range of light levels. The photosynthetic system contains c. 15-25% of total cellular N. Under low growth light, N (re)cycling through PSII repair is only c. 1% of the cellular N assimilation rate. As growth light increases to inhibitory levels, N metabolite cycling through PSII repair increases to c. 14% of the cellular N assimilation rate. Cells growing under the assumed future 750 ppmv pCO2 show higher growth rates under optimal light, coinciding with a lowered N metabolic cost to maintain photosynthesis, but then suffer greater photoinhibition of growth under excess light, coincident with rising costs to maintain photosynthesis. We predict this quantitative trait response to light will vary across taxa.

Evolution and function of light harvesting proteins

Photosynthetic eukaryotes exhibit very different light-harvesting proteins, but all contain membrane-intrinsic light-harvesting complexes (Lhcs), either as additional or sole antennae. Lhcs non-covalently bind chlorophyll a and in most cases another Chl, as well as very different carotenoids, depending on the taxon. The proteins fall into two major groups: The well-defined Lhca/b group of proteins binds typically Chl b and lutein, and the group is present in the 'green lineage'. The other group consists of Lhcr/Lhcf, Lhcz and Lhcx/LhcSR proteins. The former are found in the so-called Chromalveolates, where they mostly bind Chl c and carotenoids very efficient in excitation energy transfer, and in their red algae ancestors. Lhcx/LhcSR are present in most Chromalveolates and in some members of the green lineage as well. Lhcs function in light harvesting, but also in photoprotection, and they influence the organisation of the thylakoid membrane. The different functions of the Lhc subfamilies are discussed in the light of their evolution.

Photophysiological responses of marine diatoms to elevated CO2 and decreased pH: a review

Diatoms dominate nearly half of current oceanic productivity, so their responses to ocean acidification are of general concern regarding future oceanic carbon sequestration. Community, mesocosm and laboratory studies show a range of diatom growth and photophysiological responses to increasing pCO2. Nearly 20 studies on effects of elevated pCO2 on diatoms have shown stimulations, no effects or inhibitions of growth rates. These differential responses could result from differences in experimental setups, cell densities, levels of light and temperature, but also from taxon-specific physiology. Generally, ocean acidification treatments of lowered pH with elevated CO2 stimulate diatom growth under low to moderate levels of light, but lead to growth inhibition when combined with excess light. Additionally, diatom cell sizes and their co-varying metabolic rates can influence responses to increasing pCO2 and decreasing pH, although cell size effects are confounded with taxonomic specificities in cell structures and metabolism. Here we summarise known diatom growth and photophysiological responses to increasing pCO2 and decreasing pH, and discuss some reasons for the diverse responses observed across studies.

Excitation relaxation dynamics and energy transfer in fucoxanthin-chlorophyll a/c-protein complexes, probed by time-resolved fluorescence

In algae, light-harvesting complexes contain specific chlorophylls (Chls) and keto-carotenoids; Chl a, Chl c, and fucoxanthin (Fx) in diatoms and brown algae; Chl a, Chl c, and peridinin in photosynthetic dinoflagellates; and Chl a, Chl b, and siphonaxanthin in green algae. The Fx-Chl a/c-protein (FCP) complex from the diatom Chaetoceros gracilis contains Chl c1, Chl c2, and the keto-carotenoid, Fx, as antenna pigments, in addition to Chl a. In the present study, we investigated energy transfer in the FCP complex associated with photosystem II (FCPII) of C. gracilis. For these investigations, we analyzed time-resolved fluorescence spectra, fluorescence rise and decay curves, and time-resolved fluorescence anisotropy data. Chl a exhibited different energy forms with fluorescence peaks ranging from 677 nm to 688 nm. Fx transferred excitation energy to lower-energy Chl a with a time constant of 300fs. Chl c transferred excitation energy to Chl a with time constants of 500-600fs (intra-complex transfer), 600-700fs (intra-complex transfer), and 4-6ps (inter-complex transfer). The latter process made a greater contribution to total Chl c-to-Chl a transfer in intact cells of C. gracilis than in the isolated FCPII complexes. The lower-energy Chl a received excitation energy from Fx and transferred the energy to higher-energy Chl a. This article is part of a special issue entitled: photosynthesis research for sustainability: keys to produce clean energy.

Comparison of oligomeric states and polypeptide compositions of fucoxanthin chlorophyll a/c-binding protein complexes among various diatom species

Fucoxanthin chlorophyll a/c-binding protein (FCP) is a unique light-harvesting apparatus in diatoms. Several biochemical characteristics of FCP oligomer and trimer from different diatom species have been reported previously. However, the integration of information about molecular organizations and polypeptides of FCP through a comparison among diatoms has not been published. In this study, we used two-dimensional clear-native/SDS-PAGE to compare the oligomeric states and polypeptide compositions of FCP complexes from four diatoms: Chaetoceros gracilis, Thalassiosira pseudonana, Cyclotella meneghiniana, and Phaeodactylum tricornutum. FCP oligomer was found in C. gracilis, T. pseudonana, and C. meneghiniana, but not in P. tricornutum. The oligomerization varied among the three diatoms, although a predominant subunit having similar molecular weight was recovered in each FCP oligomer. These results suggest that the predominant subunit is involved in the formation of high FCP oligomerization in each diatom. In contrast, FCP trimer was found in all the diatoms. The trimerizations were quite similar, whereas the polypeptide compositions were markedly different. On the basis of this information and that from mass spectrometric analyses, the gene products in each FCP complex were identified in T. pseudonana and P. tricornutum. Based on these results, we discuss the role of FCP oligomer and trimer from the four diatoms.

Photosystem II protein clearance and FtsH function in the diatom Thalassiosira pseudonana

All oxygenic photoautotrophs suffer photoinactivation of their Photosystem II complexes, at a rate driven by the instantaneous light level. To maintain photosynthesis, PsbA subunits are proteolytically removed from photoinactivated Photosystem II complexes, primarily by a membrane-bound FtsH protease. Diatoms thrive in environments with fluctuating light, such as coastal regions, in part because they enjoy a low susceptibility to photoinactivation of Photosystem II. In a coastal strain of the diatom Thalassiosira pseudonana growing across a range of light levels, active Photosystem II represents only about 42 % of the total Photosystem II protein, with the remainder attributable to photoinactivated Photosystem II awaiting recycling. The rate constant for removal of PsbA protein increases with growth light, in parallel with an increasing content of the FtsH protease relative to the substrate PsbA. An offshore strain of Thalassiosira pseudonana, originating from a more stable light environment, had a lower content of FtsH and slower rate constants for removal of PsbA. We used this data to generate the first estimates for in vivo proteolytic degradation of photoinactivated PsbA per FtsH6 protease, at ~3.9 × 10(-2) s(-1), which proved consistent across growth lights and across the onshore and offshore strains.