Changes in the LHCI aggregation state during iron repletion in the unicellular red alga Rhodella violacea

Ion and metabolite transport in the chloroplast of algae: lessons from land plants

Chloroplasts are endosymbiotic organelles and play crucial roles in energy supply and metabolism of eukaryotic photosynthetic organisms (algae and land plants). They harbor channels and transporters in the envelope and thylakoid membranes, mediating the exchange of ions and metabolites with the cytosol and the chloroplast stroma and between the different chloroplast subcompartments. In secondarily evolved algae, three or four envelope membranes surround the chloroplast, making more complex the exchange of ions and metabolites. Despite the importance of transport proteins for the optimal functioning of the chloroplast in algae, and that many land plant homologues have been predicted, experimental evidence and molecular characterization are missing in most cases. Here, we provide an overview of the current knowledge about ion and metabolite transport in the chloroplast from algae. The main aspects reviewed are localization and activity of the transport proteins from algae and/or of homologues from other organisms including land plants. Most chloroplast transporters were identified in the green alga Chlamydomonas reinhardtii, reside in the envelope and participate in carbon acquisition and metabolism. Only a few identified algal transporters are located in the thylakoid membrane and play role in ion transport. The presence of genes for putative transporters in green algae, red algae, diatoms, glaucophytes and cryptophytes is discussed, and roles in the chloroplast are suggested. A deep knowledge in this field is required because algae represent a potential source of biomass and valuable metabolites for industry, medicine and agriculture.

The metabolic status drives acclimation of iron deficiency responses in Chlamydomonas reinhardtii as revealed by proteomics based hierarchical clustering and reverse genetics

Iron is a crucial cofactor in numerous redox-active proteins operating in bioenergetic pathways including respiration and photosynthesis. Cellular iron management is essential to sustain sufficient energy production and minimize oxidative stress. To produce energy for cell growth, the green alga Chlamydomonas reinhardtii possesses the metabolic flexibility to use light and/or carbon sources such as acetate. To investigate the interplay between the iron-deficiency response and growth requirements under distinct trophic conditions, we took a quantitative proteomics approach coupled to innovative hierarchical clustering using different "distance-linkage combinations" and random noise injection. Protein co-expression analyses of the combined data sets revealed insights into cellular responses governing acclimation to iron deprivation and regulation associated with photosynthesis dependent growth. Photoautotrophic growth requirements as well as the iron deficiency induced specific metabolic enzymes and stress related proteins, and yet differences in the set of induced enzymes, proteases, and redox-related polypeptides were evident, implying the establishment of distinct response networks under the different conditions. Moreover, our data clearly support the notion that the iron deficiency response includes a hierarchy for iron allocation within organelles in C. reinhardtii. Importantly, deletion of a bifunctional alcohol and acetaldehyde dehydrogenase (ADH1), which is induced under low iron based on the proteomic data, attenuates the remodeling of the photosynthetic machinery in response to iron deficiency, and at the same time stimulates expression of stress-related proteins such as NDA2, LHCSR3, and PGRL1. This finding provides evidence that the coordinated regulation of bioenergetics pathways and iron deficiency response is sensitive to the cellular and chloroplast metabolic and/or redox status, consistent with systems approach data.

Structure and function of photosystem I and its application in biomimetic solar-to-fuel systems

Photosystem I (PSI) is one of the most efficient biological macromolecular complexes that converts solar energy into condensed energy of chemical bonds. Despite high structural complexity, PSI operates with a quantum yield close to 1.0 and to date, no man-made synthetic system approached this remarkable efficiency. This review highlights recent developments in dissecting molecular structure and function of the prokaryotic and eukaryotic PSI. It also overviews progress in the application of this complex as a natural photocathode for production of hydrogen within the biomimetic solar-to-fuel nanodevices.

FLUORESCENCE EMISSION SPECTRA OF MARINE AND BRACKISH-WATER ECOTYPES OF FUCUS VESICULOSUS AND FUCUS RADICANS (PHAEOPHYCEAE) REVEAL DIFFERENCES IN LIGHT-HARVESTING APPARATUS(1)

The Bothnian Sea in the northerly part of the Baltic Sea is a geologically recent brackish-water environment, and rapid speciation is occurring in the algal community of the Bothnian Sea. We measured low-temperature fluorescence emission spectra from the Bothnian Sea and the Norwegian Sea ecotypes of Fucus vesiculosus L., a marine macroalga widespread in the Bothnian Sea. Powdered, frozen thallus was used to obtain undistorted emission spectra. The spectra were compared with spectra measured from the newly identified species Fucus radicans Bergström et L. Kautsky, which is a close relative of F. vesiculosus and endemic to the Bothnian Sea. The spectrum of variable fluorescence was used to identify fluorescence peaks originating in PSI and PSII in this chl c-containing alga. The spectra revealed much higher PSII emission, compared to PSI emission, in the Bothnian Sea ecotype of F. vesiculosus than in F. radicans or in the Norwegian Sea ecotype of F. vesiculosus. The results suggest that more light-harvesting chl a/c proteins serve PSII in the Bothnian Sea ecotype of F. vesiculosus than in the two other algal strains. Treatment of the Bothnian Sea ecotype of F. vesiculosus in high salinity (10, 20, and 35 practical salinity units) for 1 week did not lead to spectral changes, indicating that the measured features of the Bothnian Sea F. vesiculosus are stable and not simply a direct result of exposure to low salinity.

Carotenoid biosynthesis in diatoms

Diatoms are ubiquitous and constitute an important group of the phytoplankton community having a major contribution to the total marine primary production. These microalgae exhibit a characteristic golden-brown colour due to a high amount of the xanthophyll fucoxanthin that plays a major role in the light-harvesting complex of photosystems. In the water column, diatoms are exposed to light intensities that vary quickly from lower to higher values. Xanthophyll cycles prevent photodestruction of the cells in excessive light intensities. In diatoms, the diadinoxanthin-diatoxanthin cycle is the most important short-term photoprotective mechanism. If the biosynthetic pathways of chloroplast pigments have been extensively studied in higher plants and green algae, the research on carotenoid biosynthesis in diatoms is still in its infancy. In this study, the data on the biosynthetic pathway of diatom carotenoids are reviewed. The early steps occur through the 2-C-methyl-D: -erythritol 4-phosphate (MEP) pathway. Then a hypothetical pathway is suggested from dimethylallyl diphosphate (DMAPP) and isopentenyl pyrophosphate (IPP). Most of the enzymes of the pathway have not been so far isolated from diatoms, but candidate genes for each of them were identified using protein similarity searches of genomic data.

The supramolecular architecture, function, and regulation of thylakoid membranes in red algae: an overview

Red algae are a group of eukaryotic photosynthetic organisms. Phycobilisomes (PBSs), which are composed of various types of phycobiliproteins and linker polypeptides, are the main light-harvesting antennae in red algae, as in cyanobacteria. Two morphological types of PBSs, hemispherical- and hemidiscoidal-shaped, are found in different red algae species. PBSs harvest solar energy and efficiently transfer it to photosystem II (PS II) and finally to photosystem I (PS I). The PS I of red algae uses light-harvesting complex of PS I (LHC I) as a light-harvesting antennae, which is phylogenetically related to the LHC I found in higher plants. PBSs, PS II, and PS I are all distributed throughout the entire thylakoid membrane, a pattern that is different from the one found in higher plants. Photosynthesis processes, especially those of the light reactions, are carried out by the supramolecular complexes located in/on the thylakoid membranes. Here, the supramolecular architecture, function and regulation of thylakoid membranes in red algal are reviewed.

Iron stabilizes thylakoid protein-pigment complexes in Indian mustard during Cd-phytoremediation as revealed by BN-SDS-PAGE and ESI-MS/MS

Two-dimensional BN-SDS-PAGE, ESI-MS/MS and electron microscopy (EM) were used to study the role of iron (Fe) under cadmium (Cd) stress in retention of thylakoidal multiprotein complexes (MPCs) and chloroplast ultrastructure of Indian mustard, a moderate hyperaccumulator plant. Mustard was grown hydroponically with or without iron for 17 days and then exposed to CdCl2 for 3 days. Fe deficiency led to an increase in oxidative stress and damage to chloroplast/thylakoids accompanied by a decrease in chlorophyll content; exposure of plants to Cd further enhanced the oxidative stress and Cd accumulation (more in -Fe plants). However, the presence of iron aided plants in the suppression of oxidative stress and retention of chloroplasts and chlorophylls under Cd stress. Proteomic analyses by 2D BN-SDS-PAGE and mass spectrometry showed that Fe deficiency considerably decreased the amount of LHCII trimer, ATPase-F1 portion, cyt b6/f and RuBisCO. No or less reduction, was observed for PSI(RCI+LHCI), the PSII-core monomer, and the PSII subcomplex, while an increase in the LHCII monomer was noted. Under iron deficiency, Cd proved to be very deleterious to MPCs, except for the PSII subcomplex, the LHCII monomer and free proteins which were increased. Iron proved to be very protective in retaining almost all the complexes. MPCs showed greater susceptibility to Cd than Fe deficiency, mainly at the level of RuBisCO and cyt b6/f; an increase in the amount of the PSII subcomplex, LHCII monomer and free proteins indicates differences in the mechanisms affected by Fe deficiency and Cd stress when compared to Fe-fed plants. This study furthers our understanding of the sites actually damaged in MPCs under Fe deficiency and Cd stress. A role emerges for iron in the protection of MPCs and, hence, of the chloroplast. The present study also indicates the importance of iron for efficient phytoextraction/phytoremediation.

PGRL1 participates in iron-induced remodeling of the photosynthetic apparatus and in energy metabolism in Chlamydomonas reinhardtii

PGRL1 RNA and protein levels are increased in iron-deficient Chlamydomonas reinhardtii cells. In an RNAi strain, which accumulates lower PGRL1 levels in both iron-replete and -starved conditions, the photosynthetic electron transfer rate is decreased, respiratory capacity in iron-sufficient conditions is increased, and the efficiency of cyclic electron transfer under iron-deprivation is diminished. Pgrl1-kd cells exhibit iron deficiency symptoms at higher iron concentrations than wild-type cells, although the cells are not more depleted in cellular iron relative to wild-type cells as measured by mass spectrometry. Thiol-trapping experiments indicate iron-dependent and redox-induced conformational changes in PGRL1 that may provide a link between iron metabolism and the partitioning of photosynthetic electron transfer between linear and cyclic flow. We propose, therefore, that PGRL1 in C. reinhardtii may possess a dual function in the chloroplast; that is, iron sensing and modulation of electron transfer.

Comparative quantitative proteomics to investigate the remodeling of bioenergetic pathways under iron deficiency in Chlamydomonas reinhardtii

The basic question addressed in this study is how energy metabolism is adjusted to cope with iron deficiency in Chlamydomonas reinhardtii. To investigate the impact of iron deficiency on bioenergetic pathways, comparative proteomics was combined with spectroscopic as well as voltametric oxygen measurements to assess protein dynamics linked to functional properties of respiratory and photosynthetic machineries. Although photosynthetic electron transfer is largely compromised under iron deficiency, our quantitative and spectroscopic data revealed that the functional antenna size of photosystem II (PSII) significantly increased. Concomitantly, stress-related chloroplast polypeptides, like 2-cys peroxiredoxin and a stress-inducible light-harvesting protein, LhcSR3, as well as a novel light-harvesting protein and several proteins of unknown function were induced under iron-deprivation. Respiratory oxygen consumption did not decrease and accordingly, polypeptides of respiratory complexes, harboring numerous iron-sulfur clusters, were only slightly diminished or even increased under low iron. Consequently, iron-deprivation induces a transition from photoheterotrophic to primarily heterotrophic metabolism, indicating that a hierarchy for iron allocations within organelles of a single cell exists that is closely linked with the metabolic state of the cell.

Presence of 'PSI free' LHCI and monomeric LHCII and subsequent effects on fluorescence characteristics in lincomycin treated maize

The cause of the strong non-photochemical fluorescence quenching was examined in maize (Zea mays L.) plants that were treated with lincomycin during the 72 h period of greening. They were deficient in core complexes but seemed to contain the full complement of antennae. The following results were obtained: (1) High F(o) could not be attributed to the dark reduction of Q(A) but to the presence of a high amount of not properly organized antenna complexes due to the inhibited synthesis of reaction centres. (2) On illumination fluorescence intensity dropped considerably below F(o) within 20 s, and reached a steady state still below F(o). (3) Slowly relaxing part of non-photochemical quenching was significantly higher than in control plants. (4) De-epoxidation state was constant, and corresponded to the maximal value of the control. (5) Free Lhca1/4 dimers could be detected in all submembrane fractions, including the grana, obtained by digitonin fractionation. (6) Increase in the 679 and 700 nm fluorescence emissions could be attributed to the monomerisation of part of LHCII and to the presence of free Lhca2 or LHCII aggregates, respectively. (7) LHCII or PSII+LHCII and Lhca1/4 interaction may contribute to the increase of long-wavelength fluorescence in the granal fraction. We assume that the elevated fluorescence quenching of monomeric LHCII as well as the interaction between LHCII or PSII+LHCII and Lhca1/4 can be considered as an explanation for the extensive non-photochemical fluorescence quenching in lincomycin treated plants. The permanent presence of zeaxanthin may have contributed to the fast formation of quenching.

Self-assembling peptide detergents stabilize isolated photosystem I on a dry surface for an extended time

We used a class of designed peptide detergents to stabilize photosystem I (PS-I) upon extended drying under N2 on a gold-coated-Ni-NTA glass surface. PS-I is a chlorophyll-containing membrane protein complex that is the primary reducer of ferredoxin and the electron acceptor of plastocyanin. We isolated the complex from the thylakoids of spinach chloroplasts using a chemical detergent. The chlorophyll molecules associated with the PS-I complex provide an intrinsic steady-state emission spectrum between 650 and 800 nm at -196.15 degrees C that reflects the organization of the pigment-protein interactions. In the absence of detergents, a large blue shift of the fluorescence maxima from approximately 735 nm to approximately 685 nm indicates a disruption in light-harvesting subunit organization, thus revealing chlorophyll-protein interactions. The commonly used membrane protein-stabilizing detergents, N-dodecyl-beta-D-maltoside and N-octyl-beta-D-glucoside, only partially stabilized the approximately 735-nm complex with approximately 685-nm spectroscopic shift. However, prior to drying, addition of the peptide detergent acetyl-AAAAAAK at increasing concentration significantly stabilized the PS-I complex. Moreover, in the presence of acetyl-AAAAAAK, the PS-I complex is stable in a dried form at room temperature for at least 3 wk. Another peptide detergent, acetyl-VVVVVVD, also stabilized the complex but to a lesser extent. These observations suggest that the peptide detergents may effectively stabilize membrane proteins in the solid-state. These designed peptide detergents may facilitate the study of diverse types of membrane proteins.

Structural and functional organization of the peripheral light-harvesting system in photosystem I

This review centers on the structural and functional organization of the light-harvesting system in the peripheral antenna of Photosystem I (LHC I) and its energy coupling to the Photosystem I (PS I) core antenna network in view of recently available structural models of the eukaryotic Photosystem I-LHC I complex, eukaryotic LHC II complexes and the cyanobacterial Photosystem I core. A structural model based on the 3D homology of Lhca4 with LHC II is used for analysis of the principles of pigment arrangement in the LHC I peripheral antenna, for prediction of the protein ligands for the pigments that are unique for LHC I and for estimates of the excitonic coupling in strongly interacting pigment dimers. The presence of chlorophyll clusters with strong pigment-pigment interactions is a structural feature of PS I, resulting in the characteristic red-shifted fluorescence. Analysis of the interactions between the PS I core antenna and the peripheral antenna leads to the suggestion that the specific function of the red pigments is likely to be determined by their localization with respect to the reaction center. In the PS I core antenna, the Chl clusters with a different magnitude of low energy shift contribute to better spectral overlap of Chls in the reaction center and the Chls of the antenna network, concentrate the excitation around the reaction center and participate in downhill enhancement of energy transfer from LHC II to the PS I core. Chlorophyll clusters forming terminal emitters in LHC I are likely to be involved in photoprotection against excess energy.

In the Unicellular Red Alga Rhodella violacea Iron Deficiency Induces an Accumulation of Uncoupled LHC

Iron plays a key role in the synthesis and functioning of the photosynthetic apparatus. Conditions of partial iron deficiency that lead to a relatively stable phenotype were established and the effects of starvation studied in the unicellular red alga, Rhodella violacea. Synthesis of the photosynthetic pigments were found to decrease, with phycobiliproteins being affected to a lesser extent than chlorophyll a. Biophysical, biochemical and immunological approaches were used to show that the PSI content is highly diminished and the PSII/PSI stoichiometry increased by a factor of 5 compared to standard conditions. Meanwhile light-harvesting complex (LHC) was still assembled in the thylakoid membranes at unchanged levels. The use of translation inhibitors for either nuclear- or plastid-encoded polypeptides revealed that uncoupled LHC may be responsible for the high wavelength-fluorescence contribution observed around 700-710 nm. There is no evidence for the synthesis of new chlorophyll-protein complexes.