Organization and Function of Chlorophyll in Membranes of Cyanobacteria during Iron Starvation

The structural basis for light harvesting in organisms producing phycobiliproteins

Cyanobacteria, red algae, and cryptophytes produce 2 classes of proteins for light harvesting: water-soluble phycobiliproteins (PBP) and membrane-intrinsic proteins that bind chlorophylls (Chls) and carotenoids. In cyanobacteria, red algae, and glaucophytes, phycobilisomes (PBS) are complexes of brightly colored PBP and linker (assembly) proteins. To date, 6 structural classes of PBS have been described: hemiellipsoidal, block-shaped, hemidiscoidal, bundle-shaped, paddle-shaped, and far-red-light bicylindrical. Two additional antenna complexes containing single types of PBP have also been described. Since 2017, structures have been reported for examples of all of these complexes except bundle-shaped PBS by cryogenic electron microscopy. PBS range in size from about 4.6 to 18 mDa and can include ∼900 polypeptides and bind >2000 chromophores. Cyanobacteria additionally produce membrane-associated proteins of the PsbC/CP43 superfamily of Chl a/b/d-binding proteins, including the iron-stress protein IsiA and other paralogous Chl-binding proteins (CBP) that can form antenna complexes with Photosystem I (PSI) and/or Photosystem II (PSII). Red and cryptophyte algae also produce CBP associated with PSI but which belong to the Chl a/b-binding protein superfamily and which are unrelated to the CBP of cyanobacteria. This review describes recent progress in structure determination for PBS and the Chl proteins of cyanobacteria, red algae, and cryptophytan algae.

Unveiling large charge transfer character of PSII in an iron-deficient cyanobacterial membrane: A Stark fluorescence spectroscopy study

In this work, we applied Stark fluorescence spectroscopy to an iron-stressed cyanobacterial membrane to reveal key insights about the electronic structures and excited state dynamics of the two important pigment-protein complexes, IsiA and PSII, both of which prevail simultaneously within the membrane during iron deficiency and whose fluorescence spectra are highly overlapped and hence often hardly resolved by conventional fluorescence spectroscopy. Thanks to the ability of Stark fluorescence spectroscopy, the fluorescence signatures of the two complexes could be plausibly recognized and disentangled. The systematic analysis of the SF spectra, carried out by employing standard Liptay formalism with a realistic spectral deconvolution protocol, revealed that the IsiA in an intact membrane retains almost identical excited state electronic structures and dynamics as compared to the isolated IsiA we reported in our earlier study. Moreover, the analysis uncovered that the excited state of the PSII subunit of the intact membrane possesses a significantly large CT character. The observed notably large magnitude of the excited state CT character may signify the supplementary role of PSII in regulative energy dissipation during iron deficiency.

Function of iron-stress-induced protein A in cyanobacterial cells with monomeric and trimeric photosystem I

The acclimation of cyanobacteria to iron deficiency is crucial for their survival in natural environments. In response to iron deficiency, many cyanobacterial species induce the production of a pigment-protein complex called iron-stress-induced protein A (IsiA). IsiA proteins associate with photosystem I (PSI) and can function as light-harvesting antennas or dissipate excess energy. They may also serve as chlorophyll storage during iron limitation. In this study, we examined the functional role of IsiA in cells of Synechocystis sp. PCC 6803 grown under iron limitation conditions by measuring the cellular IsiA content and its capability to transfer energy to PSI. We specifically tested the effect of the oligomeric state of PSI by comparing wild-type (WT) Synechocystis sp. PCC 6803 with mutants lacking specific subunits of PSI, namely PsaL/PsaI (PSI subunits XI/VIII) and PsaF/PsaJ (PSI subunits III/IX). Time-resolved fluorescence spectroscopy revealed that IsiA formed functional PSI3-IsiA18 supercomplexes, wherein IsiA effectively transfers energy to PSI on a timescale of 10 ps at room temperature-measured in isolated complexes and in vivo-confirming the primary role of IsiA as an accessory light-harvesting antenna to PSI. However, a notable fraction (40%) remained unconnected to PSI, supporting the notion of a dual functional role of IsiA. Cells with monomeric PSI under iron deficiency contained, on average, only 3 to 4 IsiA complexes bound to PSI. These results show that IsiA can transfer energy to trimeric and monomeric PSI but to varying degrees and that the acclimatory production of IsiA under iron stress is controlled by its ability to perform its light-harvesting function.

Regulation and Functional Complexity of the Chlorophyll-Binding Protein IsiA

As the oldest known lineage of oxygen-releasing photosynthetic organisms, cyanobacteria play the key roles in helping shaping the ecology of Earth. Iron is an ideal transition metal for redox reactions in biological systems. Cyanobacteria frequently encounter iron deficiency due to the environmental oxidation of ferrous ions to ferric ions, which are highly insoluble at physiological pH. A series of responses, including architectural changes to the photosynthetic membranes, allow cyanobacteria to withstand this condition and maintain photosynthesis. Iron-stress-induced protein A (IsiA) is homologous to the cyanobacterial chlorophyll (Chl)-binding protein, photosystem II core antenna protein CP43. IsiA is the major Chl-containing protein in iron-starved cyanobacteria, binding up to 50% of the Chl in these cells, and this Chl can be released from IsiA for the reconstruction of photosystems during the recovery from iron limitation. The pigment–protein complex (CPVI-4) encoded by isiA was identified and found to be expressed under iron-deficient conditions nearly 30years ago. However, its precise function is unknown, partially due to its complex regulation; isiA expression is induced by various types of stresses and abnormal physiological states besides iron deficiency. Furthermore, IsiA forms a range of complexes that perform different functions. In this article, we describe progress in understanding the regulation and functions of IsiA based on laboratory research using model cyanobacteria.

Functional Diversity of TonB-Like Proteins in the Heterocyst-Forming Cyanobacterium Anabaena sp. PCC 7120

The TonB-dependent transport of scarcely available substrates across the outer membrane is a conserved feature in Gram-negative bacteria. The plasma membrane-embedded TonB-ExbB-ExbD accomplishes complex functions as an energy transducer by physically interacting with TonB-dependent outer membrane transporters (TBDTs). TonB mediates structural rearrangements in the substrate-loaded TBDTs that are required for substrate translocation into the periplasm. In the model heterocyst-forming cyanobacterium Anabaena sp. strain PCC 7120, four TonB-like proteins have been identified. Out of these TonB3 accomplishes the transport of ferric schizokinen, the siderophore which is secreted by Anabaena to scavenge iron. In contrast, TonB1 (SjdR) is exceptionally short and not involved in schizokinen transport. The proposed function of SjdR in peptidoglycan structuring eliminates the protein from the list of TonB proteins in Anabaena. Compared with the well-characterized properties of SjdR and TonB3, the functions of TonB2 and TonB4 are yet unknown. Here, we examined tonB2 and tonB4 mutants for siderophore transport capacities and other specific phenotypic features. Both mutants were not or only slightly affected in schizokinen transport, whereas they showed decreased nitrogenase activity in apparently normal heterocysts. Moreover, the cellular metal concentrations and pigment contents were altered in the mutants, most pronouncedly in the tonB2 mutant. This strain showed an altered susceptibility toward antibiotics and SDS and formed cell aggregates when grown in liquid culture, a phenotype associated with an elevated lipopolysaccharide (LPS) production. Thus, the TonB-like proteins in Anabaena appear to take over distinct functions, and the mutation of TonB2 strongly influences outer membrane integrity.

IMPORTANCE The genomes of many organisms encode more than one TonB protein, and their number does not necessarily correlate with that of TonB-dependent outer membrane transporters. Consequently, specific as well as redundant functions of the different TonB proteins have been identified. In addition to a role in uptake of scarcely available nutrients, including iron complexes, TonB proteins are related to virulence, flagellum assembly, pilus localization, or envelope integrity, including antibiotic resistance. The knowledge about the function of TonB proteins in cyanobacteria is limited. Here, we compare the four TonB proteins of Anabaena sp. strain PCC 7120, providing evidence that their functions are in part distinct, since mutants of these proteins exhibit specific features but also show some common impairments.

Reciprocal Effect of Copper and Iron Regulation on the Proteome of Synechocystis sp. PCC 6803

Cyanobacteria can acclimate to changing copper and iron concentrations in the environment via metal homeostasis, but a general mechanism for interpreting their dynamic relationships is sparse. In this study, we assessed growth and chlorophyll fluorescence of Synechocystis sp. PCC 6803 and investigated proteomic responses to copper and iron deductions. Results showed that copper and iron exerted reciprocal effect on the growth and photosynthesis of Synechocystis sp. PCC 6803 at combinations of different concentrations. And some proteins involved in the uptake of copper and iron and the photosynthetic electron transport system exhibit Cu-Fe proteomic association. The protein abundance under copper and iron deduction affected the photosynthetic electronic activity of Synechocystis sp. PCC 6803 and eventually affected the growth and photosynthesis. Based on these results, we hypothesize that the Cu-Fe proteomic association of Synechocystis sp. PCC 6803 can be elucidated via the uptake system of outer membrane-periplasmic space-inner plasma membrane-thylakoid membrane, and this association is mainly required to maintain electron transfer. This study provides a broader view regarding the proteomic association between Cu and Fe in cyanobacteria, which will shed light on the role of these two metal elements in cyanobacterial energy metabolism and biomass accumulation.

New insights into the function of the proteins IsiC and IsiD from Synechocystis sp. PCC 6803 under iron limitation

Iron is a common cofactor in biological processes such as respiration, photosynthesis, and nitrogen fixation. The genes isiC and isiD encode unknown proteins, and the growth of ΔisiC and ΔisiD mutants is inhibited under iron-deficient conditions. To study the regulatory mechanisms of IsiC and IsiD during iron starvation, we carried out transcriptome and metabolome sequencing. The Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis showed that the photosynthesis, nitrogen metabolism, and ABC transporter pathways play a vital role in regulating iron deficiency. Upon iron repletion, IsiC and IsiD also have a regulatory effect on these pathways. Additionally, KEGG analysis of the differential metabolites of wild type (WT) and mutants showed that they were all enriched in starch and sucrose metabolism after iron limitation. Weighted gene co-expression network analysis (WGCNA) constructed a co-expression network of differentially expressed genes with phenotypes and metabolites, and finally identified five modules. The turquoise module was positively correlated with iron deficiency. In contrast, the WT and blue module exhibited a negative correlation, and the mutants ΔisiC and ΔisiD were positively correlated with the gray and brown modules, respectively. WGCNA also analyzed the relationship between metabolites and phenotypes, and the green module was related to iron starvation. The co-expression network determined the hub genes and metabolites of each module. This study lays a foundation for a better understanding of cyanobacteria in response to iron deficiency. KEY POINTS: • Nitrogen metabolism and ABC transporters are involved in iron regulation. • Starch and sucrose metabolism is related to the regulation of iron deficiency. • WGCNA analyzes the correlation between genes and metabolites.

From the Ocean to the Lab-Assessing Iron Limitation in Cyanobacteria: An Interface Paper

Iron is an essential, yet scarce, nutrient in marine environments. Phytoplankton, and especially cyanobacteria, have developed a wide range of mechanisms to acquire iron and maintain their iron-rich photosynthetic machinery. Iron limitation studies often utilize either oceanographic methods to understand large scale processes, or laboratory-based, molecular experiments to identify underlying molecular mechanisms on a cellular level. Here, we aim to highlight the benefits of both approaches to encourage interdisciplinary understanding of the effects of iron limitation on cyanobacteria with a focus on avoiding pitfalls in the initial phases of collaboration. In particular, we discuss the use of trace metal clean methods in combination with sterile techniques, and the challenges faced when a new collaboration is set up to combine interdisciplinary techniques. Methods necessary for producing reliable data, such as High Resolution Inductively Coupled Plasma Mass Spectrometry (HR-ICP-MS), Flow Injection Analysis Chemiluminescence (FIA-CL), and 77K fluorescence emission spectroscopy are discussed and evaluated and a technical manual, including the preparation of the artificial seawater medium Aquil, cleaning procedures, and a sampling scheme for an iron limitation experiment is included. This paper provides a reference point for researchers to implement different techniques into interdisciplinary iron studies that span cyanobacteria physiology, molecular biology, and biogeochemistry.

Structure of a cyanobacterial photosystem I surrounded by octadecameric IsiA antenna proteins

Iron-stress induced protein A (IsiA) is a chlorophyll-binding membrane-spanning protein in photosynthetic prokaryote cyanobacteria, and is associated with photosystem I (PSI) trimer cores, but its structural and functional significance in light harvesting remains unclear. Here we report a 2.7-Å resolution cryo-electron microscopic structure of a supercomplex between PSI core trimer and IsiA from a thermophilic cyanobacterium Thermosynechococcus vulcanus. The structure showed that 18 IsiA subunits form a closed ring surrounding a PSI trimer core. Detailed arrangement of pigments within the supercomplex, as well as molecular interactions between PSI and IsiA and among IsiAs, were resolved. Time-resolved fluorescence spectra of the PSI–IsiA supercomplex showed clear excitation-energy transfer from IsiA to PSI, strongly indicating that IsiA functions as an energy donor, but not an energy quencher, in the supercomplex. These structural and spectroscopic findings provide important insights into the excitation-energy-transfer and subunit assembly mechanisms in the PSI–IsiA supercomplex.

Akita et al. present the latest approach to solve IsiA–PSI supercomplex molecular structure with increased resolution using cryo-EM and time-resolved fluorescence studies. With 2.7 Å resolution, they reveal molecular interactions between PSI and IsiA subunits and that IsiA functions as an energy donor in the supercomplex.

Proteomic responses to ocean acidification of the marine diazotroph Trichodesmium under iron-replete and iron-limited conditions

Growth and dinitrogen (N₂) fixation of the globally important diazotrophic cyanobacteria Trichodesmium are often limited by iron (Fe) availability in surface seawaters. To systematically examine the combined effects of Fe limitation and ocean acidification (OA), T. erythraeum strain IMS101 was acclimated to both Fe-replete and Fe-limited concentrations under ambient and acidified conditions. Proteomic analysis showed that OA affected a wider range of proteins under Fe-limited conditions compared to Fe-replete conditions. OA also led to an intensification of Fe deficiency in key cellular processes (e.g., photosystem I and chlorophyll a synthesis) in already Fe-limited T. erythraeum. This is a result of reallocating Fe from these processes to Fe-rich nitrogenase to compensate for the suppressed N₂ fixation. To alleviate the Fe shortage, the diazotroph adopts a series of Fe-based economic strategies (e.g., upregulating Fe acquisition systems for organically complexed Fe and particulate Fe, replacing ferredoxin by flavodoxin, and using alternative electron flow pathways to produce ATP). This was more pronounced under Fe-limited-OA conditions than under Fe limitation only. Consequently, OA resulted in a further decrease of N₂- and carbon-fixation rates in Fe-limited T. erythraeum. In contrast, Fe-replete T. erythraeum induced photosystem I (PSI) expression to potentially enhance the PSI cyclic flow for ATP production to meet the higher demand for energy to cope with the stress caused by OA. Our study provides mechanistic insight into the holistic response of the globally important N₂-fixing marine cyanobacteria Trichodesmium to acidified and Fe-limited conditions of future oceans.

The structure of the stress-induced photosystem I-IsiA antenna supercomplex

Photochemical conversion in oxygenic photosynthesis takes place in two large protein-pigment complexes named photosystem II and photosystem I (PSII and PSI, respectively). Photosystems associate with antennae in vivo to increase the size of photosynthetic units to hundreds or thousands of pigments. Regulation of the interactions between antennae and photosystems allows photosynthetic organisms to adapt to their environment. In low-iron environments, cyanobacteria express IsiA, a PSI antenna, critical to their survival. Here we describe the structure of the PSI-IsiA complex isolated from the mesophilic cyanobacterium Synechocystis sp. PCC 6803. This 2-MDa photosystem-antenna supercomplex structure reveals more than 700 pigments coordinated by 51 subunits, as well as the mechanisms facilitating the self-assembly and association of IsiA with multiple PSI assemblies.

Acclimation of Oxygenic Photosynthesis to Iron Starvation Is Controlled by the sRNA IsaR1

Oxygenic photosynthesis crucially depends on proteins that possess Fe²⁺ or Fe/S complexes as co-factors or prosthetic groups. Here, we show that the small regulatory RNA (sRNA) IsaR1 (Iron-Stress-Activated RNA 1) plays a pivotal role in acclimation to low-iron conditions. The IsaR1 regulon consists of more than 15 direct targets, including Fe²⁺-containing proteins involved in photosynthetic electron transfer, detoxification of anion radicals, citrate cycle, and tetrapyrrole biogenesis. IsaR1 is essential for maintaining physiological levels of Fe/S cluster biogenesis proteins during iron deprivation. Consequently, IsaR1 affects the acclimation of the photosynthetic apparatus to iron starvation at three levels: (1) directly, via posttranscriptional repression of gene expression; (2) indirectly, via suppression of pigment; and (3) Fe/S cluster biosynthesis. Homologs of IsaR1 are widely conserved throughout the cyanobacterial phylum. We conclude that IsaR1 is a critically important riboregulator. These findings provide a new perspective for understanding the regulation of iron homeostasis in photosynthetic organisms.

Dynamic Changes of IsiA-Containing Complexes during Long-Term Iron Deficiency in Synechocystis sp. PCC 6803

Iron stress-induced protein A (IsiA), a major chlorophyll-binding protein in the thylakoid membrane, is significantly induced under iron deficiency conditions. Using immunoblot analysis and 77 K fluorescence spectroscopy combined with sucrose gradient fractionation, we monitored dynamic changes of IsiA-containing complexes in Synechocystis sp. PCC 6803 during exposure to long-term iron deficiency. Within 3 days of exposure to iron deficiency conditions, the initially induced free IsiA proteins preferentially conjugated to PS I trimer to form IsiA₁₈-PS I trimers, which serve as light energy collectors for efficiently transmitting energy to PS I. With prolonged iron deficiency, IsiA proteins assembled either into IsiA aggregates or into two other types of IsiA-PS I supercomplexes, namely IsiA-PS I high fluorescence supercomplex (IHFS) and IsiA-PS I low fluorescence supercomplex (ILFS). Further analysis revealed a role for IsiA as an energy dissipater in the IHFS and as an energy collector in the ILFS. The trimeric structure of PS I mediated by PsaL was found to be indispensable for the formation of IHFS/ILFS. Dynamic changes in IsiA-containing complexes in cyanobacteria during long-term iron deficiency may represent an adaptation to iron limitation stress for flexible light energy distribution, which balances electron transfer between PS I and PS II, thus minimizing photooxidative damage.

Capsular polysaccharides facilitate enhanced iron acquisition by the colonial cyanobacterium Microcystis sp. isolated from a freshwater lake

Microcystis sp., especially in its colonial form, is a common dominant species during cyanobacterial blooms in many iron-deficient water bodies. It is still not entirely clear, however, how the colonial forms of Microcystis acclimate to iron-deficient habitats, and the responses of unicellular and colonial forms to iron-replete and iron-deficient conditions were examined here. Growth rates and levels of photosynthetic pigments declined to a greater extent in cultures of unicellular Microcystis than in cultures of the colonial form in response to decreasing iron concentrations, resulting in the impaired photosynthetic performance of unicellular Microcystis as compared to colonial forms as measured by variable fluorescence and photosynthetic oxygen evolution. These results indicate that the light-harvesting ability and photosynthetic capacity of colonial Microcystis was less affected by iron deficiency than the unicellular form. The carotenoid contents and nonphotochemical quenching of colonial Microcystis were less reduced than those of the unicellular form under decreasing iron concentrations, indicating that the colonial morphology enhanced photoprotection and acclimation to iron-deficient conditions. Furthermore, large amounts of iron were detected in the capsular polysaccharides (CPS) of the colonies, and more iron was found to be attached to the colonial Microcystis CPS under decreasing iron conditions as compared to unicellular cultures. These results demonstrated that colonial Microcystis can acclimate to iron deficiencies better than the unicellular form, and that CPS plays an important role in their acclimation advantage in iron-deficient waters.

Iron Deficiency Induces a Partial Inhibition of the Photosynthetic Electron Transport and a High Sensitivity to Light in the Diatom Phaeodactylum tricornutum

Iron limitation is the major factor controlling phytoplankton growth in vast regions of the contemporary oceans. In this study, a combination of thermoluminescence (TL), chlorophyll fluorescence, and P700 absorbance measurements have been used to elucidate the effects of iron deficiency in the photosynthetic electron transport of the marine diatom P. tricornutum. TL was used to determine the effects of iron deficiency on photosystem II (PSII) activity. Excitation of iron-replete P. tricornutum cells with single turn-over flashes induced the appearance of TL glow curves with two components with different peaks of temperature and contributions to the total signal intensity: the B band (23°C, 63%), and the AG band (40°C, 37%). Iron limitation did not significantly alter these bands, but induced a decrease of the total TL signal. Far red excitation did not increase the amount of the AG band in iron-limited cells, as observed for iron-replete cells. The effect of iron deficiency on the photosystem I (PSI) activity was also examined by measuring the changes in P700 redox state during illumination. The electron donation to PSI was substantially reduced in iron-deficient cells. This could be related with the important decline on cytochrome c 6 content observed in these cells. Iron deficiency also induced a marked increase in light sensitivity in P. tricornutum cells. A drastic increase in the level of peroxidation of chloroplast lipids was detected in iron-deficient cells even when grown under standard conditions at low light intensity. Illumination with a light intensity of 300 μE m(-2) s(-1) during different time periods caused a dramatic disappearance in TL signal in cells grown under low iron concentration, this treatment not affecting to the signal in iron-replete cells. The results of this work suggest that iron deficiency induces partial blocking of the electron transfer between PSII and PSI, due to a lower concentration of the electron donor cytochrome c 6. This decreased electron transfer may induce the over-reduction of the plastoquinone pool and consequently the appearance of acceptor side photoinhibition in PSII even at low light intensities. The functionality of chlororespiratory electron transfer pathway under iron restricted conditions is also discussed.

Multiple modes of iron uptake by the filamentous, siderophore-producing cyanobacterium, Anabaena sp. PCC 7120

Iron is a member of a small group of nutrients that limits aquatic primary production. Mechanisms for utilizing iron have to be efficient and adapted according to the ecological niche. In respect to iron acquisition cyanobacteria, prokaryotic oxygen evolving photosynthetic organisms can be divided into siderophore- and non-siderophore-producing strains. The results presented in this paper suggest that the situation is far more complex. To understand the bioavailability of different iron substrates and the advantages of various uptake strategies, we examined iron uptake mechanisms in the siderophore-producing cyanobacterium Anabaena sp. PCC 7120. Comparison of the uptake of iron complexed with exogenous (desferrioxamine B, DFB) or to self-secreted (schizokinen) siderophores by Anabaena sp. revealed that uptake of the endogenous produced siderophore complexed to iron is more efficient. In addition, Anabaena sp. is able to take up dissolved, ferric iron hydroxide species (Fe') via a reductive mechanism. Thus, Anabaena sp. exhibits both, siderophore- and non-siderophore-mediated iron uptake. While assimilation of Fe' and FeDFB are not induced by iron starvation, FeSchizokinen uptake rates increase with increasing iron starvation. Consequently, we suggest that Fe' reduction and uptake is advantageous for low-density cultures, while at higher densities siderophore uptake is preferred.

A Comparative Study of Iron Uptake Rates and Mechanisms amongst Marine and Fresh Water Cyanobacteria: Prevalence of Reductive Iron Uptake

In this contribution, we address the question of iron bioavailability to cyanobacteria by measuring Fe uptake rates and probing for a reductive uptake pathway in diverse cyanobacterial species. We examined three Fe-substrates: dissolved inorganic iron (Fe') and the Fe-siderophores Ferrioxamine B (FOB) and FeAerobactin (FeAB). In order to compare across substrates and strains, we extracted uptake rate constants (k_in = uptake rate/[Fe-substrate]). Fe' was the most bioavailable Fe form to cyanobacteria, with k_in values higher than those of other substrates. When accounting for surface area (SA), all strains acquired Fe' at similar rates, as their k_in/SA were similar. We also observed homogeneity in the uptake of FOB among strains, but with 10,000 times lower k_in/SA values than Fe'. Uniformity in k_in/SA suggests similarity in the mechanism of uptake and indeed, all strains were found to employ a reductive step in the uptake of Fe' and FOB. In contrast, different uptake pathways were found for FeAB along with variations in k_in/SA. Our data supports the existence of a common reductive Fe uptake pathway amongst cyanobacteria, functioning alone or in addition to siderophore-mediated uptake. Cyanobacteria combining both uptake strategies benefit from increased flexibility in accessing different Fe-substrates.

Identification of common motifs in the regulation of light harvesting: The case of cyanobacteria IsiA

When cyanobacteria are grown under iron-limited or other oxidative stress conditions the iron stress inducible pigment-protein IsiA is synthesized in variable amounts. IsiA accumulates in aggregates inside the photosynthetic membrane that strongly dissipate chlorophyll excited state energy. In this paper we applied Stark fluorescence (SF) spectroscopy at 77K to IsiA aggregates to gain insight into the nature of the emitting and energy dissipating state(s). Our study shows that two emitting states are present in the system, one emitting at 684 nm and the other emitting at about 730 nm. The new 730 nm state exhibits strongly reduced fluorescence (F) together with a large charge transfer character. We discuss these findings in the light of the energy dissipation mechanisms involved in the regulation of photosynthesis in plants, cyanobacteria and diatoms. Our results suggest that photosynthetic organisms have adopted common mechanisms to cope with the deleterious effects of excess light under unfavorable growth conditions.

Two essential FtsH proteases control the level of the Fur repressor during iron deficiency in the cyanobacterium Synechocystis sp. PCC 6803

The cyanobacterium Synechocystis sp. PCC 6803 expresses four different FtsH protease subunits (FtsH1-4) that assemble into specific homo- and heterocomplexes. The FtsH2/FtsH3 complex is involved in photoprotection but the physiological roles of the other complexes, notably the essential FtsH1/FtsH3 complex, remain unclear. Here we show that the FtsH1 and FtsH3 proteases are involved in the acclimation of cells to iron deficiency. A mutant conditionally depleted in FtsH3 was unable to induce normal expression of the IsiA chlorophyll-protein and FutA1 iron transporter upon iron deficiency due to a block in transcription, which is regulated by the Fur transcriptional repressor. Levels of Fur declined in the WT and the FtsH2 null mutant upon iron depletion but not in the FtsH3 downregulated strain. A similar stabilizing effect on Fur was also observed in a mutant conditionally depleted in the FtsH1 subunit. Moreover, a mutant overexpressing FtsH1 showed reduced levels of Fur and enhanced accumulation of both IsiA and FutA1 even under iron sufficiency. Analysis of GFP-tagged derivatives and biochemical fractionation supported a common location for FtsH1 and FtsH3 in the cytoplasmic membrane. Overall we propose that degradation of the Fur repressor mediated by the FtsH1/FtsH3 heterocomplex is critical for acclimation to iron depletion.

Functional role of PilA in iron acquisition in the cyanobacterium Synechocystis sp. PCC 6803

Cyanobacteria require large quantities of iron to maintain their photosynthetic machinery; however, in most environments iron is present in the form of insoluble iron oxides. Whether cyanobacteria can utilize these sources of iron, and the potential molecular mechanisms involved remains to be defined. There is increasing evidence that pili can facilitate electron donation to extracellular electron acceptors, like iron oxides in non-photosynthetic bacteria. In these organisms, the donation of electrons to iron oxides is thought to be crucial for maintaining respiration in the absence of oxygen. Our study investigates if PilA1 (major pilin protein) may also provide a mechanism to convert insoluble ferric iron into soluble ferrous iron. Growth experiments supported by spectroscopic data of a strain deficient in pilA1 indicate that the presence of the pilA1 gene enhances the ability to grow on iron oxides. These observations suggest a novel function of PilA1 in cyanobacterial iron acquisition.

Suitability of a cytotoxicity assay for detection of potentially harmful compounds produced by freshwater bloom-forming algae

Detecting harmful bioactive compounds produced by bloom-forming pelagic algae is important to assess potential risks to public health. We investigated the application of a cell-based bioassay: the rainbow trout gill-w1 cytotoxicity assay (RCA) that detects changes in cell metabolism. The RCA was used to evaluate the cytotoxic effects of (1) six natural freshwater lake samples from cyanobacteria-rich lakes in central Ontario, Canada; (2) analytical standards of toxins and noxious compounds likely to be produced by the algal communities in these lakes; and (3) complex mixtures of compounds produced by cyanobacterial and chrysophyte cultures. RCA provided a measure of lake water toxicity that could not be reproduced using toxin or noxious compound standards. RCA was not sensitive to toxins and only sensitive to noxious compounds at concentrations higher than reported environmental averages (EC₅₀≥10³nM). Cultured algae produced bioactive compounds that had recognizable dose dependent and toxic effects as indicated by RCA. Toxicity of these bioactive compounds depended on taxa (cyanobacteria, not chrysophytes), growth stage (stationary phase more toxic than exponential phase), location (intracellular more toxic than extracellular) and iron status (cells in high-iron treatment more toxic than cells in low-iron treatment). The RCA provides a new avenue of exploration and potential for the detection of natural lake algal toxic and noxious compounds.

How does iron deficiency disrupt the electron flow in photosystem I of lettuce leaves?

The changes observed photosystem I activity of lettuce plants exposed to iron deficiency were investigated. Photooxidation/reduction kinetics of P700 monitored as ΔA820 in the presence and absence of electron transport inhibitors and acceptors demonstrated that deprivation in iron decreased the population of active photo-oxidizable P700. In the complete absence of iron, the addition of plant inhibitors (DCMU and MV) could not recover the full PSI activity owing to the abolition of a part of P700 centers. In leaves with total iron deprivation (0μM Fe), only 15% of photo-oxidizable P700 remained. In addition, iron deficiency appeared to affect the pool size of NADP(+) as shown by the decline in the magnitude of the first phase of the photooxidation kinetics of P700 by FR-light. Concomitantly, chlorophyll content gradually declined with the iron concentration added to culture medium. In addition, pronounced changes were found in chlorophyll fluorescence spectra. Also, the global fluorescence intensity was affected. The above changes led to an increased rate of cyclic electron transport around PSI mainly supported by stromal reductants.

Cross-species transcriptional network analysis reveals conservation and variation in response to metal stress in cyanobacteria

BACKGROUND

As one of the most dominant bacterial groups on Earth, cyanobacteria play a pivotal role in the global carbon cycling and the Earth atmosphere composition. Understanding their molecular responses to environmental perturbations has important scientific and environmental values. Since important biological processes or networks are often evolutionarily conserved, the cross-species transcriptional network analysis offers a useful strategy to decipher conserved and species-specific transcriptional mechanisms that cells utilize to deal with various biotic and abiotic disturbances, and it will eventually lead to a better understanding of associated adaptation and regulatory networks.

RESULTS

In this study, the Weighted Gene Co-expression Network Analysis (WGCNA) approach was used to establish transcriptional networks for four important cyanobacteria species under metal stress, including iron depletion and high copper conditions. Cross-species network comparison led to discovery of several core response modules and genes possibly essential to metal stress, as well as species-specific hub genes for metal stresses in different cyanobacteria species, shedding light on survival strategies of cyanobacteria responding to different environmental perturbations.

CONCLUSIONS

The WGCNA analysis demonstrated that the application of cross-species transcriptional network analysis will lead to novel insights to molecular response to environmental changes which will otherwise not be achieved by analyzing data from a single species.

Photophysiological expressions of iron stress in phytoplankton

Iron is essential for all life, but it is particularly important to photoautotrophs because of the many iron-dependent electron transport components in photosynthetic membranes. Since the proliferation of oxygenic photosynthesis in the Archean ocean, iron has been a scarce commodity, and it is now recognized as a limiting resource for phytoplankton over broad expanses of the open ocean and even in some coastal/continental shelf waters. Iron stress does not impair photochemical or carbon fixation efficiencies, and in this respect it resembles the highly tuned photosynthetic systems of steady-state macronutrient-limited phytoplankton. However, iron stress does present unique photophysiological challenges, and phytoplankton have responded to these challenges through major architectural changes in photosynthetic membranes. These evolved responses include overexpression of photosynthetic pigments and iron-economic pathways for ATP synthesis, and they result in diagnostic fluorescence properties that allow a broad appraisal of iron stress in the field and even the detection of iron stress from space.

Iron deprivation in Synechocystis: inference of pathways, non-coding RNAs, and regulatory elements from comprehensive expression profiling

Iron is an essential cofactor in many metabolic reactions. Mechanisms controlling iron homeostasis need to respond rapidly to changes in extracellular conditions, but they must also keep the concentration of intracellular iron under strict control to avoid the generation of damaging reactive oxygen species. Due to its role as a redox carrier in photosynthesis, the iron quota in cyanobacteria is about 10 times higher than in model enterobacteria. The molecular details of how such a high quota is regulated are obscure. Here we present experiments that shed light on the iron regulatory system in cyanobacteria. We measured time-resolved changes in gene expression after iron depletion in the cyanobacterium Synechocystis sp. PCC 6803 using a comprehensive microarray platform, monitoring both protein-coding and non-coding transcripts. In total, less than a fifth of all protein-coding genes were differentially expressed during the first 72 hr. Many of these proteins are associated with iron transport, photosynthesis, or ATP synthesis. Comparing our data with three previous studies, we identified a core set of 28 genes involved in iron stress response. Among them were genes important for assimilation of inorganic carbon, suggesting a link between the carbon and iron regulatory networks. Nine of the 28 genes have unknown functions and constitute key targets for further functional analysis. Statistical and clustering analyses identified 10 small RNAs, 62 antisense RNAs, four 5′UTRs, and seven intragenic elements as potential novel components of the iron regulatory network in Synechocystis. Hence, our genome-wide expression profiling indicates an unprecedented complexity in the iron regulatory network of cyanobacteria.

Restricted capacity for PSI-dependent cyclic electron flow in ΔpetE mutant compromises the ability for acclimation to iron stress in Synechococcus sp. PCC 7942 cells

Exposure of wild type (WT) and plastocyanin coding petE gene deficient mutant (ΔpetE) of Synechococcus cells to low iron growth conditions was accompanied by similar iron-stress induced blue-shift of the main red Chl a absorption peak and a gradual decrease of the Phc/Chl ratio, although ΔpetE mutant was more sensitive when exposed to iron deficient conditions. Despite comparable iron stress induced phenotypic changes, the inactivation of petE gene expression was accompanied with a significant reduction of the growth rates compared to WT cells. To examine the photosynthetic electron fluxes in vivo, far-red light induced P700 redox state transients at 820nm of WT and ΔpetE mutant cells grown under iron sufficient and iron deficient conditions were compared. The extent of the absorbance change (ΔA(820)/A(820)) used for quantitative estimation of photooxidizable P700(+) indicated a 2-fold lower level of P700(+) in ΔpetE compared to WT cells under control conditions. This was accompanied by a 2-fold slower re-reduction rate of P700(+) in the ΔpetE indicating a lower capacity for cyclic electron flow around PSI in the cells lacking plastocyanin. Thermoluminescence (TL) measurements did not reveal significant differences in PSII photochemistry between control WT and ΔpetE cells. However, exposure to iron stress induced a 4.5 times lower level of P700(+), 2-fold faster re-reduction rate of P700(+) and a temperature shift of the TL peak corresponding to S(2)/S(3)Q(B)(-) charge recombination in WT cells. In contrast, the iron-stressed ΔpetE mutant exhibited only a 40% decrease of P700(+) and no significant temperature shift in S(2)/S(3)Q(B)(-) charge recombination. The role of mobile electron carriers in modulating the photosynthetic electron fluxes and physiological acclimation of cyanobacteria to low iron conditions is discussed. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: from Natural to Artificial.

THE CYANOBACTERIAL CHLOROPHYLL-BINDING-PROTEIN ISIA ACTS TO INCREASE THE IN VIVO EFFECTIVE ABSORPTION CROSS-SECTION OF PSI UNDER IRON LIMITATION(1)

Iron availability limits primary production in >30% of the world's oceans; hence phytoplankton have developed acclimation strategies. In particular, cyanobacteria express IsiA (iron-stress-induced) under iron stress, which can become the most abundant chl-binding protein in the cell. Within iron-limited oceanic regions with significant cyanobacterial biomass, IsiA may represent a significant fraction of the total chl. We spectroscopically measured the effective cross-section of the photosynthetic reaction center PSI (σPSI ) in vivo and biochemically quantified the absolute abundance of PSI, PSII, and IsiA in the model cyanobacterium Synechocystis sp. PCC 6803. We demonstrate that accumulation of IsiA results in a ∼60% increase in σPSI , in agreement with the theoretical increase in cross-section based on the structure of the biochemically isolated IsiA-PSI supercomplex from cyanobacteria. Deriving a chl budget, we suggest that IsiA plays a primary role as a light-harvesting antenna for PSI. On progressive iron-stress in culture, IsiA continues to accumulate without a concomitant increase in σPSI , suggesting that there may be a secondary role for IsiA. In natural populations, the potential physiological significance of the uncoupled pool of IsiA remains to be established. However, the functional role as a PSI antenna suggests that a large fraction of IsiA-bound chl is directly involved in photosynthetic electron transport.

Inhibition of photosynthetic oxygen evolution and electron transfer from the quinone acceptor QA- to QB by iron deficiency

The effect of iron deficiency on photosynthetic electron transport in Photosystem II (PS II) was studied in leaves and thylakoid membranes of lettuce (Lactuca sativa, Romaine variety) plants. PS II electron transport was characterized by oxygen evolution and chlorophyll fluorescence parameters. Iron deficiency in the culture medium was shown to affect water oxidation and the advancement of the S-states. A decrease of maximal quantum yield of PS II and an increase of fluorescence intensity at step J and I of OJIP kinetics were also observed. Thermoluminescence measurements revealed that charge recombination between the quinone acceptor of PS II, Q(B), and the S(2) state of the Mn-cluster was strongly perturbed. Also the dark decay of Chl fluorescence after a single turnover white flash was greatly retarded indicating a slower rate of Q(A)(-) reoxidation.

TonB-dependent transporters and their occurrence in cyanobacteria

Background

Different iron transport systems evolved in Gram-negative bacteria during evolution. Most of the transport systems depend on outer membrane localized TonB-dependent transporters (TBDTs), a periplasma-facing TonB protein and a plasma membrane localized machinery (ExbBD). So far, iron chelators (siderophores), oligosaccharides and polypeptides have been identified as substrates of TBDTs. For iron transport, three uptake systems are defined: the lactoferrin/transferrin binding proteins, the porphyrin-dependent transporters and the siderophore-dependent transporters. However, for cyanobacteria almost nothing is known about possible TonB-dependent uptake systems for iron or other substrates.

Results

We have screened all publicly available eubacterial genomes for sequences representing (putative) TBDTs. Based on sequence similarity, we identified 195 clusters, where elements of one cluster may possibly recognize similar substrates. For Anabaena sp. PCC 7120 we identified 22 genes as putative TBDTs covering almost all known TBDT subclasses. This is a high number of TBDTs compared to other cyanobacteria. The expression of the 22 putative TBDTs individually depends on the presence of iron, copper or nitrogen.

Conclusion

We exemplified on TBDTs the power of CLANS-based classification, which demonstrates its importance for future application in systems biology. In addition, the tentative substrate assignment based on characterized proteins will stimulate the research of TBDTs in different species. For cyanobacteria, the atypical dependence of TBDT gene expression on different nutrition points to a yet unknown regulatory mechanism. In addition, we were able to clarify a hypothesis of the absence of TonB in cyanobacteria by the identification of according sequences.

Alr0397 is an outer membrane transporter for the siderophore schizokinen in Anabaena sp. strain PCC 7120

Iron uptake in proteobacteria by TonB-dependent outer membrane transporters represents a well-explored subject. In contrast, the same process has been scarcely investigated in cyanobacteria. The heterocyst-forming cyanobacterium Anabaena sp. strain PCC 7120 is known to secrete the siderophore schizokinen, but its transport system has remained unidentified. Inspection of the genome of strain PCC 7120 shows that only one gene encoding a putative TonB-dependent iron transporter, namely alr0397, is positioned close to genes encoding enzymes involved in the biosynthesis of a hydroxamate siderophore. The expression of alr0397, which encodes an outer membrane protein, was elevated under iron-limited conditions. Inactivation of this gene caused a moderate phenotype of iron starvation in the mutant cells. The characterization of the mutant strain showed that Alr0397 is a TonB-dependent schizokinen transporter (SchT) of the outer membrane and that alr0397 expression and schizokinen production are regulated by the iron homeostasis of the cell.

Effects of iron limitation on the expression of metabolic genes in the marine cyanobacterium Trichodesmium erythraeum IMS101

Iron deficiency in axenic cultures of Trichodesmium erythraeum IMS101 led to significant declines in both nitrogen fixation rates and photochemical energy conversion efficiency, accompanied by downregulation of genes encoding the major iron-binding proteins, including psbA and psbE of photosystem II, psaA and psaC of photosystem I, petB and petC of the cytochrome b(6)f complex, and nifH. However, the iron-starved cultures remained viable and expression of the metalloprotein genes was partially or fully restored within 3 days following the addition of iron. Both physiological and molecular responses revealed that expression and synthesis of the nitrogen fixation and photosynthetic machinery follow the hierarchy of iron demand; that is, nitrogen fixation was far more susceptible to iron limitation than photosynthesis. Consequently, the nifH transcript exhibited a 1-2 day shorter half-life and two to three times faster degradation rate than that of the photosynthetic genes. Our results suggest that the changes in gene expression are related to the redox state in the shared photosynthetic/respiratory pathway which, when faced with short-term iron deficiency, signals Trichodesmium to selectively sacrifice nitrogen fixation to conserve iron for photosynthetic and respiratory electron transport. The observed functional and compositional alterations represent the compromises in gene expression and acclimation capacity between two basic metabolic pathways competing for iron when it is limiting.

The induction of CP43′ by iron-stress in Synechococcus sp. PCC 7942 is associated with carotenoid accumulation and enhanced fatty acid unsaturation

Comparative lipid analysis demonstrated reduced amount of PG (50%) and lower ratio of MGDG/DGDG in iron-stressed Synechococcus sp. PCC 7942 cells compared to cells grown under iron sufficient conditions. In parallel, the monoenoic (C:1) fatty acids in MGDG, DGDG and PG increased from 46.8%, 43.7% and 45.6%, respectively in control cells to 51.6%, 48.8% and 48.7%, respectively in iron-stressed cells. This suggests increased membrane dynamics, which may facilitate the diffusion of PQ and keep the PQ pool in relatively more oxidized state in iron-stressed compared to control cells. This was confirmed by chlorophyll fluorescence and thermoluminescence measurements. Analysis of carotenoid composition demonstrated that the induction of isiA (CP43') protein in response to iron stress is accompanied by significant increase of the relative abundance of all carotenoids. The quantity of carotenoids calculated on a Chl basis increased differentially with nostoxanthin, cryptoxanthin, zeaxanthin and beta-carotene showing 2.6-, 3.1-, 1.9- and 1.9-fold increases, respectively, while the relative amount of caloxanthin was increased only by 30%. HPLC analyses of the pigment composition of Chl-protein complexes separated by non-denaturating SDS-PAGE demonstrated even higher relative carotenoids content, especially of cryptoxanthin, in trimer and monomer PSI Chl-protein complexes co-migrating with CP43' from iron-stressed cells than in PSI complexes from control cells where CP43' is not present. This implies a carotenoid-binding role for the CP43' protein which supports our previous suggestion for effective energy quenching and photoprotective role of CP43' protein in cyanobacteria under iron stress.

Light-induced energy dissipation in iron-starved cyanobacteria: roles of OCP and IsiA proteins

In response to iron deficiency, cyanobacteria synthesize the iron stress-induced chlorophyll binding protein IsiA. This protein protects cyanobacterial cells against iron stress. It has been proposed that the protective role of IsiA is related to a blue light-induced nonphotochemical fluorescence quenching (NPQ) mechanism. In iron-replete cyanobacterial cell cultures, strong blue light is known to induce a mechanism that dissipates excess absorbed energy in the phycobilisome, the extramembranal antenna of cyanobacteria. In this photoprotective mechanism, the soluble Orange Carotenoid Protein (OCP) plays an essential role. Here, we demonstrate that in iron-starved cells, blue light is unable to quench fluorescence in the absence of the phycobilisomes or the OCP. By contrast, the absence of IsiA does not affect the induction of fluorescence quenching or its recovery. We conclude that in cyanobacteria grown under iron starvation conditions, the blue light-induced nonphotochemical quenching involves the phycobilisome OCP-related energy dissipation mechanism and not IsiA. IsiA, however, does seem to protect the cells from the stress generated by iron starvation, initially by increasing the size of the photosystem I antenna. Subsequently, the IsiA converts the excess energy absorbed by the phycobilisomes into heat through a mechanism different from the dynamic and reversible light-induced NPQ processes.

Frequency-Domain Spectroscopic Study of the PS I−CP43‘ Supercomplex from the Cyanobacterium Synechocystis PCC 6803 Grown under Iron Stress Conditions

Absorption, fluorescence excitation, emission, and hole-burning (HB) spectra were measured at liquid helium temperatures for the PS I-CP43' supercomplexes of Synechocystis PCC 6803 grown under iron stress conditions and for respective trimeric PS I cores. Results are compared with those of room temperature, time-domain experiments (Biochemistry 2003, 42, 3893) as well as with the low-temperature steady-state experiments on PS I-CP43' supercomplexes of Synechococcus PCC 7942 (Biochim. Biophys. Acta 2002, 1556, 265). In contrast to the CP43' of Synechococcus PCC 7942, CP43' of Synechocystis PCC 6803 possesses two low-energy states analogous to the quasidegenerate states A and B of CP43 of photosystem II (J. Phys. Chem. B 2000, 104, 11805). Energy transfer between the CP43' and the PS I core occurs, to a significant degree, through the state A, characterized with a broader site distribution function (SDF). It is demonstrated that the low temperature (T = 5 K) excitation energy transfer (EET) time between the state A of CP43' (IsiA) and the PS I core in PS I-CP43' supercomplexes from Synechocystis PCC 6803 is about 60 ps, which is significantly slower than the EET observed at room temperature. Our results are consistent with fast (< or =10 ps) energy transfer from state B to state A in CP43'. Energy absorbed by the CP43' manifold has, on average, a greater chance of being transferred to the reaction center (RC) and utilized for charge separation than energy absorbed by the PS I core antenna. This indicates that energy is likely transferred from the CP43' to the RC along a well-defined path and that the "red antenna states" of the PS I core are localized far away from that path, most likely on the B7-A32 and B37-B38 dimers in the vicinity of the PS I trimerization domain (near PsaL subunit). We argue that the A38-A39 dimer does not contribute to the red antenna region.

Controls on tropical Pacific Ocean productivity revealed through nutrient stress diagnostics

In situ enrichment experiments have shown that the growth of bloom-forming diatoms in the major high-nitrate low-chlorophyll (HNLC) regions of the world's oceans is limited by the availability of iron. Yet even the largest of these manipulative experiments represents only a small fraction of an ocean basin, and the responses observed are strongly influenced by the proliferation of rare species rather than the growth of naturally dominant populations. Here we link unique fluorescence attributes of phytoplankton to specific physiological responses to nutrient stress, and use these relationships to evaluate the factors that constrain phytoplankton growth in the tropical Pacific Ocean on an unprecedented spatial scale. On the basis of fluorescence measurements taken over 12 years, we delineate three major ecophysiological regimes in this region. We find that iron has a key function in regulating phytoplankton growth in both HNLC and oligotrophic waters near the Equator and further south, whereas nitrogen and zooplankton grazing are the primary factors that regulate biomass production in the north. Application of our findings to the interpretation of satellite chlorophyll fields shows that productivity in the tropical Pacific basin may be 1.2-2.5 Pg C yr(-1) lower than previous estimates have suggested, a difference that is comparable to the global change in ocean production that accompanied the largest El Niño to La Niña transition on record.

Iron deficiency in cyanobacteria causes monomerization of photosystem I trimers and reduces the capacity for state transitions and the effective absorption cross section of photosystem I in vivo

The induction of the isiA (CP43') protein in iron-stressed cyanobacteria is accompanied by the formation of a ring of 18 CP43' proteins around the photosystem I (PSI) trimer and is thought to increase the absorption cross section of PSI within the CP43'-PSI supercomplex. In contrast to these in vitro studies, our in vivo measurements failed to demonstrate any increase of the PSI absorption cross section in two strains (Synechococcus sp. PCC 7942 and Synechocystis sp. PCC 6803) of iron-stressed cells. We report that iron-stressed cells exhibited a reduced capacity for state transitions and limited dark reduction of the plastoquinone pool, which accounts for the increase in PSII-related 685 nm chlorophyll fluorescence under iron deficiency. This was accompanied by lower abundance of the NADP-dehydrogenase complex and the PSI-associated subunit PsaL, as well as a reduced amount of phosphatidylglycerol. Nondenaturating polyacrylamide gel electrophoresis separation of the chlorophyll-protein complexes indicated that the monomeric form of PSI is favored over the trimeric form of PSI under iron stress. Thus, we demonstrate that the induction of CP43' does not increase the PSI functional absorption cross section of whole cells in vivo, but rather, induces monomerization of PSI trimers and reduces the capacity for state transitions. We discuss the role of CP43' as an effective energy quencher to photoprotect PSII and PSI under unfavorable environmental conditions in cyanobacteria in vivo.

Effect of iron on growth and ultrastructure of Acaryochloris marina

The cyanobacterial genus Acaryochloris is the only known group of oxygenic phototrophs that contain chlorophyll d rather than chlorophyll a as the major photosynthetic pigment. Studies on this organism are still in their earliest stages, and biochemical analysis has rapidly outpaced growth optimization. We have investigated culture growth of the major strains of Acaryochloris marina (MBIC11017 and MBIC10697) by using several published and some newly developed growth media. It was determined that heavy addition of iron significantly enhanced culture longevity. These high-iron cultures showed an ultrastructure with thylakoid stacks that resemble traditional cyanobacteria (unlike previous studies). These cultures also show a novel reversal in the pigment ratios of the photosystem II signature components chlorophyll a and pheophytin a, as opposed to those in previous studies.

Biogenesis of chlorophyll-binding proteins under iron stress in Synechocystis sp. PCC 6803

The biogenesis of chlorophyll-binding proteins under iron stress has been investigated in vivo in a chlN deletion mutant of Synechocystis sp. PCC 6803. The chlN gene encodes one subunit of the light-independent protochlorophyllide reductase. The mutant is unable to synthesis chlorophyll in darkness, causing chlorophyll biosynthesis to become light dependent. When the mutant was propagated in darkness, essentially no chlorophyll and photosystems were detected. Upon return of the chlN deletion mutant to light, 77 K fluorescence emission spectra and oxygen evolution of greening cells under iron-sufficient or -deficient conditions were measured. The gradual blue shift of the photosystem I (PS I) peak upon greening under iron stress suggested the structural alteration of newly synthesized PS I. Furthermore, the rate of biogenesis of PS II was delayed under iron stress, which might be due to the presence of IsiA.

Constitutive Expression of Small Heat Shock Protein in an htpG Disruptant of the Cyanobacterium Synechococcus sp. PCC 7942

In cyanobacteria, a disruptant of hspA encoding a small heat shock protein homologue, shows decreased cell growth rates at moderately high temperatures, and loss of both basal and acquired thermo-tolerances, which resemble the phenotype of an htpG disruptant. In vitro studies have shown that both small heat shock protein and Hsp90 can bind and keep non-native proteins in a refolding-competent state under denaturing conditions. The aim of the present study is to elucidate whether constitutive expression of HspA can functionally replace HtpG, a prokaryotic homolog of Hsp90, in the cyanobacterium Synechococcus sp. PCC 7942. HspA did not improve the viability of the htpG disruptant at a lethal temperature, although it did that of the wild type. It did not improve an iron-starved phenotype of the mutant under normal growth conditions, a novel phenotype found in the present study. These results suggest that cellular function of HtpG may differ significantly from that of HspA.

Structure and functional role of supercomplexes of IsiA and Photosystem I in cyanobacterial photosynthesis

Cyanobacteria express large quantities of the iron stress-inducible protein IsiA under iron deficiency. IsiA can assemble into numerous types of single or double rings surrounding Photosystem I. These supercomplexes are functional in light-harvesting, empty IsiA rings are effective energy dissipaters. Electron microscopy studies of these supercomplexes show that Photosystem I trimers bind 18 IsiA copies in a single ring, whereas monomers may bind up to 35 copies in two rings. Work on mutants indicates that the PsaF/J and PsaL subunits facilitate the formation of closed rings around Photosystem I monomers but are not obligatory components in the formation of Photosystem I-IsiA supercomplexes.

The dependence of algal H2 production on Photosystem II and O2 consumption activities in sulfur-deprived Chlamydomonas reinhardtii cells

Chlamydomonas reinhardtii cultures, deprived of inorganic sulfur, undergo dramatic changes during adaptation to the nutrient stress [Biotechnol. Bioeng. 78 (2002) 731]. When the capacity for Photosystem II (PSII) O(2) evolution decreases below that of respiration, the culture becomes anaerobic [Plant Physiol. 122 (2000) 127]. We demonstrate that (a) the photochemical activity of PSII, monitored by in situ fluorescence, also decreases slowly during the aerobic period; (b) at the exact time of anaerobiosis, the remaining PSII activity is rapidly down regulated; and (c) electron transfer from PSII to PSI abruptly decreases at that point. Shortly thereafter, the PSII photochemical activity is partially restored, and H(2) production starts. Hydrogen production, which lasts for 3-4 days, is catalyzed by an anaerobically induced, reversible hydrogenase. While most of the reductants used directly for H(2) gas photoproduction come from water, the remaining electrons must come from endogenous substrate degradation through the NAD(P)H plastoquinone (PQ) oxido-reductase pathway. We propose that the induced hydrogenase activity provides a sink for electrons in the absence of other alternative pathways, and its operation allows the partial oxidation of intermediate photosynthetic carriers, including the PQ pool, between PSII and PSI. We conclude that the reduced state of this pool, which controls PSII photochemical activity, is one of the main factors regulating H(2) production under sulfur-deprived conditions. Residual O(2) evolved under these conditions is probably consumed mostly by the aerobic oxidation of storage products linked to mitochondrial respiratory processes involving both the cytochrome oxidase and the alternative oxidase. These functions maintain the intracellular anaerobic conditions required to keep the hydrogenase enzyme in the active, induced form.

Microarray analysis and redox control of gene expression in the cyanobacterium Synechocystis sp. PCC 6803

Expression profiles of Synechocystis sp. PCC 6803 genes in response to growth in iron-deficient versus iron-sufficient media and after 30 min treatment with H(2)O(2) were determined using a full-genome microarray. We used an anova model that accounted for slide and replicate (random) effects as well as dye (a fixed) effects to identify statistically significant, differentially expressed genes that changed by 1.25-fold or greater during each of these experiments. We utilized this microarray data to identify gene clusters that were regulated under these stresses, because we are interested in cellular redox control and the way in which the cell responds to oxidative stresses. We are particularly interested in using differential expression to help determine the function of genes involved in redox control and cluster analysis aids this process. We concentrated on four gene clusters, two of which were similarly affected by both stresses, and two that were only differentially regulated by one of the stresses. We also analysed the regulatory genes that responded to these oxidative stresses and discussed the changes in transcription of the RNA polymerase sigma factors and the other regulatory proteins, many of which represent two-component regulatory systems.

Microarray analysis of the genome-wide response to iron deficiency and iron reconstitution in the cyanobacterium Synechocystis sp. PCC 6803

A full-genome microarray of the (oxy)photosynthetic cyanobacterium Synechocystis sp. PCC 6803 was used to identify genes that were transcriptionally regulated by growth in iron (Fe)-deficient versus Fe-sufficient media. Transcript accumulation for 3,165 genes in the genome was analyzed using an analysis of variance model that accounted for slide and replicate (random) effects and dye (a fixed) effect in testing for differences in the four time periods. We determined that 85 genes showed statistically significant changes in the level of transcription (P </= 0.05/3,165 = 0.0000158) across the four time points examined, whereas 781 genes were characterized as interesting (P </= 0.05 but greater than 0.0000158; 731 of these had a fold change >1.25 x). The genes identified included those known previously to be Fe regulated, such as isiA that encodes a novel chlorophyll-binding protein responsible for the pigment characteristics of low-Fe (LoFe) cells. ATP synthetase and phycobilisome genes were down-regulated in LoFe, and there were interesting changes in the transcription of genes involved in chlorophyll biosynthesis, in photosystem I and II assembly, and in energy metabolism. Hierarchical clustering demonstrated that photosynthesis genes, as a class, were repressed in LoFe and induced upon the re-addition of Fe. Specific regulatory genes were transcriptionally active in LoFe, including two genes that show homology to plant phytochromes (cph1 and cph2). These observations established the existence of a complex network of regulatory interactions and coordination in response to Fe availability.

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

Red algae are well suited to study the effects of iron deficiency on light-harvesting complex for photosystem I (LHCI), since they are totally devoid of light-harvesting complex for photosystem II (LHCII). Iron starvation results in a reduction of the pigment content, an increase of the fluorescence yield and a new emission band at 705 nm in the 77 K fluorescence emission spectra. These changes reflect the accumulation of uncoupled, aggregated LHCI in iron-depleted cells. Reconnection of LHCI to de novo synthesized reaction center I (RCI) is the first event, which takes place after iron addition. The changes in the aggregation state of LHCI are likely to occur also in brown and green algae.