Phycobilisome diffusion is required for light-state transitions in cyanobacteria

Phycobilisomes are the major accessory light-harvesting complexes of cyanobacteria and red algae. Studies using fluorescence recovery after photobleaching on cyanobacteria in vivo have shown that the phycobilisomes are mobile complexes that rapidly diffuse on the thylakoid membrane surface. By contrast, the PSII core complexes are completely immobile. This indicates that the association of phycobilisomes with reaction centers must be transient and unstable. Here, we show that when cells of the cyanobacterium Synechococcus sp. PCC7942 are immersed in buffers of high osmotic strength, the diffusion coefficient for the phycobilisomes is greatly decreased. This suggests that the interaction between phycobilisomes and reaction centers becomes much less transient under these conditions. We discuss the possible reasons for this. State transitions are a rapid physiological adaptation mechanism that regulates the way in which absorbed light energy is distributed between PSI and PSII. Immersing cells in high osmotic strength buffers inhibits state transitions by locking cells into whichever state they were in prior to addition of the buffer. The effect on state transitions is induced at the same buffer concentrations as the effect on phycobilisome diffusion. This implies that phycobilisome diffusion is required for state transitions. The main physiological role for phycobilisome mobility may be to allow such flexibility in light harvesting.     

Isolation and characterization of disc-shaped phycobilisomes from the red alga rhodella violacea

Disc-shaped phycobilisomes were purified from Triton X100 treated cell homogenates of the unicellular marine red alga, Rhodella violacea. Their absorption spectrum had principal maxima at 544 and 568 nm (B-phycoerythrin), 624 nm (C-phycocyanin) and a distinct shoulder at 652 nm (allophycocyanin). Intermolecular energy transfer within the phycobilisomes was clearly demonstrated by fluorescence data. Excited at 546 nm intact phycobilisomes showed a main fluorescence emission maximum at 665 nm, a minor one at 577 nm and a shoulder at 730 nm.Dissociated phycobilisomes revealed a composition of 58% B-phycoerythrin, 25% C-phycocyanin and 17% allophycocyanin under the cultural conditions used. Analytical methods resolved no other components than phycobiliproteins. In addition to the defined C-phycocyanin and two isoproteins of B-phycoerythrin a stable heterogeneous aggregate of B-phycoerythrin/C-phycocyanin was separated in considerable amounts.In the electron microscope negatively stained phycobilisomes appeared as elliptical aggregates having dimensions slightly above the values found in ultrathin sections and a detailed subunit structure. All observations and data suggest a new rhodophytan phycobilisome type in Rhodella violacea.Abbreviations PBS phycobilisome(s) - PE B-phycoerythrin - PC C-phycocyanin - APC allophycocyanin - C concentration (mg/ml) - E extinction     

The Supramolecular Structure of Photosystem II — Phycobilisome-Complexes of Porphyridium cruentum

The structure and arrangement of phycobilisomes of the unicellular red alga Porphyridium cruentum is compared with the organization of the thylakoid freeze-fracture particles in order to determine the relationship between phycobilisomes and photosystem II. The hemi-ellipsoidal phycobilisomes, 20 nm thick, are predominantly organized into rows; their centre to centre periodicity is 30–40 nm, so that they are well separated by a gap of 10–20 nm. The phycobilisomes are cleaved by a central faint furrow, parallel to the long axis from top to base. The organization of the exoplasmic particles in rows is similar to the arrangement of the phycobilisomes so that a structural relationship between both systems, previously demonstrated in cyanobacteria, is evident. Within the rows, the 10 nm EF-particles are grouped in tetrameric complexes separated by distances similar to those observed for phycobilisomes. We propose that the tetrameric EF-particle complexes correspond to tetrameric photosystem II complexes which bind one hemi-ellipsoidal phycobilisome on the stroma exposed surface of the thylakoid. A hypothetical model of this photosystem II-phycobilisome complex is presented.     

The phycobilisome, a light-harvesting complex responsive to environmental conditions.

Photosynthetic organisms can acclimate to their environment by changing many cellular processes, including the biosynthesis of the photosynthetic apparatus. In this article we discuss the phycobilisome, the light-harvesting apparatus of cyanobacteria and red algae. Unlike most light-harvesting antenna complexes, the phycobilisome is not an integral membrane complex but is attached to the surface of the photosynthetic membranes. It is composed of both the pigmented phycobiliproteins and the nonpigmented linker polypeptides; the former are important for absorbing light energy, while the latter are important for stability and assembly of the complex. The composition of the phycobilisome is very sensitive to a number of different environmental factors. Some of the filamentous cyanobacteria can alter the composition of the phycobilisome in response to the prevalent wavelengths of light in the environment. This process, called complementary chromatic adaptation, allows these organisms to efficiently utilize available light energy to drive photosynthetic electron transport and CO2 fixation. Under conditions of macronutrient limitation, many cyanobacteria degrade their phycobilisomes in a rapid and orderly fashion. Since the phycobilisome is an abundant component of the cell, its degradation may provide a substantial amount of nitrogen to nitrogen-limited cells. Furthermore, degradation of the phycobilisome during nutrient-limited growth may prevent photodamage that would occur if the cells were to absorb light under conditions of metabolic arrest. The interplay of various environmental parameters in determining the number of phycobilisomes and their structural characteristics and the ways in which these parameters control phycobilisome biosynthesis are fertile areas for investigation.     

Blue light reduces photosynthetic efficiency of cyanobacteria through an imbalance between photosystems I and II

Several studies have described that cyanobacteria use blue light less efficiently for photosynthesis than most eukaryotic phototrophs, but comprehensive studies of this phenomenon are lacking. Here, we study the effect of blue (450 nm), orange (625 nm), and red (660 nm) light on growth of the model cyanobacterium Synechocystis sp. PCC 6803, the green alga Chlorella sorokiniana and other cyanobacteria containing phycocyanin or phycoerythrin. Our results demonstrate that specific growth rates of the cyanobacteria were similar in orange and red light, but much lower in blue light. Conversely, specific growth rates of the green alga C. sorokiniana were similar in blue and red light, but lower in orange light. Oxygen production rates of Synechocystis sp. PCC 6803 were five-fold lower in blue than in orange and red light at low light intensities but approached the same saturation level in all three colors at high light intensities. Measurements of 77 K fluorescence emission demonstrated a lower ratio of photosystem I to photosystem II (PSI:PSII ratio) and relatively more phycobilisomes associated with PSII (state 1) in blue light than in orange and red light. These results support the hypothesis that blue light, which is not absorbed by phycobilisomes, creates an imbalance between the two photosystems of cyanobacteria with an energy excess at PSI and a deficiency at the PSII-side of the photosynthetic electron transfer chain. Our results help to explain why phycobilisome-containing cyanobacteria use blue light less efficiently than species with chlorophyll-based light-harvesting antennae such as Prochlorococcus, green algae and terrestrial plants.     

Photosynthetic pigment localization and thylakoid membrane morphology are altered in Synechocystis 6803 phycobilisome mutants

Cyanobacteria are oxygenic photosynthetic prokaryotes that are the progenitors of the chloroplasts of algae and plants. These organisms harvest light using large membrane-extrinsic phycobilisome antenna in addition to membrane-bound chlorophyll-containing proteins. Similar to eukaryotic photosynthetic organisms, cyanobacteria possess thylakoid membranes that house photosystem (PS) I and PSII, which drive the oxidation of water and the reduction of NADP+, respectively. While thylakoid morphology has been studied in some strains of cyanobacteria, the global distribution of PSI and PSII within the thylakoid membrane and the corresponding location of the light-harvesting phycobilisomes are not known in detail, and such information is required to understand the functioning of cyanobacterial photosynthesis on a larger scale. Here, we have addressed this question using a combination of electron microscopy and hyperspectral confocal fluorescence microscopy in wild-type Synechocystis species PCC 6803 and a series of mutants in which phycobilisomes are progressively truncated. We show that as the phycobilisome antenna is diminished, large-scale changes in thylakoid morphology are observed, accompanied by increased physical segregation of the two photosystems. Finally, we quantified the emission intensities originating from the two photosystems in vivo on a per cell basis to show that the PSI:PSII ratio is progressively decreased in the mutants. This results from both an increase in the amount of photosystem II and a decrease in the photosystem I concentration. We propose that these changes are an adaptive strategy that allows cells to balance the light absorption capabilities of photosystems I and II under light-limiting conditions.     

The structure of Gloeobacter violaceus and its phycobilisomes

The fine structure of the atypical cyanobacterium Gloeobacter violaceus has been studied on frozen-etched replicas and compared to that of a typical unicellular strain: Synechocystis 6701. The complementary fracture faces of G. violaceus cytoplasmic membrane contain particles less numerous and more heterogenous in size than either the cytoplasmic membrane or the thylakoid membranes of Synechocystis. The most frequently observed particles of the exoplasmic fracture (EF) face of the G. violaceus cytoplasmic membrane are 11 nm in diameter and occasionally form short alignments. This particle class is similar in appearance to the numerous, aligned EF particles of Synechocystis thylakoid membranes. In replicas of cross-fractured G. violaceus, a layer 50–70 nm thick, composed of rod-like elements, underlies the inner surface of the cytoplasmic membrane. The rods, 12–14 nm in diameter, are oriented perpendicularly to the cytoplasmic membrane and show a 6 nm repeat along their length.Isolated phycobilisomes of G. violaceus appear, after fixation and negative staining, as bundles of 6 parallel rodshaped elements connected to an ill-defined basal structure. The bundles are 40–45 nm wide and 75–90 nm long. The rods are 10–12 nm in width; their length varies between 50 and 70 nm. These rods are morphologically similar to those observed at the periphery of hemidiscoidal phycobilisomes of other cyanobacteria, with a strong repeat at 6 nm intervals and a weaker one at 3 nm intervals along their length.The calculated molar ratio of phycobiliproteins in isolated G. violaceus phycobilisomes corresponds to 1:3.9:2.9 for allophycocyanin, phycocyanin and phycoerythrin respectively. When excited at 500 nm, isolated phycobilisomes exhibit a major fluorescence emission band centered at 663 nm.Abbreviations PBS phycobilisome(s) - PBP phycobiliprotein(s) - AP allophycocyanin - PC phycocyanin - PE phycoerythrin - K–PO₄ buffer KH₂PO₄ titrated with KOH to a given pH     

Growth under Red Light Enhances Photosystem II Relative to Photosystem I and Phycobilisomes in the Red Alga Porphyridium cruentum

Acclimation of the photosynthetic apparatus to light absorbed primarily by photosystem I (PSI) or by photosystem II (PSII) was studied in the unicellular red alga Porphyridium cruentum (ATCC 50161). Cultures grown under green light of 15 microeinsteins per square meter per second (PSII light; absorbed predominantly by the phycobilisomes) exhibited a PSII/PSI ratio of 0.26 ± 0.05. Under red light (PSI light; absorbed primarily by chlorophyll) of comparable quantum flux, cells contained nearly five times as many PSII per PSI (1.21 ± 0.10), and three times as many PSII per cell. About 12% of the chlorophyll was attributed to PSII in green light, 22% in white light, and 39% in red light-grown cultures. Chlorophyll antenna sizes appeared to remain constant at about 75 chlorophyll per PSII and 140 per PSI. Spectral quality had little effect on cell content or composition of the phycobilisomes, thus the number of PSII per phycobilisome was substantially greater in red light-grown cultures (4.2 ± 0.6) than in those grown under green (1.6 ± 0.3) or white light (2.9 ± 0.1). Total photosystems (PSI + PSII) per phycobilisome remained at about eight in each case. Carotenoid content and composition was little affected by the spectral composition of the growth light. Zeaxanthin comprised more than 50% (mole/mole), β-carotene about 40%, and cryptoxanthin about 4% of the carotenoid pigment. Despite marked changes in the light-harvesting apparatus, red and green light-grown cultures have generation times equal to that of cultures grown under white light of only one-third the quantum flux.     

PHOTOREGULATION OF PHYCOBILISOME STRUCTURE DURING COMPLEMENTARY CHROMATIC ADAPTATION IN THE MARINE CYANOPHYTE PHORMIDIUM SP. C861

Changes in the molecular structure of phycobilisomes during complementary chromatic adaptation were studied in the marine cyanophyte Phormidium sp. C86. This strain forms phycoerythrin (PE)-less phycobilisomes under red light but synthesizes PE-rich phycobilisomes under green light. Analysis of phycobiliprotein composition and electron microscopic examination of phycobilisomes in ultra-thin sections of cells and of isolated phycobilisomes were performed for cells acclimated to red and green light, respectively. The structure of phycobilisomes formed under red light conditions was typically hemidiscoidal. Phycobilisomes in cells acclimated to green light were twice as large in size as those in cells acclimated to red light. This increase in phycobilisome size was a result of the increase in the molar ratio of antenna pigment (PE and phycocyanin) to allophycocyanin, from 3.5 to 11.3. Pigment composition and fine structure of phycobilisomes formed under green light were similar to those of “nonhemidiscoidal” phycobilisomes reported in Phormidium persicinum. These results suggest that changes occur not only in the molecular species of peripheral rods but also in the structure of rods and probably of cores in relation to their connection with rods during chromatic adaptation of Phormidium sp. C86.     

Chromatic adaptation and the events involved in phycobilisome biosynthesis

Abstract. The major light-harvesting complex in cyanobacteria and red algae is the phycobilisome, a macromolecular complex that is attached to the surface of the photosynthetic membranes. The phycobilisome is composed of a number of different chromophoric polypeptides called phycobiliproteins and nonchromophoric polypeptides called linker proteins. Several environmental parameters modulate the synthesis, assembly and degradation of phycobilisome components. In many cyanobacteria, the composition of the phycobilisome can change to accommodate the prevalent wavelengths of light in the environment. This phenomenon is called complementary chromatic adaptation. Organisms that exhibit complementary chromatic adaptation must perceive the wavelengths of light in the environment and transduce the light signals into a sequence of biochemical events that result in altering the activities of genes encoding specific phycobiliprotein and linker polypeptides. Other environmental parameters such as light intensity and nutrient status can also have marked effects on both the number and composition of the phycobilisomes. The major concern of this article is the molecular events involved in chromatic adaptation. Most of the information concerning this process has been gained from studies involving the filamentous cyanobacterium Fremyella diplosiphon . However, also briefly considered are some of the complexities involved in phycobilisome biosynthesis and degradation; they include post-translational modification of phycobilisome polypeptides, the coordinate expression of chromophore and apobiliprotein, the specific degradation of phycobilisomes when cyanobacteria are deprived of macronutrients such as nitrogen, sulphur and phosphorus, and the assembly of the individual phycobilisome components into substructures of the light harvesting complex.     

Photochemical apparatus organization in Synechococcus 6301 (Anacystis nidulans). Effect of phycobilisome mutation

The photochemical apparatus organization in Synechococcus 6301 (Cyanophyceae) was investigated under various experimental conditions. Wild type (WT) Synechococcus produced phycobilisomes (PBSs) containing normal levels of phycocyanin (Phc) and allophycocyanin (Aphc). The ratio of reaction centers(RC) RCII/RCI of 0.4 was the same in WT and the mutant strain AN112, whereas RCH/PBS was 1.9:1 in WT and 1:1 in AN112. Excitation of WT cells with broad-band 620 nm light, which is absorbed primarily by Phc and Aphc and to a much lesser extent by chlorophyll (Chl), sensitized the RC of photosystem (PS) II at about 15 times the rate it sensitized RCI. This implies that PBSs are associated exclusively with PSII complexes and that PBS excitation is not transferred to PSI. The AN112 mutant of Synechococcus produced smaller PBSs consisting of the Aphc-containing core and of only six Phc-containing hexamers, respectively. It lacked about 65% of the Phccontaining rod substractures. Under our experimental conditions, the effective absorption cross section of the mutant PBS was only about half that of the WT. In agreement, the rate of RCII excitation by 620 nm light was also about half of that measured in the WT. Thus, the rate of light absorption by PSII depends directly on PBS size and composition. The low rate of RCI excitation with 620 nm light was the same in WT and AN112 cells, apparently independent of the PBS effective absorption cross section. We propose a strict structural-functional association between PBS and PSII complex. PSI is a structurally distinct entity and it receives excitation independently from its own Chl light-harvesting antenna.Abbreviations PBS phycobilisome - Phc phycocyanin - Aphc allophycocyanin - PS photosystem - RC reaction center - P700 reaction center of PSI - Q primary electron acceptor of PSII - Chl chlorophyll - MV methyl viologen - Tris Tris(hydroxymethyl)-aminomethane - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea     

The PsbU subunit of photosystem II stabilizes energy transfer and primary photochemistry in the phycobilisome-photosystem II assembly of Synechocystis sp. PCC 6803

The PsbU subunit of photosystem II (PSII) is one of three extrinsic polypeptides associated with stabilizing the oxygen evolving machinery of photosynthesis in cyanobacteria. We investigated the influence of PsbU on excitation energy transfer and primary photochemistry by spectroscopic analysis of a PsbU-less (or deltaPsbU) mutant. The absence of PsbU was found to have multiple effects on the excited state dynamics of the phycobilisome and PSII. DeltaPsbU cells exhibited decreased variable fluorescence when excited with light absorbed primarily by allophycocyanin but not when excited with light absorbed primarily by chlorophyll a. Fluorescence emission spectra at 77 K showed evidence for impaired energy transfer from the allophycocyanin terminal phycobilisome emitters to PSII. Picosecond fluorescence decay kinetics revealed changes in both allophycocyanin and PSII associated decay components. These changes were consistent with a decrease in the coupling of phycobilisomes to PSII and an increase in the number of closed PSII reaction centers in the dark-adapted deltaPsbU mutant. Our results are consistent with the assumption that PsbU stabilizes both energy transfer and electron transport in the PBS/PSII assembly.     

In Vitro Reconstitution of the Cyanobacterial Photoprotective Mechanism Mediated by the Orange Carotenoid Protein in Synechocystis PCC 6803

In conditions of fluctuating light, cyanobacteria thermally dissipate excess absorbed energy at the level of the phycobilisome, the light-collecting antenna. The photoactive Orange Carotenoid Protein (OCP) and Fluorescence Recovery Protein (FRP) have essential roles in this mechanism. Absorption of blue-green light converts the stable orange (inactive) OCP form found in darkness into a metastable red (active) form. Using an in vitro reconstituted system, we studied the interactions between OCP, FRP, and phycobilisomes and demonstrated that they are the only elements required for the photoprotective mechanism. In the process, we developed protocols to overcome the effect of high phosphate concentrations, which are needed to maintain the integrity of phycobilisomes, on the photoactivation of the OCP, and on protein interactions. Our experiments demonstrated that, whereas the dark-orange OCP does not bind to phycobilisomes, the binding of only one red photoactivated OCP to the core of the phycobilisome is sufficient to quench all its fluorescence. This binding, which is light independent, stabilizes the red form of OCP. Addition of FRP accelerated fluorescence recovery in darkness by interacting with the red OCP and destabilizing its binding to the phycobilisome. The presence of phycobilisome rods renders the OCP binding stronger and allows the isolation of quenched OCP-phycobilisome complexes. Using the in vitro system we developed, it will now be possible to elucidate the quenching process and the chemical nature of the quencher.     

Ultrastructure of the vegetative gametophytic cells of Porphyra leucosticta (Rhodophyta) grown in red, blue and green light

The ultrastructure of the vegetative gametophytic cells of Porphyra leucosticta Thuret grown in red, blue and green light was studied both in ultrathin sections and in replicas of rapidly frozen cells. High activity of dictyosornes and mucilage sacs results in a dramatic decrease of the protoplasmic area and in thicker cell walls in red light in comparison with blue light and the control. There are numerous well‐formed phycobili‐somes in blue light, whereas not well‐formed ones are present in red and especially in green light. There are also many phycobilisomes in the intrapyrenoidal thylakoids in blue light, fewer in green light, but they are absent in red light and in the control. It seems that in red and especially in green light, the phycobilisomes have fewer rods than in blue light. In green light, chloroplasts bear numerous genophores in contrast to blue and red light. The spacings of neighboring parallel thylakoids are as follows: control 64.3 nm, blue light 90.6 nm, red light 41.3 nm, green light 43.7 nm. Due to the relatively small spacing of the neighboring parallel thylakoids in red (41.3 nm) and in green light (43.7 nm) and of the given height of phycobilisomes (35 nm), the alternate phycobilisomes attached to neighboring lamellae are forced to interdigitate. The density of phycobilisomes per square micrometer of thylakoid surface dramatically increases in blue light (800 μm^?2) in relation to red (250 μm^?2) and green light (180 μm^?2). The protoplasmic fracture face of the thylakoids reveals numerous, tightly packed, but randomly distributed particles. The particle size distribution is uniform in the two types of fracture faces, with an average diameter of about 11.5 nm. In blue light, both the phycobilisomes and exoplasmic face particles are organized into rows with a spacing of 60–70 nm. The results (changes: in the protoplasmic area; in the spacing of the thylakoids; in phycobilisome arrangement; in structure, shape and size of phycobilisomes; and in the accumulation of plastoglobuli), have shown that the monochromatic light (blue, red and green) brings about marked changes in the package effect and consequently in the efficiency of light absorption. In addition, the blue light contributes to the intense production of chlorophyll a, phycoerythrin, phycocyanin and soluble proteins, while intense production of polysaccharidic material is attributed to red light.     

A new type of complementary chromatic adaptation exemplified byPhormidium sp. C86: Changes in the number of peripheral rods and in the stoichiometry of core complexes in phycobilisomes

The marine cyanobacterium Phormidium sp. strain C86 changes the phycobilisome type depending on light quality. Red-light-adapted cells contained hemidiscoidal phycobilisomes with a photosystem II:phycobilisome ratio of 2.2, while green-light-adapted cells exhibited hemiellipsoidal phycobilisomes with a photosystem II:phycobilisome ratio of 4.4, as determined by a combined analysis of freeze-fractured thylakoid membranes and ultrathin sections and by photochemical determinations of photosystems and phycobilisomes. Core complexes of phycobilisomes of red- and green-light-adapted cells were isolated by affinity chromatography and were subsequently separated into two allophycocyanin-containing fractions. The high-molecular-weight fraction, with a sedimentation coefficient of 24 S and a calculated mol. wt. of 860,000, contained complexes of the quaternary structure (α^AP ₉β^AP ₈β^19.5AP)₂· (L_CM)₂ and tricylindrical shape, previously designated AP_CM. This fraction was similar in size in red- and green-light-adapted cells; however, differences were detected in the low-molecular-weight allophycocyanin fraction containing the "trimeric" complexes with a sedimentation coefficient of 6 S. As shown by comparison of spectral and stoichiometric data of intact phycobilisomes and isolated core complexes, the amount of the α^APB-containing core complex (α^AP ₂α^APBβ^AP ₃) · L_C ¹⁰ was greater in core fractions of green-light phycobilisomes, whereas the amount of the core complexes (α^AP ₃β^AP ₃) · L_C ¹⁰, designated AP · L_C ¹⁰, was higher in cores of red-light phycobilisomes. Phormidium sp. is the first organism examined that exhibits a new type of complementary chromatic adaptation by altering the composition of the phycobilisome core and the number and composition of peripheral rods and by changing the ratio of photosystem II to phycobilisomes. A model summarizing the structural consequences of the results is presented. Received: 5 December 1995 / Accepted: 10 April 1995     

Distinct roles of CpcG1-phycobilisome and CpcG2-phycobilisome in state transitions in a cyanobacterium <Emphasis Type="Italic">Synechocystis</Emphasis> sp. PCC 6803

State transitions in cyanobacteria regulate the relative energy transfer from phycobilisome to photosystem I and II. Although it has been shown that phycobilisome mobility is essential for phycobilisome-dependent state transitions, the biochemical mechanism is not known. Previously we reported that two distinct forms of phycobilisome are assembled with different CpcG copies, which have been referred to as “rod-core linker,” in a cyanobacterium Synechocystis sp. PCC 6803. CpcG2-phycobilisome is devoid of a typical central core, while CpcG1-phycobilisome is equivalent to the conventional phycobilisome supercomplex. Here, we demonstrated that the cpcG1 disruptant has a severe specific defect in the phycobilisome-dependent state transition. However, fluorescence recovery after photobleaching measurements showed no obvious difference in phycobilisome mobility between the wild type and the cpcG1 disruptant. This suggests that both CpcG1 and CpcG2 phycobilisomes have an unstable interaction with the reaction centres. However, only CpcG1 phycobilisomes are involved in state transitions. This suggests that state transitions require the phycobilisome core.     

Subcellular localization of ferredoxin-NADP+ oxidoreductase in phycobilisome retaining oxygenic photosysnthetic organisms

Ferredoxin-NADP⁺ oxidoreductase (FNR) catalyzing the terminal step of the linear photosynthetic electron transport was purified from the cyanobacterium Spirulina platensis and the red alga Cyanidium caldarium. FNR of Spirulina consisted of three domains (CpcD-like domain, FAD-binding domain, and NADP⁺-binding domain) with a molecular mass of 46 kDa and was localized in either phycobilisomes or thylakoid membranes. The membrane-bound FNR with 46 kDa was solublized by NaCl and the solublized FNR had an apparent molecular mass of 90 kDa. FNR of Cyanidium consisted of two domains (FAD-binding domain and NADP⁺-binding domain) with a molecular mass of 33 kDa. In Cyanidium, FNR was found on thylakoid membranes, but there was no FNR on phycobilisomes. The membrane-bound FNR of Cyanidium was not solublized by NaCl, suggesting the enzyme is tightly bound in the membrane. Although both cyanobacteria and red algae are photoautotrophic organisms bearing phycobilisomes as light harvesting complexes, FNR localization and membrane-binding characteristics were different. These results suggest that FNR binding to phycobilisomes is not characteristic for all phycobilisome retaining oxygenic photosynthetic organisms, and that the rhodoplast of red algae had possibly originated from a cyanobacterium ancestor, whose FNR lacked the CpcD-like domain.     

Photoprotection in cyanobacteria: the orange carotenoid protein (OCP)-related non-photochemical-quenching mechanism

Plants and algae have developed multiple protective mechanisms to survive under high light conditions. Thermal dissipation of excitation energy in the membrane-bound chlorophyll-antenna of photosystem II (PSII) decreases the energy arriving at the reaction center and thus reduces the generation of toxic photo-oxidative species. This process results in a decrease of PSII-related fluorescence emission, known as non-photochemical quenching (NPQ). It has always been assumed that cyanobacteria, the progenitor of the chloroplast, lacked an equivalent photoprotective mechanism. Recently, however, evidence has been presented for the existence of at least three distinct mechanisms for dissipating excess absorbed energy in cyanobacteria. One of these mechanisms, characterized by a blue-light-induced fluorescence quenching, is related to the phycobilisomes, the extramembranal antenna of cyanobacterial PSII. In this photoprotective mechanism the soluble carotenoid-binding protein (OCP) encoded by the slr1963 gene in Synechocystis sp. PCC 6803, of previously unknown function, plays an essential role. The amount of energy transferred from the phycobilisomes to the photosystems is reduced and the OCP acts as the photoreceptor and as the mediator of this antenna-related process. These are novel roles for a soluble carotenoid protein.     

Large reallocations of carbon,nitrogen, and photosynthetic reductant among phycobilisomes,photosystems, and Rubisco during light acclimation in <Emphasis Type="Italic">Synechococcus elongatus</Emphasis> strain PCC7942 are constrained in cells under low environmental inorganic carbon

Synechococcus elongatus strain PCC7942 cells were grown in high or low environmental concentrations of inorganic C (high-C_i, low-C_i) and subjected to a light shift from 50 µmol m^–2 s^–1 to 500 µmol m^–2 s^–1. We quantified photosynthetic reductant (O₂ evolution) and molar cellular contents of phycobilisomes, PSII, PSI, and ribulose-1,5-bisphosphate carboxylase oxygenase (Rubisco) through the light shift. Upon the increase in light, small initial relative decreases in phycobilisomes per cell resulted from near cessation of phycobilisome synthesis and their dilution into daughter cells. Thus, allocation of reductant to phycobilisome synthesis dropped fivefold from pre- to post-light shift. The decrease in phycobilisome synthesis liberated enough material and reductant to allow a doubling of Rubisco and up to a sixfold increase in PSII complexes per cell. Low-C_i cells had smaller initial phycobilisome pools and upon increased light; their reallocation of reductant from phycobilisome synthesis may have limited the rate and extent of light acclimation, compared to high-C_i cells. Acclimation to increased light involved large reallocations of C, N, and reductant among different components of the photosynthetic apparatus, but total allocation to the apparatus was fairly stable at ca. 50% of cellular N, and drew 25–50% of reductant from photosynthesis.