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     

ACCLIMATION OF THE PHOTOSYNTHETIC APPARATUS OF PALMARIA PALMATA (RHODOPHYTA) TO LIGHT QUALITIES THAT PREFERENTIALLY EXCITE PHOTOSYSTEM I OR PHOTOSYSTEM II1

Acclimation of the photosynthetic apparatus to light absorbed primarily by phycobilisomes (which transfer energy predominantly to photosystem II) or absorbed by chlorophyll a (mainly present in the antenna of photosystem I) was studied in the macroalga Palmaria palmata L. In addition, the influence of blue and yellow light, exciting chlorophyll a and phycobilisomes, respectively, ivas investigated. All results were compared to a white light control. Complementary chromatic adaptation in terms of an enhanced ratio of phycoerythrin to phycocyanin under green light conditions was observed. Red light (mainly absorbed by chlorophyll a) and green light (mainly absorbed by phycobilisomes) caused an increase of the antenna system, which was not preferentially excited. Yellow and blue light led to intermediate states comparable to each other and white light. Growth was reduced under all light qualities in comparison to white light, especially under conditions preferably exciting phycobilisomes (green light-adapted algae had a 58% lower growth rate compared to white light-adapted algae). Red and blue light-adapted algae showed maximal photosynthetic capacity with white light excitation and significantly lower values with green light excitation. In contrast, green and yellow light-adapted algae exhibited comparable photosynthetic capacities at all excitation wavelengths. Low-temperature fluorescence emission analysis showed an increase of photosystem II emission in red light-adapted algae and a decrease in green light-adapted algae. A small increase of photosystem I emission teas also found in green light-adapted algae, but this was much less than the photosystem II emission increase observed in red light-adapted algae (both compared to phycobilisome emission). Efficiency of energy transfer from phycobilisomes to photosystem II was higher in red than in green light-adapted algae. The opposite was found for the energy transfer efficiency from phycobilisomes to photosystem I. Zeaxanthin content increased in green and blue light-adapted algae compared to red, white, and yellow light-adapted algae. Results are discussed in comparison to published data on unicellular red algae and cyanobacteria.     

Shade adaptation in cyanobacteria

Growth of Anacystis in high light in the presence of sublethal concentrations of DCMU-type inhibitors leads to an increased synthesis of phycocyanin paralleled by a reduced rate of ³⁵S methionine incorporation into the D1 protein compared to the high light controls, as is characteristic for naturally-induced shade phenotype. On the contrary, sun phenotype is characterized by a low rate of antenna synthesis, but a high rate of ³⁵S methionine incorporation into the D1 protein.Room temperature excitation spectra of 684 nm fluorescence emission clearly demonstrate the participation of the extraordinarily high concentration of phycocyanin in artificially shade-adapted cells in excitation energy transfer to chlorophyll.It could be shown that the development of shade-type appearance is not simply the consequence of an imbalance in electron transport, since an addition of thiosulphate to cultures growing in high light in the presence of DCMU-type inhibitors can only partially prevent or revert the change from sun to artificial-herbicide-induced-shade phenotype. This is regarded as evidence that the dynamic herbicide-binding D1 protein itself may play a role as a light meter in the process of natural shade adaptation, the rate of its degradation and resynthesis possibly giving the signal for the adaptive reorganization of the photosynthetic apparatus. The chain of signal transduction remains to be established.Abbreviations atrazine 2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine - chl chlorophyll - D1 reaction center polypeptide carrying the secondary plastoquinone electron acceptor of PS II - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - PAGE polyacrylamide gel electrophoresis - PAR photosynthetically active radiation - PC phycocyanin - PCC Pasteur Culture Collection - PS photosystem - Q_B secondary plastoquinone electron acceptor of PS II - SAUG Sammlung von Algenkulturen am Pflanzenphysiologischen Institut der Universtität Göttingen - SDS sodium dodecyl sulphate Dedicated to Professor Wilhelm Menke on the occasion of his 80th birthday.     

Integration of Apo-α-Phycocyanin into Phycobilisomes and Its Association with FNRL in the Absence of the Phycocyanin α-Subunit Lyase (CpcF) in Synechocystis sp. PCC 6803

Phycocyanin is an important component of the phycobilisome, which is the principal light-harvesting complex in cyanobacteria. The covalent attachment of the phycocyanobilin chromophore to phycocyanin is catalyzed by the enzyme phycocyanin lyase. The photosynthetic properties and phycobilisome assembly state were characterized in wild type and two mutants which lack holo-α-phycocyanin. Insertional inactivation of the phycocyanin α-subunit lyase (ΔcpcF mutant) prevents the ligation of phycocyanobilin to α-phycocyanin (CpcA), while disruption of the cpcB/A/C2/C1 operon in the CK mutant prevents synthesis of both apo-α-phycocyanin (apo-CpcA) and apo-β-phycocyanin (apo-CpcB). Both mutants exhibited similar light saturation curves under white actinic light illumination conditions, indicating the phycobilisomes in the ΔcpcF mutant are not fully functional in excitation energy transfer. Under red actinic light illumination, wild type and both phycocyanin mutant strains exhibited similar light saturation characteristics. This indicates that all three strains contain functional allophycocyanin cores associated with their phycobilisomes. Analysis of the phycobilisome content of these strains indicated that, as expected, wild type exhibited normal phycobilisome assembly and the CK mutant assembled only the allophycocyanin core. However, the ΔcpcF mutant assembled phycobilisomes which, while much larger than the allophycocyanin core observed in the CK mutant, were significantly smaller than phycobilisomes observed in wild type. Interestingly, the phycobilisomes from the ΔcpcF mutant contained holo-CpcB and apo-CpcA. Additionally, we found that the large form of FNR (FNR_L) accumulated to normal levels in wild type and the ΔcpcF mutant. In the CK mutant, however, significantly less FNR_L accumulated. FNR_L has been reported to associate with the phycocyanin rods in phycobilisomes via its N-terminal domain, which shares sequence homology with a phycocyanin linker polypeptide. We suggest that the assembly of apo-CpcA in the phycobilisomes of ΔcpcF can stabilize FNR_L and modulate its function. These phycobilisomes, however, inefficiently transfer excitation energy to Photosystem II.     

Structural analysis of the intracellular domain of (pro)renin receptor fused to maltose-binding protein

In Synechocystis sp. PCC 6803, the loop domain (aa 1–70) of the phycobilisome core-membrane linker, L_CM, was found to interact with the glycosyl transferase homolog, Sll1466. Growth of a Sll1466 knock-out mutant was slightly faster in low light, but strongly inhibited in high light; the phenotype is discussed in relation to the regulation of light energy transfer to photosystem II. At the molecular level, the mutant shows the following changes compared to the wild type: (1) a smaller size and higher mobility of phycobilisomes on the thylakoid membrane, and (2) a changed lipid composition of the thylakoid membrane, especially decreased amounts of digalactosyl diacylglycerol. These results indicate a profound regulatory role for Sll1466 in regulating photosynthetic energy transfer.     

Photochemical Apparatus Organization in Anacystis nidulans (Cyanophyceae) : Effect of CO(2) Concentration during Cell Growth

Anacystis nidulans cells grown under high (3%) CO₂ partial pressure have greater phycocyanin to chlorophyll ratio (Phc/Chl) relative to cells grown under low (0.2%) CO₂ tension (Eley (1971) Plant Cell Physiol 12: 311-316). Absorbance difference spectrophotometry of A. nidulans thylakoid membranes in the ultraviolet (ΔA₃₂₀) and red (ΔA₇₀₀) regions of the spectrum reveal photosystem II/photosystem I (PSII/PSI) reaction center ratio (RCII/RCI) changes that parallel those of Phc/Chl. For cells growing under 3% CO₂, the Phc/Chl ratio was 0.48 and RCII/RCI = 0.40. At 0.2% CO₂, Phc/Chl = 0.38 and RCII/RCI = 0.24. Excitation of intact cells at 620 nm sensitized RCII at a rate approximately 20 times faster than that of RCI, suggesting that Phc excitation is delivered to RCII only. In the presence of DCMU, excitation at 620 nm induced single exponential RCII photoconversion kinetics, suggesting a one-to-one structural-functional correspondance between phycobilisome and PSII complex in the thylakoid membrane. Therefore, phycobilisomes may serve as microscopic markers for the presence of PSII in the photosynthetic membrane of A. nidulans. Neither the size of individual phycobilisomes nor the Chl light-harvesting antenna of PSI changed under the two different CO₂ tensions during cell growth. Our results are compatible with the hypothesis that, at low CO₂ concentrations, the greater relative amounts of PSI present may facilitate greater rates of ATP synthesis via cyclic electron flow. The additional ATP may be required for the active uptake of CO₂ under such conditions.     

Dynamic aspects of phycobilisome structure: Modulation of phycocyanin content of Synechococcus phycobilisomes

Phycobilisomes of the cyanobacterium Synechococcus 6301 contain the phycobiliproteins phycocyanin, allophycocyanin, and allophycocyanin B, and four major non pigmented polypeptides of 75, 33, 30, and 27 kdaltons. The molar ratio of phycocyanin to allophycocyanin in wild type phycobilisomes can be varied over about a two-fold range by alterations in culture conditions with parallel changes in the amounts of the 33 and 30 kdalton polypeptides whereas the levels of the 27 and 75 kdalton polypeptides do not vary. Two nitrosoguanidine-induced mutants, AN112 and AN135, produce abnormally small phycobilisomes, containing only 35 and 50% of the wild type level of phycocyanin. AN135 phycobilisomes contain less 33 kdalton polypeptide than wild type and the 30 kdalton polypeptide is only detected in phycobilisomes from cultures grown under conditions favoring high levels of phycocyanin. AN112 lacks both the 30 and 33 kdalton polypeptides and produces phycobilisomes of constant size and composition, independent of growth conditions. Both mutant phycobilisomes have wild type levels of 27 and 75 kdalton polypeptides relative to allophycocyanin and have normal energy transfer properties. These results indicate that modulation of phycobilisome size involves concurrent regulation of the levels of phycocyanin and of both the 30 and 33 kdalton polypeptides with no change in the composition of the allophycocyanin-containing core.Abbreviations LP cells cells grown under conditions favoring low p phycobiliprotein levels - HP cells cells grown under conditions favoring high phycobiliprotein levels - SDS sodium dodecylsulfate - EDTA ethylenediamine tetraacetic acid - NaK-PO₄ NaH₂PO₄ titrated with K₂HPO₄ to a given pH A preliminary report of some of this work was presented at the 81st Annual Meeting of the American Society for Microbiology, Dallas, Texas, March 1981     

Ultra-violet mutagenesis of Synechocystis sp. 6701: mutations in chromatic adaptation and phycobilisome assembly

Mutations affecting pigmentation of the cyanobacterium Synechocystis sp. 6701 were induced with ultraviolet light. Two mutants with phycobilisome structural changes were selected for structural studies. One mutant, UV08, was defective in chromatic adaptation and incorporated phycoerythrin into phycobilisomes in white or red light at a level typical of growth in green light. The other mutant, UV16, was defective in phycobilisome assembly: little phycocyanin was made and none was attached to the phycobilisome cores. The cores were completely free of any rod substructures and contained the major core peptides plus the 27,000 M_r linker peptide that attaches rods to the core. Micrographs of the core particles established their structural details. Phycoerythrin in UV 16 was assembled into rod structures that were not associated with core material or phycocyanin. The 30,500 M_r and 31,500 M_r linker peptides were present in the phycoerythrin rods with the 30,500 M_r protein as the major component. Phycobilisome assembly in vivo is discussed in light of this unusual mutant.Abbreviations PE phycoerythrin - PC phycocyanin - AP allophycocyanin - W white light - G green light - R red light - SDS sodium dodecyl sulfate - Na–K–PO₄ equimolar solutions of NaH₂PO₄ · H₂O and K₂HPO₄ · 3 H₂O titrated to the desired pH     

The phycobilisome core-membrane linkers from Synechocystis sp. PCC 6803 and red-algae assemble in the same topology

The phycobilisomes (PBSs) of cyanobacteria and red-algae are unique megadaltons light-harvesting protein-pigment complexes that utilize bilin derivatives for light absorption and energy transfer. Recently, the high-resolution molecular structures of red-algal PBSs revealed how the multi-domain core-membrane linker (L_CM) specifically organizes the allophycocyanin subunits in the PBS’s core. But, the topology of L_CM in these structures was different than that suggested for cyanobacterial PBSs based on lower-resolution structures. Particularly, the model for cyanobacteria assumed that the Arm2 domain of L_CM connects the two basal allophycocyanin cylinders, whereas the red-algal PBS structures revealed that Arm2 is partly buried in the core of one basal cylinder and connects it to the top cylinder. Here, we show by biochemical analysis of mutations in the apcE gene that encodes L_CM, that the cyanobacterial and red-algal L_CM topologies are actually the same. We found that removing the top cylinder linker domain in L_CM splits the PBS core longitudinally into two separate basal cylinders. Deleting either all or part of the helix-loop-helix domain at the N-terminal end of Arm2, disassembled the basal cylinders and resulted in degradation of the part containing the terminal emitter, ApcD. Deleting the following 30 amino-acids loop severely affected the assembly of the basal cylinders, but further deletion of the amino-acids at the C-terminal half of Arm2 had only minor effects on this assembly. Altogether, the biochemical data are consistent with the red-algal L_CM topology, suggesting that the PBS cores in cyanobacteria and red-algae assemble in the same way.     

Carotenoid-triggered energy dissipation in phycobilisomes of Synechocystis sp. PCC 6803 diverts excitation away from reaction centers of both photosystems

Cyanobacteria are capable of using dissipation of phycobilisome-absorbed energy into heat as part of their photoprotective strategy. Non-photochemical quenching in cyanobacteria cells is triggered by absorption of blue-green light by the carotenoid-binding protein, and involves quenching of phycobilisome fluorescence. In this study, we find direct evidence that the quenching is accompanied by a considerable reduction of energy flow to the photosystems. We present light saturation curves of photosystems’ activity in quenched and non-quenched states in the cyanobacterium Synechocystis sp. PCC 6803. In the quenched state, the quantum efficiency of light absorbed by phycobilisomes drops by about 30-40% for both photoreactions—P700 photooxidation in the photosystem II-less strain and photosystem II fluorescence induction in the photosystem I-less strain of Synechocystis. A similar decrease of the excitation pressure on both photosystems leads us to believe that the core-membrane linker allophycocyanin APC-L_CM is at or beyond the point of non-photochemical quenching. We analyze 77 K fluorescence spectra and suggest that the quenching center is formed at the level of the short-wavelength allophycocyanin trimers. It seems that both chlorophyll and APC-L_CM may dissipate excess energy via uphill energy transfer at physiological temperatures, but neither of the two is at the heart of the carotenoid-binding protein-dependent non-photochemical quenching mechanism.     

Action of selected heavy metal ions on the photosystem 2 activity of the cyanobacteriumSpirulina platensis

Addition of different concentrations of heavy metal ions (Hg²⁺, Cu²⁺, Ni²⁺ and Pb²⁺) inhibited the photosystem 2 catalyzed electron transport activity (H₂O→p-benzo-quinone) of the cyanobacteriumSpirulina platensis. Hg²⁺ caused the inhibition in electron transport activity in very low concentrations compared to the other metal ions. Hg²⁺ at this low concentration specifically altered the spectral properties of phycocyanin of the phycobilisomes in the intact cells ofSpirulina, whereas other heavy metal ions were ineffective in this sense.     

Photosynthetische carboxylierungsreaktionen verschieden pigmentierter Anacystis-zellen

Summary CO₂ exchange, ¹⁴CO₂ fixation and ¹⁴C-labelled photosynthetic products of differently pigmented Anacystis nidulans (strain L 1402-1) were studied during the induction period at +30°C. The algae were grown at +35° C in an atmosphere of 0.04 vol.-% CO₂ and measured under the same low CO₂ concentrations. Changing the culture conditions caused alterations in the pigment composition. Under normal illumination (white light; 0.6×10³ erg/ cm² s) the relation between amounts of chlorophyll a and phycocyanin was 1:7 to 1:10. In a high light intensity (30.8×10³ erg/cm² s) the phycocyanin content was reduced (1:5 to 1:2). When the cells were grown in red light of high intensity (20×10³ erg/ cm² s) phycocyanin synthesis increased; the pigment ratio varied between 1:20 and 1:33. Anacystis cells grown under strong white light were filamentous.Photosynthetic CO₂ uptake, measured with an infrared gas analyzer, was very low in algae grown in high light intensity. The pattern of ¹⁴C-labelled photosynthetic products of these algae was very similar to those of the Calvin cycle. In Anacystis cells grown under low intensities of white light or in red light ¹⁴CO₂ was, at the beginning of the light period, incorporated mainly into aspatate and glycerine/serine. The enzyme activities of NAD⁺-specific malate dehydrogenase, ribulose-1,5-diphosphate carboxylase, aspartate and alanine aminotransferase decreased with increasing phycocyanin content. NADP⁺-specific malic enzyme activities showed practically no change. In contrast, phosphoenolpyruvate carboxylase activity increased with a higher rate of phycocyanin synthesis. In another series of experiments the behaviour of the PEP carboxylase activity after breakdown of the Anacystis cells was tested in differently pigmented cultures. In all cases the enzyme activities very rapidly decreased within two hours. The results obtained are discussed with reference to the correlation of pigment composition and CO₂ fixation of the phosphoenolpyruvate system.

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Abkürzungen Asp Aspartat - Gly/Ser Glycin/Serin - PGS 3-Phosphoglycerat - ZmP Zuckermonophosphat Herrn Professor Dr. Andre Pirson in Verehrung gewidmet     

Protective dissipation of excess absorbed energy by photosynthetic apparatus of cyanobacteria: role of antenna terminal emitters

Two mechanisms of photoprotective dissipation of the excessively absorbed energy by photosynthetic apparatus of cyanobacteria are described that divert energy from reaction centers. Energy dissipation, monitored as nonphotochemical fluorescence quenching, occurs at different steps of energy transfer within the phycobilisomes or core antenna of photosystem I. Although these mechanisms differ significantly, in both cases, energy dissipates mainly from terminal emitters: allophycocyanin B or core membrane linker protein (L_CM) in phycobilisomes, or the longest-wavelength chlorophylls in photosystem I antenna. It is supposed that carotenoid-induced energy dissipation in phycobilisomes is triggered by light-induced transformation of the nonquenched state of antenna into quenched state due to conformation changes caused by orange carotinoid-binding protein (OCP)–phycobilisome interaction. Fluorescence of the longest-wavelength chlorophylls of photosystem I antenna is strongly quenched by P700 cation radical or by P700 triplet state, dependent on redox state of the acceptor side cofactors of photosystem I.     

Dynamic aspects of phycobilisome structure

In exponentially growing cells of Synechococcus sp. 6301, over 95% of the phycobiliproteins are located in phycobilisomes, and the remainder is present in the form of low molecular weight aggregates. In addition to the subunits of the phycobiliproteins (C-phycocyanin, allophycocyanin, allophycocyanin B), the phycobilisomes of this unicellular cyanobacterium contain five non-pigmented polypeptides. During the initial phase of starvation (24 h after removal of combined nitrogen from the growth medium), the phycobiliproteins in the low molecular weight fraction largely disappeared. Phycocyanin was lost more rapidly from this fraction than allophycocyanin. Simultaneous changes in the phycobilisome were (1) a decrease in sedimentation coefficient, (2) a decrease in phycocyanin: allophycocyanin ratio, (3) a shift in the fluorescence emission maximum from 673 to 676 nm, and (4) a selective complete loss of a 30,000 dalton non-pigmented polypeptide. Upon extensive nitrogen starvation (72 h), the intracellular level of phycocyanin decreased by over 30-fold. These results indicate that in the early stage of nitrogen starvation, the free phycobiliproteins of the cell are degraded, as well as a significant proportion of the phycocyanin from the periphery of the phycobilisome. However, the structures partially depleted of phycocyanin still function efficiently in energy transfer. On extended starvation, total degradation of residual phycobilisomes takes place, possibly in conjunction with the detachment of these structures from the thylakoids.None of the effects of the absence of combined nitrogen were seen when cells were starved in the presence of chloramphenicol, or in a methionine auxotroph starved for methionine.Abbreviations Used NaK-PO₄ NaH₂PO₄ titrated with K₂HPO₄ to a given pH - SDS sodium dodecyl sulfate - Tris Tris(hydroxymethyl)aminomethane     

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.     

Fluorescence changes accompanying short-term light adaptations in photosystem I and photosystem II of the cyanobacterium <Emphasis Type="Italic">Synechocystis</Emphasis> sp. PCC 6803 and phycobiliprotein-impaired mutants: State 1/State 2 transitions and carotenoid-induced quenching of phycobilisomes

The features of the two types of short-term light-adaptations of photosynthetic apparatus, State 1/State 2 transitions, and non-photochemical fluorescence quenching of phycobilisomes (PBS) by orange carotene-protein (OCP) were compared in the cyanobacterium Synechocystis sp. PCC 6803 wild type, CK pigment mutant lacking phycocyanin, and PAL mutant totally devoid of phycobiliproteins. The permanent presence of PBS-specific peaks in the in situ action spectra of photosystem I (PSI) and photosystem II (PSII), as well as in the 77 K fluorescence excitation spectra for chlorophyll emission at 690 nm (PSII) and 725 nm (PSI) showed that PBS are constitutive antenna complexes of both photosystems. The mutant strains compensated the lack of phycobiliproteins by higher PSII content and by intensification of photosynthetic linear electron transfer. The detectable changes of energy migration from PBS to the PSI and PSII in the Synechocystis wild type and the CK mutant in State 1 and State 2 according to the fluorescence excitation spectra measurements were not registered. The constant level of fluorescence emission of PSI during State 1/State 2 transitions and simultaneous increase of chlorophyll fluorescence emission of PSII in State 1 in Synechocystis PAL mutant allowed to propose that spillover is an unlikely mechanism of state transitions. Blue–green light absorbed by OCP diminished the rout of energy from PBS to PSI while energy migration from PBS to PSII was less influenced. Therefore, the main role of OCP-induced quenching of PBS is the limitation of PSI activity and cyclic electron transport under relatively high light conditions.     

ACCLIMATION OF SYNECHOCOCCUS STRAIN WH7803 TO AMBIENT CO2 CONCENTRATION AND TO ELEVATED LIGHT INTENSITY1

A CO₂ concentrating mechanism has been identified in the phycoerythrin-possessing Synechococcus sp. WH7803 and has been observed to be severely inhibited by short exposure to elevated light intensities. A light treatment of 300–2000 μmol quanta·m^?2·s^?1 resulted in a considerable decay in the variable fluorescence of PSII with time, suggesting decreased efficiency of energy transfer from the phycobilisomes, direct damage to the reaction center II, or both. Measurements of the activity of PSII and changes in fluorescence emission spectra during a light treatment of 1000 μmol quanta·m^?2·s^?1 indicated considerable reduction in the energy flow from the phycocyanin to the phycobilisome terminal acceptor and chlorophyll a. Consequently, whereas the maximal photosynthetic rate, at saturating light and Co₂ concentration, was hardly affected by a light treatment of 1000 μmol quanta·m^?2·s^?1 for 2 h, the light intensity required to reach that maximum increased with the duration of the light treatment.     

Luminescence decay kinetics in relation to quenching and stimulation of dark fluorescence from high and low CO2 adapted cells of Scenedesmus obliquus and Chlamydomonas reinhardtii

Two green algal species, Chlamydomonas reinhardtii and Scenedesmus obliquus, exhibited a relative maximum during the decay of luminescence, when adapted to low CO₂ conditions that was not observed in high CO₂ adapted cells.From the kinetics of transient changes in the level of dark fluorescence, after illumination and parallel to the luminescence maxima, it was concluded that the maximum in Scenedesmus was mainly related to a decrease in nonphotochemical quenching, whereas in Chlamydomonas the maximum was mainly related to a dark reduction of the primary PS II acceptor Q_A.ATP/ADP ratios from low CO₂ adapted Scenedesmus showed transient high levels after a dark/light transition that was not observed in high CO₂ adapted cells. After 30 s of illumination the ATP/ADP ratios however stabilized at the same steady state level as in high CO₂ adapted cells.Dark addition of HCO₃ ^- to low CO₂ adapted cells of Chlamydomonas resulted in a rapid transient quenching of luminescence that was not observed in low CO₂ adapted cells of neither species.It is concluded that the luminescence maxima present in both low CO₂ adapted Scenedesmus and Chlamydomonas reflect adaptation of the cells to low CO₂ conditions. It is further suggested that the difference in mechanistic origin of luminescence maxima in the two species reflects differences in adaptation.Abbreviations ADP adenosine-diphosphate - ATP adenosine-triphosphate - C_i inorganic carbon - F_D dark fluorescence recorded under dark adapted conditions - F₀ fluorescence with all reaction centers open - FV variable fluorescence - PS I photosystem I - PS II photosystem II - Q_A the first quinone acceptor of PS II     

Spectroscopic and Biochemical Analyses of UV Effects on Phycobiliproteins of Anabaena sp. and Nostoc carmium

The effects of UV (280–400 nm) irradiation on phycobiliprotein composition have been studied in two N₂-fixing cyanobacteria, Anabaena sp. and Nostoc carmium, isolated from rice paddy fields in India. Phycobiliproteins were isolated and separated by sucrose density gradient centrifugation. After UV exposure the top fraction mainly contained carotenoids (absorption maximum at 485 nm), which first showed an increase in intensity and absorption and then a gradual decrease with increasing UV exposure in Anabaena sp., whereas, in Nostoc carmium this fraction showed a steady increase over the whole exposure time. The bottom fraction of both organisms mainly contained phycocyanin (absorption peak at 620 nm) which showed a steady decline in intensity, as well as absorption. Fluorescence excitation at 620 nm resulted in an emission at 650 nm which underwent a shift towards shorter wave-lengths with increasing UV-exposure time, indicating a disassembly of the phycobilisomal complex and of impaired energy transfer from accessory pigments to the reaction centers. SDS PAGE analysis of the fractions revealed a loss of high molecular mass linker proteins and low molecular mass (αβ monomers indicating that the phycobiliproteins, which function as accessory pigments for the operation of photosystem II, disassemble during UV irradiation.     

Cyanobacterial phycobilisomes

Cyanobacterial phycobilisomes harvest light and cause energy migration usually toward photosystem II reaction centers. Energy transfer from phycobilisomes directly to photosystem I may occur under certain light conditions. The phycobilisomes are highly organized complexes of various biliproteins and linker polypeptides. Phycobilisomes are composed of rods and a core. The biliproteins have their bilins (chromophores) arranged to produce rapid and directional energy migration through the phycobilisomes and to chlorophyll a in the thylakoid membrane. The modulation of the energy levels of the four chemically different bilins by a variety of influences produces more efficient light harvesting and energy migration. Acclimation of cyanobacterial phycobilisomes to growth light by complementary chromatic adaptation is a complex process that changes the ratio of phycocyanin to phycoerythrin in rods of certain phycobilisomes to improve light harvesting in changing habitats. The linkers govern the assembly of the biliproteins into phycobilisomes, and, even if colorless, in certain cases they have been shown to improve the energy migration process. The Lcm polypeptide has several functions, including the linker function of determining the organization of the phycobilisome cores. Details of how linkers perform their tasks are still topics of interest. The transfer of excitation energy from bilin to bilin is considered, particularly for monomers and trimers of C-phycocyanin, phycoerythrocyanin, and allophycocyanin. Phycobilisomes are one of the ways cyanobacteria thrive in varying and sometimes extreme habitats. Various biliprotein properties perhaps not related to photosynthesis are considered: the photoreversibility of phycoviolobilin, biophysical studies, and biliproteins in evolution. Copyright 1998 Academic Press.