Sponge-derived Ageladine A affects the in vivo fluorescence emission spectra of microalgae.

In several marine hosts of microalgae, fluorescent natural products may play an important role. While the ecological function of these compounds is not well understood, an interaction of these molecules with the photosynthesis of the symbionts has been suggested. In this study, the effect of Ageladine A (Ag A), a pH-dependent fluorophore found in sponges of the genus Agelas, on microalgal fluorescence was examined. The spectra showed an accumulation of Ag A within the cells, but with variable impacts on fluorescence. While in two Synechococcus strains, fluorescence of phycoerythrin increased significantly, the fluorescence of other Synechococcus strains was not affected. In four out of the five eukaryote species examined, chlorophyll a (Chl a) fluorescence intensity was modulated. In Tisochrysis lutea, for example, the position of the fluorescence emission maximum of Chl a was shifted. The variety of these effects of Ag A on microalgal fluorescence suggests that fluorophores derived from animals could play a crucial role in shaping the composition of marine host/symbiont systems.

ß-Carotene influences the phycobilisome antenna of cyanobacterium Synechocystis sp. PCC 6803.

We investigated the relation between the carotenoid composition and the structure of phycobilisome (PBS) antenna of cyanobacterium Synechocystis sp. PCC 6803. PBS is a large soluble protein complex enhances the light harvesting efficiency of the cells. It is composed of a central allophycocyanin core and radial phycocyanin rods, but it does not contain carotenoids. However, the absence or low level of carotenoids were previously shown to lead the co-existence of unconnected rod units and assembled PBS with shorter peripheral rods. Here we show that the lack of ß-carotene, but not of xanthophylls or the distortion of photosystem structure, evoked unconnected rods. Thus, these essential ß-carotene molecules are not bound by Photosystem I or Photosystem II. Our results do not show correlation between the reactive oxygen species (ROS) and PBS distortion despite the higher singlet oxygen producing capacity and light sensitivity of the mutant cells. Reduced cellular level of those linker proteins attaching the rod units together was also observed, but the direct damage of the linkers by ROS are not supported by our data. Enzymatic PBS proteolysis induced by nitrogen starvation in carotenoid mutant cells revealed a retarded degradation of the unconnected rod units.

[Efficiency of non-Photochemical Fluorescence Quenching of Phycobilisomes by the Orange Carotenoid Protein].

We report on theoretical efficiency of non-photochemical fluorescense quenching of phycobilisomes by the orange carotenoid protein. The created 3D computer model of the three-cylindrical phycobilisomes core allowed us to determine the distances between centers of mass of all phycobilin chromophores of the core and calculate the time and an average number of energy migration steps for the resulting non-radiative excitation transfer from the phycobilisomes to photosystem II. The obtained kinetic scheme equations for a way of energy transfer confirm the incomplete interception of energy flow in the phycobilisomes core by the orange carotenoid protein. Theoretical estimation of the rate of phycobilisomes quenching is in good agreement with experimental data.

Sulfite-stress induced functional and structural changes in the complexes of photosystems I and II in a cyanobacterium, Synechococcus elongatus PCC 7942.

Excess sulfite is well known to have toxic effects on photosynthetic activities and growth in plants, however, so far, the behavior of the photosynthetic apparatus during sulfite-stress has not been characterized as to the responsible proteins or genes. Here, the effects of sulfite on photosystem complexes were investigated in a cyanobacterium, Synechococcus elongatus PCC 7942, a possible model organism of chloroplasts. Culturing of the cells for 24 h in the presence of 10 mM sulfite retarded cell growth of the wild type, concomitantly with synthesis of Chl and phycobilisome repressed. The excess sulfite simultaneously repressed photosynthesis by more than 90%, owing largely to structural destabilization and resultant inactivation of the PSII complex, which seemed to consequently retard the cell growth. Notably, the PsbO protein, one of the subunits that construct the water-splitting system of PSII, was retained at a considerable level, and disruption of the psbO gene led to higher sensitivity of photosynthesis and growth to sulfite. Meanwhile, the PSI complex showed monomerization of its trimeric configuration with little effect on the activity. The structural alterations of these PS complexes depended on light. Our data provide evidence for quantitative decreases in the photosystem complex(es) including their antenna(e), structural alterations of the PSI and PSII complexes that would modulate their functions, and a crucial role of psbO in PSII protection, in Synechococcus cells during sulfite-stress. We suggest that the reconstruction of the photosystem complexes is beneficial to cell survival.

Differences in energy transfer of a cyanobacterium, Synechococcus sp. PCC 7002, grown in different cultivation media.

Currently, cyanobacteria are regarded as potential biofuel sources. Large-scale cultivation of cyanobacteria in seawater is of particular interest because seawater is a low-cost medium. In the present study, we examined differences in light-harvesting and energy transfer processes in the cyanobacterium Synechococcus sp. PCC 7002 grown in different cultivation media, namely modified A medium (the optimal growth medium for Synechococcus sp. PCC 7002) and f/2 (a seawater medium). The concentrations of nitrate and phosphate ions were varied in both media. Higher nitrate ion and/or phosphate ion concentrations yielded high relative content of phycobilisome. The cultivation medium influenced the energy transfers within phycobilisome, from phycobilisome to photosystems, within photosystem II, and from photosystem II to photosystem I. We suggest that the medium also affects charge recombination at the photosystem II reaction center and formation of a chlorophyll-containing complex.

Specificity of the cyanobacterial orange carotenoid protein: influences of orange carotenoid protein and phycobilisome structures.

Cyanobacteria have developed a photoprotective mechanism that decreases the energy arriving at the reaction centers by increasing thermal energy dissipation at the level of the phycobilisome (PB), the extramembranous light-harvesting antenna. This mechanism is triggered by the photoactive Orange Carotenoid Protein (OCP), which acts both as the photosensor and the energy quencher. The OCP binds the core of the PB. The structure of this core differs in diverse cyanobacterial strains. Here, using two isolated OCPs and four classes of PBs, we demonstrated that differences exist between OCPs related to PB binding, photoactivity, and carotenoid binding. Synechocystis PCC 6803 (hereafter Synechocystis) OCP, but not Arthrospira platensis PCC 7345 (hereafter Arthrospira) OCP, can attach echinenone in addition to hydroxyechinenone. Arthrospira OCP binds more strongly than Synechocystis OCP to all types of PBs. Synechocystis OCP can strongly bind only its own PB in 0.8 m potassium phosphate. However, if the Synechocystis OCP binds to the PB at very high phosphate concentrations (approximately 1.4 m), it is able to quench the fluorescence of any type of PB, even those isolated from strains that lack the OCP-mediated photoprotective mechanism. Thus, the determining step for the induction of photoprotection is the binding of the OCP to PBs. Our results also indicated that the structure of PBs, at least in vitro, significantly influences OCP binding and the stabilization of OCP-PB complexes. Finally, the fact that the OCP induced large fluorescence quenching even in the two-cylinder core of Synechococcus elongatus PBs strongly suggested that OCP binds to one of the basal allophycocyanin cylinders.

Stabilization and modulation of the phycobilisome by calcium in the calciphilic freshwater red alga Bangia atropurpurea.

The bangiophycean filamentous red alga Bangia atropurpurea is distributed in freshwater habitats such as littoral and splash zones of lakes or rapid currents distant from the sea. In these habitats, the distribution and growth of this alga appear to be related to hard water rich in calcium ions. To characterize the eco-physiological properties of this calciphilic red alga, we examined the effects of long-term and short-term Ca(2+) depletion on photosynthetic growth of the thallus and on the phycobilisome. Long-term culture experiments suggested that higher Ca(2+) concentrations (>50mgL(-1)) were required to sustain thallus growth and pigmentation of cells. In short-term Ca(2+)-depletion treatments, fluorescence derived from phycoerythrin (PE) fluctuated, although the absorption spectra of the thalli did not change. After 30 min of Ca(2+) depletion, the fluorescence lifetime of PE became markedly longer, indicating that the energy transfer from PE to phycocyanin (PC) was suppressed. The fluorescence lifetime of PE returned to its original value within a short time after 4h of Ca(2+) depletion, however, energy transfer from PE to PC was still suppressed. This suggested that the excitation energy absorbed by PE was quenched during prolonged Ca(2+) depletion. The efficient energy transfer from PC and allophycocyanin were unchanged during these treatments.

Protein-protein interactions in carotenoid triggered quenching of phycobilisome fluorescence in Synechocystis sp. PCC 6803.

An inquiry into the effect of temperature on carotenoid triggered quenching of phycobilisome (PBS) fluorescence in a photosystem II-deficient mutant of Synechocystis sp. results in identification of two temperature-dependent processes: one is responsible for the quenching rate, and one determines the yield of PBS fluorescence. Non-Arrhenius behavior of the light-on quenching rate suggests that carotenoid-absorbed light triggers a process that bears a strong resemblance to soluble protein folding, showing temperature-dependent enthalpy of activated complex formation. The response of PBS fluorescence yield to hydration changing additives and to passing of the membrane lipid phase transition point indicates that the pool size of PBSs subject to quenching depends on the state of some membrane component.

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.

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.

Light-induced excitation energy redistribution in Spirulina platensis cells: "spillover" or "mobile PBSs"?

State transitions induced by light and redox were investigated by observing the 77 K fluorescence spectra for the intact cells of Spirulina platensis. To clarify if phycobilisomes (PBSs) take part in the state transition, the contributions of PBSs to light-induced state transition were studied in untreated cells and the cells treated by betaine which fixed PBSs firmly on the thylakoid membranes. It was observed that the betaine-treated cells did not show any light-induced state transition. This result definitely confirmed that the light-induced excitation energy regulation between the two photosystems is mainly dependent on a spatial movement of PBSs on the thylakoid membranes, which makes PBS cores partially decoupled from photosystem II (PSII) while PBS rods more strongly coupled with photosystem I (PSI) during the transition from state 1 to state 2. On the other hand, an energy exchange between the two photosystems was observed in both untreated and betaine-treated cells during redox-induced state transition. These observations suggested that two different mechanisms were involved in the light-induced state transition and the redox-induced one. The former involves only a physical movement of PBSs, while the latter involves not only the movement of PBS but also energy spillover from PSII to PSI. A model for light-induced state transition was proposed based on the current results as well as well known knowledge.

[Effect of CHAPS on the stability of Spirulina platensis phycobilisomes].

The Spirulina platensis phycobilisomes were isolated by sucrose density gradients ultracentrifugation, and the fluorescence emission maximum of the phycobilisomes at room temperature was at 671 nm. The effects of ionic strength and the zwitterionic detergent CHAPS on the stability of the Spirulina platensis phycobilisomes were studied by room temperature fluorescence spectrum. The phycobilisomes were stable in 1.0 mol x L(-1) phosphate buffer solution, and their fluorescence emission maximum could remain unchanged for 7 days. The fluorescence emission maximum of phycobilisomes was blue-shifted to 648 nm when the concentration of the phosphate buffer solution was diluted to 0.1 mol x L(-1) with deionized water, which suggested that the phycobilisomes had been dissociated. The phycobilisomes were readily dissociated in phosphate buffer solutions of low concentrations (< 0.6 mol x L(-1)) and the speed of the dissociation increased with decreasing the concentration of the phosphate buffer solution. The fluorescence emission maximum of the phycobilisomes in 1.0 mol x L(-1) phosphate buffer solution was blue-shifted to 648 nm when 10 mmol x L(-1) CHAPS was added into the phycobilisomes solution, suggesting that CHAPS could dissociate phycobilisomes under high ionic strength conditions. The results might be useful for isolating intact substructures of phycobilisomes.