Phycobilisomes Research Articles

Diversity and Evolution of Pigment Types in Marine Synechococcus Cyanobacteria.

Synechococcus cyanobacteria are ubiquitous and abundant in the marine environment and contribute to an estimated 16% of the ocean net primary productivity. Their light-harvesting complexes, called phycobilisomes (PBS), are composed of a conserved allophycocyanin core, from which radiates six to eight rods with variable phycobiliprotein and chromophore content. This variability allows Synechococcus cells to optimally exploit the wide variety of spectral niches existing in marine ecosystems. Seven distinct pigment types or subtypes have been identified so far in this taxon based on the phycobiliprotein composition and/or the proportion of the different chromophores in PBS rods. Most genes involved in their biosynthesis and regulation are located in a dedicated genomic region called the PBS rod region. Here, we examine the variability of gene content and organization of this genomic region in a large set of sequenced isolates and natural populations of Synechococcus representative of all known pigment types. All regions start with a tRNA-PheGAA and some possess mobile elements for DNA integration and site-specific recombination, suggesting that their genomic variability relies in part on a “tycheposon”-like mechanism. Comparison of the phylogenies obtained for PBS and core genes revealed that the evolutionary history of PBS rod genes differs from the core genome and is characterized by the co-existence of different alleles and frequent allelic exchange. We propose a scenario for the evolution of the different pigment types and highlight the importance of incomplete lineage sorting in maintaining a wide diversity of pigment types in different Synechococcus lineages despite multiple speciation events.

Photoprotective energy quenching in the red alga Porphyridium purpureum occurs at the core antenna of the photosystem II but not at its reaction center

Photosynthetic organisms have evolved light-harvesting antennae over time. In cyanobacteria, external phycobilisomes (PBSs) are the dominant antennae, whereas in green algae and higher plants, PBSs have been replaced by proteins of the Lhc family that are integrated in the membrane. Red algae represent an evolutionary intermediate between these two systems, as they employ both PBSs and membrane LHCR proteins as light-harvesting units. Understanding how red algae cope with light is not only interesting for biotechnological applications, but is also of evolutionary interest. For example, energy-dependent quenching (qE) is an essential photoprotective mechanism widely used by species from cyanobacteria to higher plants to avoid light damage; however, the quenching mechanism in red algae remains largely unexplored. Here, we used both pulse amplitude-modulated (PAM) and time-resolved chlorophyll fluorescence to characterize qE kinetics in the red alga Porphyridium purpureum. PAM traces confirmed that qE in P. purpureum is activated by a decrease in the thylakoid lumen pH, whereas time-resolved fluorescence results further revealed the quenching site and ultrafast quenching kinetics. We found that quenching exclusively takes place in the photosystem II (PSII) complexes and preferentially occurs at PSII’s core antenna rather than at its reaction center, with an overall quenching rate of 17.6 ± 3.0 ns−1. In conclusion, we propose that qE in red algae is not a reaction center type of quenching, and that there might be a membrane-bound protein that resembles PsbS of higher plants or LHCSR of green algae that senses low luminal pH and triggers qE in red algae.

Environmental Tuning of Homologs of the Orange Carotenoid Protein-Encoding Gene in the Cyanobacterium Fremyella diplosiphon.

The orange carotenoid protein (OCP) family of proteins are light-activated proteins that function in dissipating excess energy absorbed by accessory light-harvesting complexes, i.e., phycobilisomes (PBSs), in cyanobacteria. Some cyanobacteria contain multiple homologs of the OCP-encoding gene (ocp). Fremyella diplosiphon, a cyanobacterium studied for light-dependent regulation of PBSs during complementary chromatic acclimation (CCA), contains several OCP homologs – two full-length OCPs, three Helical Carotenoid Proteins (HCPs) with homology to the N-terminus of OCP, and one C-terminal domain-like carotenoid protein (CCP) with homology to the C-terminus of OCP. We examined whether these homologs are distinctly regulated in response to different environmental factors, which could indicate distinct functions. We observed distinct patterns of expression for some OCP, HCP, and CCP encoding genes, and have evidence that light-dependent aspects of ocp homolog expression are regulated by photoreceptor RcaE which controls CCA. RcaE-dependent transcriptional regulator RcaC is also involved in the photoregulation of some hcp genes. Apart from light, additional environmental factors associated with cellular redox regulation impact the mRNA levels of ocp homologs, including salt, cold, and disruption of electron transport. Analyses of conserved sequences in the promoters of ocp homologs were conducted to gain additional insight into regulation of these genes. Several conserved regulatory elements were found across multiple ocp homolog promoters that potentially control differential transcriptional regulation in response to a range of environmental cues. The impact of distinct environmental cues on differential accumulation of ocp homolog transcripts indicates potential functional diversification of this gene family in cyanobacteria. These genes likely enable dynamic cellular protection in response to diverse environmental stress conditions in F. diplosiphon.

Antenna Modification Leads to Enhanced Nitrogenase Activity in a High Light-Tolerant Cyanobacterium.

ABSTRACTBiological nitrogen fixation is an energy-intensive process that contributes significantly toward supporting life on this planet. Among nitrogen-fixing organisms, cyanobacteria remain unrivaled in their ability to fuel the energetically expensive nitrogenase reaction with photosynthetically harnessed solar energy. In heterocystous cyanobacteria, light-driven, photosystem I (PSI)-mediated ATP synthesis plays a key role in propelling the nitrogenase reaction. Efficient light transfer to the photosystems relies on phycobilisomes (PBS), the major antenna protein complexes. PBS undergo degradation as a natural response to nitrogen starvation. Upon nitrogen availability, these proteins are resynthesized back to normal levels in vegetative cells, but their occurrence and function in heterocysts remain inconclusive. Anabaena 33047 is a heterocystous cyanobacterium that thrives under high light, harbors larger amounts of PBS in its heterocysts, and fixes nitrogen at higher rates compared to other heterocystous cyanobacteria. To assess the relationship between PBS in heterocysts and nitrogenase function, we engineered a strain that retains large amounts of the antenna proteins in its heterocysts. Intriguingly, under high light intensities, the engineered strain exhibited unusually high rates of nitrogenase activity compared to the wild type. Spectroscopic analysis revealed altered PSI kinetics in the mutant with increased cyclic electron flow around PSI, a route that contributes to ATP generation and nitrogenase activity in heterocysts. Retaining higher levels of PBS in heterocysts appears to be an effective strategy to enhance nitrogenase function in cyanobacteria that are equipped with the machinery to operate under high light intensities.

Photosynthesis and Cell Growth Trigger Degradation of Phycobilisomes during Nitrogen Limitation.

Under nitrogen (N)-limited conditions, the non-N2-fixing cyanobacterium Synechocystis sp. PCC 6803 (Synechocystis 6803) actively grows during early stages of starvation by performing photosynthesis but gradually stops the growth and eventually enters dormancy to withstand long-term N limitation. During N limitation, Synechocystis 6803 cells degrade the large light-harvesting antenna complex phycobilisomes (PBSs) presumably to avoid excess light absorption and to reallocate available N to essential functions for growth and survival. These two requirements may be driving forces for PBS degradation during N limitation, but how photosynthesis and cell growth affect PBS degradation remains unclear. To address this question, we examined involvements of photosynthesis and cell growth in PBS degradation during N limitation. Treatment of photosynthesis inhibitors and shading suppressed PBS degradation and caused non-bleaching of cells during N limitation. Limitations of photosynthesis after initial gene responses to N limitation suppressed PBS degradation, implying that photosynthesis affects PBS degradation in a post-translational manner. In addition, limitations of cell growth by inhibition of peptidoglycan and fatty acid biosynthesis, low growth temperature and phosphorous starvation suppressed PBS degradation during N limitation. Because decreased photosynthetic activity led to decreased cell growth, and vice versa, photosynthesis and cell growth would inseparably intertwine each other and affect PBS degradation during N limitation in a complex manner. Our data shed light on the coordination mechanisms among photosynthesis, cell growth and PBS degradation during N limitation.

Time-Resolved Fluorescence Spectroscopy Study of Energy Transfer Dynamics in Phycobilisomes from Cyanobacteria Thermosynechococcus vulcanus NIES 2134 and Synechocystis sp. PCC 6803

As the largest light-harvesting complex in cyanobacteria, phycobilisomes (PBSs) show high efficiency and a high rate of energy transfer, owing to an elegant antenna-like assembly. To understand the structural influence on the dynamic process of the energy transfer in PBSs, two cyanobacterium species Thermosynechococcus vulcanus NIES 2134 (T. 2134) and Synechocystis sp. PCC 6803 (S. 6803) with different rod–core-linked assemblies were chosen for this study. The dynamic process of the energy transfer in both PBSs was investigated through time-resolved fluorescence spectroscopy (TRFS) with a time resolution of sub-picosecond. Via the fluorescence decay curves deconvolution, the pathways and related rates of the excitation energy transfer (EET) were determined. Three time components, i.e., 10, 80, and 1250 ps, were identified in the EET in the PBSs of T. 2134 and three, i.e., 9, 115, and 1680 ps, in the EET in the PBSs of S. 6803. In addition, a comparison of the dynamic process of the energy transfer between the two cyanobacteria revealed how the PBS assembly affects the energy transfer in PBSs. The findings will provide insight into future time-resolved crystallography.

Elucidation of the essential amino acids involved in the binding of the cyanobacterial Orange Carotenoid Protein to the phycobilisome

The Orange Carotenoid Protein (OCP) is a soluble photoactive protein involved in cyanobacterial photoprotection. It is formed by the N-terminal domain (NTD) and C-terminal (CTD) domain, which establish interactions in the orange inactive form and share a ketocarotenoid molecule. Upon exposure to intense blue light, the carotenoid molecule migrates into the NTD and the domains undergo separation. The free NTD can then interact with the phycobilisome (PBS), the extramembrane cyanobacterial antenna, and induces thermal dissipation of excess absorbed excitation energy. The OCP and PBS amino acids involved in their interactions remain undetermined. To identify the OCP amino acids essential for this interaction, we constructed several OCP mutants (23) with modified amino acids located on different NTD surfaces. We demonstrated that only the NTD surface that establishes interactions with the CTD in orange OCP is involved in the binding of OCP to PBS. All amino acids surrounding the carotenoid β1 ring in the OCPR-NTD (L51, P56, G57, N104, I151, R155, N156) are important for binding OCP to PBS. Additionally, modification of the amino acids influences OCP photoactivation and/or recovery rates, indicating that they are also involved in the translocation of the carotenoid.

Structural insight into the mechanism of energy transfer in cyanobacterial phycobilisomes

Phycobilisomes (PBS) are the major light-harvesting machineries for photosynthesis in cyanobacteria and red algae and they have a hierarchical structure of a core and peripheral rods, with both consisting of phycobiliproteins and linker proteins. Here we report the cryo-EM structures of PBS from two cyanobacterial species, Anabaena 7120 and Synechococcus 7002. Both PBS are hemidiscoidal in shape and share a common triangular core structure. While the Anabaena PBS has two additional hexamers in the core linked by the 4th linker domain of ApcE (LCM). The PBS structures predict that, compared with the PBS from red algae, the cyanobacterial PBS could have more direct routes for energy transfer to ApcD. Structure-based systematic mutagenesis analysis of the chromophore environment of ApcD and ApcF subunits reveals that aromatic residues are critical to excitation energy transfer (EET). The structures also suggest that the linker protein could actively participate in the process of EET in both rods and the cores. These results provide insights into the organization of chromophores and the mechanisms of EET within cyanobacterial PBS.

State of the phycobilisome determines effective absorption cross-section of Photosystem II in Synechocystis sp. PCC 6803

Quenching of excess excitation energy is necessary for the photoprotection of light-harvesting complexes. In cyanobacteria, quenching of phycobilisome (PBS) excitation energy is induced by the Orange Carotenoid Protein (OCP), which becomes photoactivated under high light conditions. A decrease in energy transfer efficiency from the PBSs to the reaction centers decreases photosystem II (PS II) activity. However, quantitative analysis of OCP-induced photoprotection in vivo is complicated by similar effects of both photochemical and non-photochemical quenching on the quantum yield of the PBS fluorescence overlapping with the emission of chlorophyll. In the present study, we have analyzed chlorophyll a fluorescence induction to estimate the effective cross-section of PS II and compared the effects of reversible OCP-dependent quenching of PBS fluorescence with reduction of PBS content upon nitrogen starvation or mutations of key PBS components. This approach allowed us to estimate the dependency of the rate constant of PS II primary electron acceptor reduction on the amount of PBSs in the cell. We found that OCP-dependent quenching triggered by blue light affects approximately half of PBSs coupled to PS II, indicating that under normal conditions, the concentration of OCP is not sufficient for quenching of all PBSs coupled to PS II.

In situ cryo-ET structure of phycobilisome-photosystem II supercomplex from red alga.

Phycobilisome (PBS) is the main light-harvesting antenna in cyanobacteria and red algae. How PBS transfers the light energy to photosystem II (PSII) remains to be elucidated. Here we report the in situ structure of the PBS-PSII supercomplex from Porphyridium purpureum UTEX 2757 using cryo-electron tomography and subtomogram averaging. Our work reveals the organized network of hemiellipsoidal PBS with PSII on the thylakoid membrane in the native cellular environment. In the PBS-PSII supercomplex, each PBS interacts with six PSII monomers, of which four directly bind to the PBS, and two bind indirectly. Additional three 'connector' proteins also contribute to the connections between PBS and PSIIs. Two PsbO subunits from adjacent PSII dimers bind with each other, which may promote stabilization of the PBS-PSII supercomplex. By analyzing the interaction interface between PBS and PSII, we reveal that αLCM and ApcD connect with CP43 of PSII monomer and that αLCM also interacts with CP47' of the neighboring PSII monomer, suggesting the multiple light energy delivery pathways. The in situ structures illustrate the coupling pattern of PBS and PSII and the arrangement of the PBS-PSII supercomplex on the thylakoid, providing the near-native 3D structural information of the various energy transfer from PBS to PSII.

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 (LCM ) specifically organizes the allophycocyanin subunits in the PBS's core. But, the topology of LCM 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 LCM 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 LCM , that the cyanobacterial and red-algal LCM topologies are actually the same. We found that removing the top cylinder linker domain in LCM 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 LCM topology, suggesting that the PBS cores in cyanobacteria and red-algae assemble in the same way.

Direct Energy Transfer from Allophycocyanin-Free Rod-Type CpcL-Phycobilisome to Photosystem I.

Phycobilisomes (PBSs) are photosynthetic antenna megacomplexes comprising pigment-binding proteins (cores and rods) joined with linker proteins. A rod-type PBS that does not have a core is connected to photosystem I (PSI) by a CpcL linker protein, which stabilizes a red-form of the phycocyanobilin (red-PCB) in the rod. However, quantitative information on the energy transfer from red-type PBS to PSI has not been determined. Herein, the isolated supercomplex of the rod-type PBS and the PSI tetramer from Anabaena sp. PCC 7120 were probed by time-resolved spectroscopy at 77 K and by decay-associated spectral analysis to show that red-PCB mediates the fast and efficient (time constant = 90 ps, efficiency = 95%) transfer of excitation energy from PCB to chlorophyll a (Chl a). According to the Förster energy transfer mechanism, this high efficiency corresponds to a 4 nm distance between red-PCB and Chl a, suggesting that β-84 PCB in the rod acts as red-PCB.

Difference in light use strategy in red alga between Griffithsia pacifica and Porphyridium purpureum

Phycobilisomes (PBSs) are the largest light-harvesting antenna in red algae, and feature high efficiency and rate of energy transfer even in a dim environment. To understand the influence of light on the energy transfer in PBSs, two red algae Griffithsia pacifica and Porphyridium purpureum living in different light environment were selected for this research. The energy transfer dynamics in PBSs of the two red algae were studied in time-resolved fluorescence spectroscopy in sub-picosecond resolution. The energy transfer pathways and the related transfer rates were uncovered by deconvolution of the fluorescence decay curve. Four time-components, i.e., 8 ps, 94 ps, 970 ps, and 2288 ps were recognized in the energy transfer in PBSs of G. pacifica, and 10 ps, 74 ps, 817 ps and 1292 ps in P. purpureum. In addition, comparison in energy transfer dynamics between the two red algae revealed that the energy transfer was clearly affected by lighting environment. The findings help us to understand the energy transfer mechanisms of red algae for adaptation to a natural low light environment.

Proteome analysis of response to different spectral light irradiation in Synechocystis sp. PCC 6803

In cyanobacteria, it is known that the excitation ratios of photosystem (PS) I and PSII changes with the wavelength of irradiated light due to mobile phycobilisome (PBS) and spillover, affecting the photosynthetic ATP/NADPH synthesis ratio and metabolic flux state. However, the mechanisms by which these changes are controlled have not been well studied. In this study, we performed a targeted proteomic analysis of Synechocystis sp. PCC 6803 under different spectral light conditions to clarify the regulation mechanisms of mobile PBS, spillover and metabolisms under different light qualities at the protein level. The results showed an increase in the amount of proteins mainly involved in CO2 fixation under Red1 light conditions with a high specific growth rate, suggesting that the rate of intracellular metabolism is controlled by the rate of carbon uptake, not by changes in the amount of each enzyme. Correlation analysis between protein levels and PSI/PSII excitation ratios revealed that PsbQUY showed high correlations and significantly increased under Blue and Red2 light conditions, where the PSI/PSII excitation ratio was higher due to spillover. In the strains lacking the genes encoding these proteins, a decrease in the PSI/PSII excitation ratio was observed, suggesting that PsbQUY contribute to spillover occurrence. SignificanceIn cyanobacteria, the photosynthetic apparatus's responses, such as state transition [mobile PBS and spillover], occur due to the intensity and wavelength of irradiated light, resulting in changes in photosynthetic electron transport and metabolic flux states. Previous studies have analyzed the response of Synechocystis sp. PCC 6803 to light intensity from various directions, but only spectroscopic analysis of the photosynthetic apparatus has been done on the response to changes in the wavelength of irradiated light. This study analyzed the response mechanisms of mobile PBS, spillover, photosynthetic, and metabolic systems in Synechocystis sp. PCC 6803 under six different spectral light conditions by a targeted proteomic analysis. As a result, many proteins were successfully quantified, and the metabolic enzymes and photosynthetic apparatus were analyzed using an integrated approach. Principal component and correlation analyses and volcano plots revealed that the PSII subunits PsbQ, PsbU, and PsbY have a strong correlation with the PSI/PSII excitation ratio and contribute to spillover occurrence. Thus, statistical analysis based on proteome data revealed that PsbQ, PsbU, and PsbY are involved in spillover, as revealed by spectroscopic analysis.

Fluorescence recovery after photobleaching: analyses of cyanobacterial phycobilisomes reveal intrinsic fluorescence recovery.

Fluorescence recovery after photobleaching (FRAP) has been used to study the dynamics of the cyanobacterial photosynthesis apparatus since 1997. Fluorescence recovery of cyanobacteria during FRAP was conventionally interpreted as a result of phycobilisome (PBS) diffusion on the surface of the thylakoid membrane. The mechanism of state transition in cyanobacteria has been widely attributed to PBS diffusion. However, in red algae, another PBS-containing group, the intrinsic photoprocess was found to contribute greatly to the fluorescence recovery of PBS, which raises questions concerning the role of FRAP in red algal PBS. Therefore, it is important to re-evaluate the nature of PBS fluorescence recovery in cyanobacteria. In the present study, four cyanobacterial strains with different phenotypes and PBS compositions were used to investigate their FRAP characteristics. Fluorescence recovery of PBS was observed in wholly photobleached cells in all four cyanobacterial strains, in which the contribution of PBS diffusion to the fluorescence recovery was not possible. Moreover, the fluorescence recovered in isolated PBSs and PBS-thylakoid membranes after photobleaching further demonstrated the intrinsic photoprocess nature of fluorescence recovery. These findings suggest that the intrinsic photoprocess contributed to the fluorescence recovery following photobleaching when measured by the FRAP method.

Engineering Cells, Membranes and Light Harvesting/Photosystem II Super-Complexes in Bio-Photoelectrochemical Cells

Photosynthetic organisms, membranes and complexes are attractive starting materials for solar energy conversion (SEC). Our overall goal is to develop methods to perform SEC using these materials in simple, inexpensive and a fashion that will be non-polluting and will not compete with the growth of food materials. I will describe here how the remarkable photocatalytic activity of the photosynthetic apparatus can provide overall water splitting with oxygen and hydrogen production in Bio-Photo-Electro-Chemical (BPEC) cells via the simplest and cleanest of processes wither in the absence or presence of added electron transport molecules. With plant thylakoids, electrons are shuttled by FeCN to a transparent FTO electrode, yielding a photocurrent density of 0.5 mA·cm-2. Hydrogen evolution occurs at the cathode at a bias as low as 0.8 V. A tandem cell comprising the BPEC cell with the thylakoid membranes and a Si photovoltaic module achieves overall water splitting with solar to hydrogen conversion efficiency of 0.3% (Pinhassi et al. Nature Communications 2016). With cyanobacterial cells, electrons from the respiratory system are energized by the photosynthetic system and transferred directly to a graphite electrode, utilizing endogenous electron carriers, especially NADPH (Saper et al. Nature Communications 2018 and Shlosberg et al. iScience 2020). The current produced can be used for hydrogen production at low additional bias for significantly longer durations than the plant thylakoids.Another route is to use isolated photosynthetic complexes and chemically connect them to the electrochemical cell. One of the issues with this strategy is the small number of light absorbing chromophores connected to the complex, lowering the photochemical efficiency. Connection of isolated light harvesting (LH) complexes can help to overcome this deficiency. Photosystem II (PSII) is the only enzyme that catalyzes light-induced water oxidation being the basis for its application as a biophotoanode in various bio-photovoltaics and photo-bioelectrochemical cells. However, the absorption spectrum of PSII limits the quantum efficiency in the range of visible light, due to a gap in the green absorption region of chlorophylls (500 - 600 nm). To overcome this limitation, we have used two strategies: stabilizing the interaction between PSII and Phycobilisomes (PBSs) –LH complex or direct connection to modified gold-nanoparticles (Au-NPs) that serve both as LH and electron conduit. Integration of the PBS-PSII super-complexes within an Os-complex-modified hydrogel on macro-porous indium tin oxide electrodes (MP-ITO) resulted in notably improved, wavelength dependent, incident photon-to-electron conversion efficiencies (Hartmann et al. J. Mat. Chem. A. 2020). With the Au-NPs, record currents were obtained, with strong improvement in the SPR range of wavelengths that are not absorbed by chlorophylls.

Structural implications for a phycobilisome complex from the thermophilic cyanobacterium Thermosynechococcus vulcanus

Phycobilisomes (PBSs) are huge, water-soluble light-harvesting complexes used by oxygenic photosynthetic organisms. The structures of some subunits of the PBSs, including allophycocyanin (APC) and phycocyanin (PC), have been solved by X-ray crystallography previously. However, there are few reports on the overall structures of PBS complexes in photosynthetic organisms. Here, we report the overall structure of the PBS complex isolated from the cyanobacterium Thermosynechococcus vulcanus, determined by negative-staining electron microscopy (EM). Intact PBS complexes were purified by trehalose density gradient centrifugation with a high-concentration phosphate buffer and then subjected to a gradient-fixation preparation using glutaraldehyde. The final map constructed by the single-particle analysis of EM images showed a hemidiscoidal structure of the PBS, consisting of APC cores and peripheral PC rods. The APC cores are composed of five cylinders: A1, A2, B, C1, and C2. Each of the cylinders is composed of three (A1 and A2), four (B), or two (C1 and C2) APC trimers. In addition, there are eight PC rods in the PBS: one bottom pair (Rb and Rb'), one top pair (Rt and Rt'), and two side pairs (Rs1/Rs1′ and Rs2/Rs2′). Comparison with the overall structures of PBSs from other organisms revealed structural characteristics of T. vulcanus PBS.

Ultrafast energy transfer dynamics of phycobilisome from Thermosynechococcus vulcanus, as revealed by ps fluorescence and fs pump-probe spectroscopies.

Cyanobacterial photosynthetic systems efficiently capture sunlight using the pigment-protein megacomplexes, phycobilisome (PBS). The energy is subsequently transferred to photosystem I (PSI) and II (PSII), to produce electrochemical potentials. In the present study, we performed picosecond (ps) time-resolved fluorescence and femtosecond (fs) pump-probe spectroscopies on the intact PBS from a thermophilic cyanobacterium, Thermosynechococcus vulcanus, to reveal excitation energy transfer dynamics in PBS. The photophysical properties of the intact PBS were well characterized by spectroscopic measurements covering wide temporal range from femtoseconds to nanoseconds. The ps fluorescence measurements excited at 570nm, corresponding to the higher energy of the phycocyanin (PC) absorption band, demonstrated the excitation energy transfer from the PC rods to the allophycocyanin (APC) core complex as well as the energy transfer in the APC core complex. Then, the fs pump-probe measurements revealed the detailed energy transfer dynamics in the PC rods taking place in an ultrafast time scale. The results obtained in this study provide the full picture of the funnel-type excitation energy transfer with rate constants of (0.57ps)-1 → (7.3ps)-1 → (53ps)-1 → (180ps)-1 → (1800ps)-1.

Structure of Phycobilisomes.

Phycobilisomes (PBSs) are extremely large chromophore-protein complexes on the stromal side of the thylakoid membrane in cyanobacteria and red algae. The main function of PBSs is light harvesting, and they serve as antennas and transfer the absorbed energy to the reaction centers of two photosynthetic systems (photosystems I and II). PBSs are composed of phycobiliproteins and linker proteins. How phycobiliproteins and linkers are organized in PBSs and how light energy is efficiently harvested and transferred in PBSs are the fundamental questions in the study of photosynthesis. In this review, the structures of the red algae Griffithsia pacifica and Porphyridium purpureum are discussed in detail, along with the functions of linker proteins in phycobiliprotein assembly and in fine-tuning the energy state of chromophores.

The Role of Selected Wavelengths of Light in the Activity of Photosystem II in Gloeobacter violaceus.

Gloeobacter violaceus is a cyanobacteria species with a lack of thylakoids, while photosynthetic antennas, i.e., phycobilisomes (PBSs), photosystem II (PSII), and I (PSI), are located in the cytoplasmic membrane. We verified the hypothesis that blue–red (BR) light supplemented with a far-red (FR), ultraviolet A (UVA), and green (G) light can affect the photosynthetic electron transport chain in PSII and explain the differences in the growth of the G. violaceus culture. The cyanobacteria were cultured under different light conditions. The largest increase in G. violaceus biomass was observed only under BR + FR and BR + G light. Moreover, the shape of the G. violaceus cells was modified by the spectrum with the addition of G light. Furthermore, it was found that both the spectral composition of light and age of the cyanobacterial culture affect the different content of phycobiliproteins in the photosynthetic antennas (PBS). Most likely, in cells grown under light conditions with the addition of FR and G light, the average antenna size increased due to the inactivation of some reaction centers in PSII. Moreover, the role of PSI and gloeorhodopsin as supplementary sources of metabolic energy in the G. violaceus growth is discussed.