Phycobiliproteins in Prochlorococcus marinus: Biosynthesis of pigments and their assembly into proteins

Light Color Regulation of Photosynthetic Antennae Biogenesis in Marine Phytoplankton

Photosynthesis in the world's oceans is primarily conducted by phytoplankton, microorganisms that use many different pigments for light capture. Synechococcus is a unicellular cyanobacterium estimated to be the second most abundant marine phototroph, with a global population of 7 × 1026 cells. This group's success is partly due to the pigment diversity in their photosynthetic light harvesting antennae, which maximize photon capture for photosynthesis. Many Synechococcus isolates adjust their antennae composition in response to shifts in the blue:green ratio of ambient light. This response was named type 4 chromatic acclimation (CA4). Research has made significant progress in understanding CA4 across scales, from its global ecological importance to its molecular mechanisms. Two forms of CA4 exist, each correlated with the occurrence of one of two distinct but related genomic islands. Several genes in these islands are differentially transcribed by the ambient blue:green light ratio. The encoded proteins control the addition of different pigments to the antennae proteins in blue versus green light, altering their absorption characteristics to maximize photon capture. These genes are regulated by several putative transcription factors also encoded in the genomic islands. Ecologically, CA4 is the most abundant of marine Synechococcus pigment types, occurring in over 40% of the population oceanwide. It predominates at higher latitudes and at depth, suggesting that CA4 is most beneficial under sub-saturating photosynthetic light irradiances. Future CA4 research will further clarify the ecological role of CA4 and the molecular mechanisms controlling this globally important form of phenotypic plasticity.

The heat and irradiation driven degradation of the diagnostic pigments in marine phytoplankton and the compositions of special degradation products

Marine phytoplankton stands as one of the most crucial components of marine ecosystems, so tracking it using appropriate biomarkers holds significant meaning. Chlorins are a sort of degradation products derived from the diagnostic pigment of marine phytoplankton and serve as valuable biomarkers for describing the temporal and spatial distribution of phytoplankton. However, previous research has not qualitatively or quantitatively studied multiple Chlorins, nor has it clearly revealed the conditions of their formation. Thus, this study investigated the chemical structure and formation mechanism of Chlorins in Chlorophytes, Prochlorococcus, and Chrysophytes communities by experiencing specific heat and irradiation conditions. According to the standardized redundancy analysis results, Phytin-a and its analogues with shorter phytyl chain are sensitive to irradiation intensity, and different Phytin-a analogues represent different degradation pathway of Chl-a. The composition and concentration of Phide-a and its further degradation products emerged as the primary thermosensitive components, capable of indicating and reversing temperature fluctuations within the environment. During the degradation processes of Chl-a, the carotenoids of each phytoplankton can also affect the degradation direction of Chl-a. Phycourobilin can cause Chl-a in the Prochlorococcus group to transform into special Phide-a analogues relying on low irradiation intensity. Zeaxanthin and Diatoxanthin dominate the conversion of Chl-a in Chrysophytes, and tend to cause heat-driven degradation of pigments. Zeaxanthin and Prasinoxanthin can also mediate the degradation of Chl-a in Chlorophytes. Thus, Chlorophyll and the composition of its Chlorins briefly reveal the rates of degradation and light intensity, and the carotenoids and their Chlorins can represent the temperature condition in the environment. These results imply that Chlorins can be utilized as biomarkers to infer the distribution and abundance of phytoplankton communities, reflecting the environmental factors in which phytoplankton live.

Chromatic acclimation in cyanobacteria renders robust photosynthesis and fitness in dynamic light environment: Recent advances and future perspectives

Cyanobacteria are photoautotrophic organisms that use light and water as a source of energy and electrons, respectively, to fix atmospheric carbon dioxide and release oxygen as a by-product during photosynthesis. However, photosynthesis and fitness of organisms are challenged by seasonal and diurnal fluctuations in light environments. Also, the distribution of cyanobacteria in a water column is subject to changes in the light regime. The quality and quantity of light change significantly in low and bright light environments that either limit photochemistry or result in photoinhibition due to an excess amount of light reaching reaction centers. Therefore, cyanobacteria have to adjust their light-harvesting machinery and cell morphology for the optimal harvesting of light. This adjustment of light-harvesting involves remodeling of the light-harvesting complex called phycobilisome or incorporation of chlorophyll molecules such as chlorophyll d and f into their light-harvesting machinery. Thus, photoacclimation responses of cyanobacteria at the level of pigment composition and cell morphology maximize their photosynthetic ability and fitness under a dynamic light environment. Cyanobacteria exhibit different types of photoacclimation responses that are commonly known as chromatic acclimation (CA). In this work, we discuss different types of CA reported in cyanobacteria and present a molecular mechanism of well-known type 3 CA where phycoerythrin and phycocyanin of phycobilisome changes according to light signals. We also include other aspects of type 3 CA that have been recently studied at a molecular level and highlight the importance of morphogenes, cytoskeleton, and carboxysome proteins. In summary, CA gives a unique competitive benefit to cyanobacteria by increasing their resource utilization ability and fitness.

Phycobilisomes and Phycobiliproteins in the Pigment Apparatus of Oxygenic Photosynthetics: From Cyanobacteria to Tertiary Endosymbiosis

Eukaryotic photosynthesis originated in the course of evolution as a result of the uptake of some unstored cyanobacterium and its transformation to chloroplasts by an ancestral heterotrophic eukaryotic cell. The pigment apparatus of Archaeplastida and other algal phyla that emerged later turned out to be arranged in the same way. Pigment-protein complexes of photosystem I (PS I) and photosystem II (PS II) are characterized by uniform structures, while the light-harvesting antennae have undergone a series of changes. The phycobilisome (PBS) antenna present in cyanobacteria was replaced by Chl a/b- or Chl a/c-containing pigment-protein complexes in most groups of photosynthetics. In the form of PBS or phycobiliprotein aggregates, it was inherited by members of Cyanophyta, Cryptophyta, red algae, and photosynthetic amoebae. Supramolecular organization and architectural modifications of phycobiliprotein antennae in various algal phyla in line with the endosymbiotic theory of chloroplast origin are the subject of this review.

Chromatic Acclimation in Cyanobacteria: A Diverse and Widespread Process for Optimizing Photosynthesis

Chromatic acclimation (CA) encompasses a diverse set of molecular processes that involve the ability of cyanobacterial cells to sense ambient light colors and use this information to optimize photosynthetic light harvesting. The six known types of CA, which we propose naming CA1 through CA6, use a range of molecular mechanisms that likely evolved independently in distantly related lineages of the Cyanobacteria phylum. Together, these processes sense and respond to the majority of the photosynthetically relevant solar spectrum, suggesting that CA provides fitness advantages across a broad range of light color niches. The recent discoveries of several new CA types suggest that additional CA systems involving additional light colors and molecular mechanisms will be revealed in coming years. Here we provide a comprehensive overview of the currently known types of CA and summarize the molecular details that underpin CA regulation.

Cloning of pcB and pcA Gene from Gracilariopsis lemaneiformis and Expression of a Fluorescent Phycocyanin in Heterologous Host

In order to study the assembly mechanism of phycocyanin in red algae, the apo-phycocyanin genes (pcB and pcA) were cloned from Gracilariopsis lemaneiformis. The full length of phycocyanin β-subunit (pcB) contained 519 nucleotides encoding a protein of 172 amino acids, and the full length of phycocyanin α-subunit(pcA) contained 489 nucleotides encoding a protein of 162 amino acids. Expression vector pACYCDuet-pcB-pcA was constructed and transformed into E. coli BL21 with pET-ho-pcyA (containing ho and pcyA gene to synthesize phycocyanobilin). The recombinant strain showed fluorescence activity, indicating the expression of optically active phycocyanin in E. coli. To further investigate the possible binding sites between phycocyanobilin and apo-phycocyanin, Cys-82 and Cys-153 of the β subunit and the Cys-84 of the α subunit were respectively mutated, and four mutants were obtained. All mutant strains had lower fluorescence intensity than the non-mutant strains, which indicated that these mutation sites could be the active binding sites between apo-phycocyanin and phycocyanobilin (PCB). This research provides a supplement for the comprehensive understanding of the assembly mechanism of optically active phycocyanin in red algae.

New horizons in culture and valorization of red microalgae

Research on marine microalgae has been abundantly published and patented these last years leading to the production and/or the characterization of some biomolecules such as pigments, proteins, enzymes, biofuels, polyunsaturated fatty acids, enzymes and hydrocolloids. This literature focusing on metabolic pathways, structural characterization of biomolecules, taxonomy, optimization of culture conditions, biorefinery and downstream process is often optimistic considering the valorization of these biocompounds. However, the accumulation of knowledge associated with the development of processes and technologies for biomass production and its treatment has sometimes led to success in the commercial arena. In the history of the microalgae market, red marine microalgae are well positioned particularly for applications in the field of high value pigment and hydrocolloid productions. This review aims to establish the state of the art of the diversity of red marine microalgae, the advances in characterization of their metabolites and the developments of bioprocesses to produce this biomass.

Paramagnetism and Fluorescence of Zinc(II) Tripyrrindione: A Luminescent Radical Based on a Redox-Active Biopyrrin

The ability of bilins and other biopyrrins to form fluorescent zinc complexes has been known for more than a century; however, the exact identity of the emissive species remains uncertain in many cases. Herein, we characterize the hitherto elusive zinc complex of tripyrrin-1,14-dione, an analogue of several orange urinary pigments. As previously observed for its Pd(II), Cu(II), and Ni(II) complexes, tripyrrindione binds Zn(II) as a dianionic radical and forms a paramagnetic complex carrying an unpaired electron on the ligand π-system. This species is stable at room temperature and undergoes quasi-reversible ligand-based redox chemistry. Although the complex is isolated as a coordination dimer in the solid state, optical absorption and electron paramagnetic resonance spectroscopic studies indicate that the monomer is prevalent in a tetrahydrofuran solution. The paramagnetic Zn(II) tripyrrindione complex is brightly fluorescent (λ_abs = 599 nm, λ_em = 644 nm, Φ_F = 0.23 in THF), and its study provides a molecular basis for the observation, made over several decades since the 1930s, of fluorescent behavior of tripyrrindione pigments in the presence of zinc salts. The zinc-bound tripyrrindione radical is thus a new addition to the limited number of stable radicals that are fluorescent at room temperature.

Polymicrobial Sepsis Chronic Immunoparalysis Is Defined by Diminished Ag-Specific T Cell-Dependent B Cell Responses

Immunosuppression is one hallmark of sepsis, decreasing the host response to the primary septic pathogens and/or secondary nosocomial infections. CD4 T cells and B cells are among the array of immune cells that experience reductions in number and function during sepsis. “Help” from follicular helper (Tfh) CD4 T cells to B cells is needed for productive and protective humoral immunity, but there is a paucity of data defining the effect of sepsis on a primary CD4 T cell-dependent B cell response. Using the cecal ligation and puncture (CLP) mouse model of sepsis induction, we observed reduced antibody production in mice challenged with influenza A virus or TNP-KLH in alum early (2 days) and late (30 days) after CLP surgery compared to mice subjected to sham surgery. To better understand how these CD4 T cell-dependent B cell responses were altered by a septic event, we immunized mice with a Complete Freund's Adjuvant emulsion containing the MHC II-restricted peptide 2W1S₅₆₋₆₈ coupled to the fluorochrome phycoerythrin (PE). Immunization with 2W1S-PE/CFA results in T cell-dependent B cell activation, giving us the ability to track defined populations of antigen-specific CD4 T cells and B cells responding to the same immunogen in the same mouse. Compared to sham mice, differentiation and class switching in PE-specific B cells were blunted in mice subjected to CLP surgery. Similarly, mice subjected to CLP had reduced expansion of 2W1S-specific T cells and Tfh differentiation after immunization. Our data suggest CLP-induced sepsis impacts humoral immunity by affecting the number and function of both antigen-specific B cells and CD4 Tfh cells, further defining the period of chronic immunoparalysis after sepsis induction.

Photoinhibition in marine picocyanobacteria

Marine Synechococcus and Prochlorococcus cyanobacteria have different antenna compositions although they are genetically near to each other, and different strains thrive in very different illumination conditions. We measured growth and photoinhibition of PSII in two low-light and one high-light Prochlorococcus strains and in one Synechococcus strain. All strains were found to be able to shortly utilize moderate or even high light, but the low-light strains bleached rapidly in moderate light. Measurements of photoinhibition in the presence of the antibiotic lincomycin showed that a low-light Prochlorococcus strain was more sensitive than a high-light strain and both were more sensitive than the marine Synechococcus. The action spectrum of photoinhibition showed an increase from blue to ultraviolet wavelengths in all strains, suggesting contribution of manganese absorption to photoinhibition, but blue light caused less photoinhibition in marine cyanobacteria than expected on the basis of earlier results from plants and cyanobacteria. The visible-light part of the action spectrum resembled the absorption spectrum of the organism, suggesting that photosynthetic antenna pigments, especially divinyl chlorophylls, have a more important role as photoreceptors of visible-light photoinhibition in marine cyanobacteria than in other photoautotrophs.

Photoinactivation of Photosystem II in Prochlorococcus and Synechococcus

The marine picocyanobacteria Synechococcus and Prochlorococcus numerically dominate open ocean phytoplankton. Although evolutionarily related they are ecologically distinct, with different strategies to harvest, manage and exploit light. We grew representative strains of Synechococcus and Prochlorococcus and tracked their susceptibility to photoinactivation of Photosystem II under a range of light levels. As expected blue light provoked more rapid photoinactivation than did an equivalent level of red light. The previous growth light level altered the susceptibility of Synechococcus, but not Prochlorococcus, to this photoinactivation. We resolved a simple linear pattern when we expressed the yield of photoinactivation on the basis of photons delivered to Photosystem II photochemistry, plotted versus excitation pressure upon Photosystem II, the balance between excitation and downstream metabolism. A high excitation pressure increases the generation of reactive oxygen species, and thus increases the yield of photoinactivation of Photosystem II. Blue photons, however, retained a higher baseline photoinactivation across a wide range of excitation pressures. Our experiments thus uncovered the relative influences of the direct photoinactivation of Photosystem II by blue photons which dominates under low to moderate blue light, and photoinactivation as a side effect of reactive oxygen species which dominates under higher excitation pressure. Synechococcus enjoyed a positive metabolic return upon the repair or the synthesis of a Photosystem II, across the range of light levels we tested. In contrast Prochlorococcus only enjoyed a positive return upon synthesis of a Photosystem II up to 400 μmol photons m-2 s-1. These differential cost-benefits probably underlie the distinct photoacclimation strategies of the species.

Shedding new light on viral photosynthesis

Viruses infecting the environmentally important marine cyanobacteria Prochlorococcus and Synechococcus encode 'auxiliary metabolic genes' (AMGs) involved in the light and dark reactions of photosynthesis. Here, we discuss progress on the inventory of such AMGs in the ever-increasing number of viral genome sequences as well as in metagenomic datasets. We contextualise these gene acquisitions with reference to a hypothesised fitness gain to the phage. We also report new evidence with regard to the sequence and predicted structural properties of viral petE genes encoding the soluble electron carrier plastocyanin. Viral copies of PetE exhibit extensive modifications to the N-terminal signal peptide and possess several novel residues in a region responsible for interaction with redox partners. We also highlight potential knowledge gaps in this field and discuss future opportunities to discover novel phage-host interactions involved in the photosynthetic process.

Occurrence of a functionally stable photoharvesting single peptide allophycocyanin α-subunit (16.4 kDa) in the cyanobacterium Nostoc sp. R76DM

We report the occurrence of a functionally stable single peptide APC α-subunit in cyanobacterium Nostoc sp. R76DM.

A stable and functional single peptide phycoerythrin (15.45 kDa) from Lyngbya sp. A09DM

A functional and stable truncated-phycoerythrin (T-PE) was found as a result of spontaneous in vitro truncation. Truncation was noticed to occur during storage of purified native-phycoerythrin (N-PE) isolated from Lyngbya sp. A09DM. SDS and native-PAGE analysis revealed the truncation of N-PE, containing α (19.0 kDa)--and β (21.5 kDa)--subunits to the only single peptide of ∼15.45 kDa (T-PE). The peptide mass fingerprinting (PMF) and MS/MS analysis indicated that T-PE is the part of α-subunit of N-PE. UV-visible absorption peak of N-PE was found to split into two peaks (540 and 565 nm) after truncation, suggesting the alterations in its folded state. The emission spectra of both N-PE and T-PE show the emission band centered at 581 nm (upon excitation at 559 nm) suggested the maintenance of fluorescence even after significant truncation. Urea-induced denaturation and Gibbs-free energy (ΔGD°) calculations suggested that the folding and structural stability of T-PE was almost similar to that of N-PE. Presented bunch of evidences revealed the truncation in N-PE without perturbing its folding, structural stability and functionality (fluorescence), and thereby suggested its applicability in fluorescence based biomedical techniques where smaller fluorescence molecules are more preferable.

Protein co-migration database (PCoM -DB) for Arabidopsis thylakoids and Synechocystis cells

Protein-protein interactions are critical for most cellular processes; however, many remain to be identified. Here, to comprehensively identify protein complexes in photosynthetic organisms, we applied the recently developed approach of blue native PAGE (BN-PAGE) coupled with LC-MS/MS to the thylakoid proteins of Arabidopsis thaliana and the whole cell proteins of whole cell proteins of Synechocystis sp. PCC 6803. We identified 245 proteins from the purified Arabidopsis thylakoid membranes and 1,458 proteins from the whole cells of Synechocystis using the method. Next, we generated protein migration profiles that were assessed by plotting the label-free estimations of protein abundances versus migration distance in BN-PAGE. Comparisons between the migration profiles of the major photosynthetic complexes and their band patterns showed that the protein migration profiles were well correlated. Thus, the protein migration profiles allowed us to estimate the molecular size of each protein complex and to identify co-migrated proteins with the proteins of interest by determining the protein pairs that contained peaks in the same gel slice. Finally, we built the protein co-migration database for photosynthetic organisms (PCoM-DB: http://pcomdb.lowtem.hokudai.ac.jp/proteins/top) to make our data publicly accessible online, which stores the analyzed data with a user-friendly interface to compare the migration profiles of proteins of interest. It helps users to find unidentified protein complexes in Arabidopsis thylakoids and Synechocystis cells. The accumulation of the data from the BN-PAGE coupled with LC-MS/MS should reveal unidentified protein complexes and should aid in understanding the adaptation and the evolution of photosynthetic organisms.

Opposite chirality of α-carotene in unusual cyanobacteria with unique chlorophylls, Acaryochloris and Prochlorococcus

Among all photosynthetic and non-photosynthetic prokaryotes, only cyanobacterial species belonging to the genera Acaryochloris and Prochlorococcus have been reported to synthesize α-carotene. We reviewed the carotenoids, including their chirality, in unusual cyanobacteria containing diverse Chls. Predominantly Chl d-containing Acaryochloris (two strains) and divinyl-Chl a and divinyl-Chl b-containing Prochlorococcus (three strains) contained β-carotene and zeaxanthin as well as α-carotene, whereas Chl b-containing Prochlorothrix (one strain) and Prochloron (three isolates) contained only β-carotene and zeaxanthin but no α-carotene as in other cyanobacteria. Thus, the capability to synthesize α-carotene seemed to have been acquired only by Acaryochloris and Prochlorococcus. In addition, we unexpectedly found that α-carotene in both cyanobacteria had the opposite chirality at C-6': (6'S)-chirality in Acaryochloris and normal (6'R)-chirality in Prochlorococcus, as reported in some green algae and land plants. The results represent the first evidence for the natural occurrence and biosynthesis of (6'S)-α-carotene. All the zeaxanthins in these species were of the usual (3R,3'R)-chirality. Therefore, based on the identification of the carotenoids and genome sequence data, we propose a biosynthetic pathway for the carotenoids, particularly α-carotene, including the participating genes and enzymes.