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In oxygenic photosynthetic organisms, light energy is captured by antenna systems and transferred to photosystem II (PSII) and photosystem I (PSI) to drive photosynthesis 1 , 2 . The antenna systems of red algae consist of soluble phycobilisomes (PBSs) and transmembrane light-harvesting complexes (LHCs) 3 . Excitation energy transfer pathways from PBS to photosystems remain unclear owing to the lack of structural information. Here we present in situ structures of PBS–PSII–PSI–LHC megacomplexes from the red alga Porphyridium purpureum at near-atomic resolution using cryogenic electron tomography and in situ single-particle analysis 4 , providing interaction details between PBS, PSII and PSI. The structures reveal several unidentified and incomplete proteins and their roles in the assembly of the megacomplex, as well as a huge and sophisticated pigment network. This work provides a solid structural basis for unravelling the mechanisms of PBS–PSII–PSI–LHC megacomplex assembly, efficient energy transfer from PBS to the two photosystems, and regulation of energy distribution between PSII and PSI. In situ structures of PBS–PSII–PSI–LHC megacomplexes from the alga P. purpureum at near-atomic resolution using cryogenic-electron tomography and in situ single-particle analysis are reported, providing interaction details between PBS, PSII and PSI.

Photosynthetic organisms have developed various light-harvesting systems to adapt to their environments 1 . Phycobilisomes are large light-harvesting protein complexes found in cyanobacteria and red algae 2 – 4 , although how the energies of the chromophores within these complexes are modulated by their environment is unclear. Here we report the cryo-electron microscopy structure of a 14.7-megadalton phycobilisome with a hemiellipsoidal shape from the red alga Porphyridium purpureum . Within this complex we determine the structures of 706 protein subunits, including 528 phycoerythrin, 72 phycocyanin, 46 allophycocyanin and 60 linker proteins. In addition, 1,598 chromophores are resolved comprising 1,430 phycoerythrobilin, 48 phycourobilin and 120 phycocyanobilin molecules. The markedly improved resolution of our structure compared with that of the phycobilisome of Griffithsia pacifica 5 enabled us to build an accurate atomic model of the P. purpureum phycobilisome system. The model reveals how the linker proteins affect the microenvironment of the chromophores, and suggests that interactions of the aromatic amino acids of the linker proteins with the chromophores may be a key factor in fine-tuning the energy states of the chromophores to ensure the efficient unidirectional transfer of energy. The cryo-electron microscopy structure of a phycobilisome from the red alga Porphyridium purpureum reveals how aromatic interactions between the linker proteins and the chromophores drive a unidirectional transfer of energy.

The scientific community continue to explore novel bioactive molecules by investigating natural origins; microalgae are photosynthetic organisms considered as a sustainable resource to use in many fields. They present a high diversity in species and richness in terms of attractive bio-compounds. The aim of this review is to (1) provide first an overview of current issues related to oxidative stress, and propose a natural metabolite derived from eukaryotic and prokaryotic microalgae; ‘polysaccharides’ as a powerful antioxidant agent, then, (2) organize the available data on the antioxidant potential of polysaccharides derived from the main microalgal groups (red microalgae, green microalgae, and cyanobacteria) and especially highlighted the key species of each group (Porphyridium sp., Chlorella sp., and Arthrospira sp., respectively), meanwhile, (3) we described the chemical composition of polysaccharides from each class, and (4) we cite briefly the most factors affecting the antioxidant activity of these molecules. Finally, we explored the major challenges and gaps found to require more investigation.Article HighlightsHuman diseases associated with oxidation are stimulating the search for new drugs, particularly from marine sources,Polysaccharides from microalgae are natural biomolecules exhibiting high antioxidant potential,Potent microalgal species producing polysaccharides are being more investigated in the biotechnological field,Biodiversity in microalgae leads to the production of various polysaccharides, mainly in terms of composition and structure, which enhance the antioxidant activity,Several factors influence the antioxidant potential of microalgae’s polysaccharides.

Nowadays, there is a growing interest in finding new coloring molecules of natural origin that can increase and diversify the offer of natural food dyes already present in the market. In the present work, a B-phycoerythrin extract from the microalgae Porphyridium cruentum was tested as a food colorant in milk-based products. Using spectroscopy and colorimetry, the extract was characterized and gave evidence of good properties and good stability in the pH range between 4.0 and 9.0. Coloring studies were conducted to demonstrate that samples carrying the pink extract could be used for simulating the pink color of marketed milk-based products. The staining factors, representing the amount of pink protein to be added to reproduce the color of strawberry commercial products, ranged between 1.6 mg/L and 49.5 mg/L, being sufficiently low in all samples. Additionally, color stability during a short period of cold storage was studied: it demonstrated that the three tested types of dairy products remained stable throughout the 11-day analysis period with no significant changes. These results prove the potential of the B-phycoerythrin extract as a natural colorant and alternative ingredient to synthetic coloring molecules.

In the present study, microwave-assisted extraction was first employed to extract the phycobiliproteins of Porphyridium purpureum (Pp). Freeze-dried Pp cells were subjected to microwave-assisted extraction (MAE) to extract phycoerythin (PE), phycocyanin (PC), and allophycocyanin (APC). MAE combined reproducibility and high extraction yields and allowed a 180- to 1,080-fold reduction of the extraction time compared to a conventional soaking process. The maximal PE extraction yield was obtained after 10-s MAE at 40 °C, and PE was thermally damaged at temperatures higher than 40 °C. In contrast, a flash irradiation for 10 s at 100 °C was the best process to efficiently extract PC and APC, as it combined a high temperature necessary to extract them from the thylakoid membrane to a short exposure to thermal denaturation. The extraction order of the three phycobiliproteins was coherent with the structure of Pp phycobilisomes. Moreover, the absorption and fluorescence properties of MAE extracted phycobiliproteins were stable for several months after the microwave treatment. Scanning electron microscopy indicated that MAE at 100 °C induced major changes in the Pp cell morphology, including fusion of the exopolysaccharidic cell walls and cytoplasmic membranes of adjacent cells. As a conclusion, MAE is a fast and high yield process efficient to extract and pre-purify phycobiliproteins, even from microalgae containing a thick exopolysaccharidic cell wall.

Porphyridium is a unicellular marine microalga that is rich in polysaccharides and has excellent biological activities. Optimizing the extraction of polysaccharides can significantly improve the value of Porphyridium biomass. In the present study, response surface methodology was employed to optimize the extraction conditions of polysaccharides, including extraction time, extraction temperature, and biomass-to-water ratio. Furthermore, microwave-assisted extraction was used to improve the yield of polysaccharides further. The results showed that increasing the extraction temperature and extraction time could enhance the yield of polysaccharides. The multiple regression analysis of RSM indicated that the model could be employed to optimize the extraction of polysaccharides. The optimal extraction time, extraction temperature, and biomass-to-water ratio were 45 min, 87 °C, and 1:63 g mL−1, respectively. Under these optimal conditions, the maximum yield of polysaccharides was 23.66% DW, which well matched the predicted yield. The results indicated that the extraction temperature was the most significant condition affecting the yield of polysaccharides. The microwave-assisted extraction could further improve the yield of polysaccharides to 25.48% DW. In conclusion, hot water with microwave-assisted extraction was effective for polysaccharide extraction in P. purpureum.

For the first time, the microalga Porphyridium cruentum was tested for its ability to produce silver nanoparticles. To characterize formed silver nanoparticles UV–vis Spectrometry, Scanning Electron Microscopy, Energy-dispersive analysis of X-rays and X-ray diffraction were used. It was shown that after biomass exposure to silver nitrate solution the extracellular formation of spherical-like nanoparticles took place. Functional groups responsible for metal binding were determined by Fourier-transform infrared spectroscopy. The complex of biochemical tests was used for biomass characterization and assessment of the changes of its main components (proteins, lipids, carbohydrates, and phycobilin) during nanoparticle formation. Obtained data indicate a significant decrease of proteins, carbohydrates, phycobiliproteins, and lipids content as well as antiradical activity of biomass. The obtained results show the necessity of determination of optimal conditions for obtaining Porphyridium cruentum biomass enriched with silver nanoparticles for its further application in the pharmaceuticals industry

The biocompatible silica precursor tetrakis(2-hydroxyethyl)orthosilicate with ethylene glycol residues was used instead of the common alcohol-containing tetraethoxysilane for the first time to prepare a biorecognition element by entrapping the marine microalga Porphyridium purpureum into a silica matrix by a one-stage sol–gel procedure at conditions (pH, ionic strength, and temperature) appropriate for living cells. We show that the microalga immobilized in this way fully maintains its viability and functionality. We furthermore show that the silica matrix had a stabilizing effect, providing microalgal survival and functionality at increased temperature. The high optical transparency of the silica matrix allowed us to study the optical properties of Porphyridium purpureum thoroughly. When irradiated by a laser, intense fluorescence of chlorophyll-a and phycoerythrin of the photosynthetic system was observed. The characteristics of this fluorescence differed notably from that observed with P. purpureum in suspension before immobilization; possible reasons for this and an underlying mechanism are discussed.

  Lynn Margulis, whose 1967 paper articulated the endosymbiotic origin of mitochondria and chloroplasts; Carl Woese, whose attempts to classify prokaryotes based on ribosomal RNA cataloging led to his championing the new kingdom of Archaea in 1977; and Ford Doolittle (Fig 1), who provided evidence for Margulis's hypothesis using Woese's methods. Though Woese and Margulis are now deceased, Doolittle, I can attest, is very much alive and, dare I say, \"kicking!\" Over time, Doolittle has cogitated on a variety of intriguing evolutionary questions, including the origin of introns, the role of lateral gene transfer in speciation, and the meaning of \"function.\" The earlier and still-dominant theory would be that bacteria evolved into blue-green algae, which were more complicated than bacteria, and blue-green algae evolved into eukaryotic algae, which are more complicated than prokaryotic algae, and everything else eukaryotic evolved from them.

Microalgae are microorganisms often surrounded by a slime layer made of secreted polymeric substances sometimes including polysaccharides. These polysaccharides, weakly described in the literature, can constitute value-added molecules in several industrial areas. The aim of this article is to show that a new tool, the BioFilm Ring Test ®, can be used to detect viscous microalgal exopolymers. Two red microalgal strains (Rhodella violacea and Porphyridium purpureum), one cyanobacterium (Arthrospira platensis) and their excreted polymeric fractions were studied. R. violacea and P. purpureum induced a positive response with the BioFilm Ring Test ® contrary to A. platensis. Finally, the understanding of the fractions viscosity involvement in the BRT response was performed by a rheological study.