Growth phase-dependent reorganization of cryptophyte photosystem I antennae.

Photosynthetic cryptophytes are eukaryotic algae that utilize membrane-embedded chlorophyll a/c binding proteins (CACs) and lumen-localized phycobiliproteins (PBPs) as their light-harvesting antennae. Cryptophytes go through logarithmic and stationary growth phases, and may adjust their light-harvesting capability according to their particular growth state. How cryptophytes change the type/arrangement of the photosynthetic antenna proteins to regulate their light-harvesting remains unknown. Here we solve four structures of cryptophyte photosystem I (PSI) bound with CACs that show the rearrangement of CACs at different growth phases. We identify a cryptophyte-unique protein, PsaQ, which harbors two chlorophyll molecules. PsaQ specifically binds to the lumenal region of PSI during logarithmic growth phase and may assist the association of PBPs with photosystems and energy transfer from PBPs to photosystems.

Molecular dissection of the soluble photosynthetic antenna from the cryptophyte alga Hemiselmis andersenii.

Cryptophyte algae have a unique phycobiliprotein light-harvesting antenna that fills a spectral gap in chlorophyll absorption from photosystems. However, it is unclear how the antenna transfers energy efficiently to these photosystems. We show that the cryptophyte Hemiselmis andersenii expresses an energetically complex antenna comprising three distinct spectrotypes of phycobiliprotein, each composed of two αß protomers but with different quaternary structures arising from a diverse α subunit family. We report crystal structures of the major phycobiliprotein from each spectrotype. Two-thirds of the antenna consists of open quaternary form phycobiliproteins acting as primary photon acceptors. These are supplemented by a newly discovered open-braced form (~15%), where an insertion in the α subunit produces ~10 nm absorbance red-shift. The final components (~15%) are closed forms with a long wavelength spectral feature due to substitution of a single chromophore. This chromophore is present on only one ß subunit where asymmetry is dictated by the corresponding α subunit. This chromophore creates spectral overlap with chlorophyll, thus bridging the energetic gap between the phycobiliprotein antenna and the photosystems. We propose that the macromolecular organization of the cryptophyte antenna consists of bulk open and open-braced forms that transfer excitations to photosystems via this bridging closed form phycobiliprotein.

The Unique Light-Harvesting System of the Algal Phycobilisome: Structure, Assembly Components, and Functions.

The phycobilisome (PBS) is the major light-harvesting apparatus in cyanobacteria and red algae. It is a large multi-subunit protein complex of several megadaltons that is found on the stromal side of thylakoid membranes in orderly arrays. Chromophore lyases catalyse the thioether bond between apoproteins and phycobilins of PBSs. Depending on the species, composition, spatial assembly, and, especially, the functional tuning of different phycobiliproteins mediated by linker proteins, PBSs can absorb light between 450 and 650 nm, making them efficient and versatile light-harvesting systems. However, basic research and technological innovations are needed, not only to understand their role in photosynthesis but also to realise the potential applications of PBSs. Crucial components including phycobiliproteins, phycobilins, and lyases together make the PBS an efficient light-harvesting system, and these provide a scheme to explore the heterologous synthesis of PBS. Focusing on these topics, this review describes the essential components needed for PBS assembly, the functional basis of PBS photosynthesis, and the applications of phycobiliproteins. Moreover, key technical challenges for heterologous biosynthesis of phycobiliproteins in chassis cells are discussed.

Magnetic Fields as Inducers of Phycobiliprotein Production by Synechococcus elongatus PCC 7942.

This study aimed to analyze the effect of magnetic field (MF) application on the metabolism of Synechococcus elongatus PCC 7942. Concentrations of biomass, carbohydrate, protein, lipid, and photosynthetic pigments (chlorophyll-a, C-phycocyanin, allophycocyanin and phycoerythrin) were determined. In cultures with MF application (30 mT for 24 h d-1), there were increases of 47.5% in total protein content, 87.4% in C-phycocyanin, and 332.8% in allophycocyanin contents, by comparison with the control. Allophycocyanin is the most affected pigment by MF application. Therefore, its biosynthetic route was investigated, and four genes related to its synthesis were found. However, the analysis of the gene expression showed no statistical differences from the control culture, which suggests that induction of such genes may occur soon after MF application with consequent stabilization over time. MF application may be a cost-effective alternative to increase production of compounds of commercial interest by cyanobacteria.

Cultivation of blue green algae (Arthrospira platensis Gomont, 1892) in wastewater for biodiesel production.

The production of biodiesel has become an important issue in the effort to reduce gas emissions due to the climate change crisis; therefore, algae have widely used to produce biodiesel for energy sustainability. The present study represented an effort to assess the ability of the alga Arthrospira platensis to produce fatty acids involved in biofuel (diesel) by cultivation in Zarrouk media enriched with different municipal wastewater concentrations. Wastewater was used in different concentrations (5, 15, 25, 35 and 100% [control]). Five fatty acids from the alga were determined and included in the present study. These were inoleic acid, palmitic acid, oleic acid, gamma-linolenic acid, and docosahexaenoic acid. Impact of different cultivation conditions were studied in terms of observed changes in growth rate, doubling time, total carbohydrate, total protein, chlorophyll a, carotenoids, phycocyanin, allophycocyanin, and phycobiliproteins. Results showed an increase in the values of growth rate, total protein content, chlorophyll a, and levels of carotenoids at all treatments except for carbohydrate content, which decreased with an increasing concentration of wastewater. The high value of doubling time (11.605 days) was recorded at treatment 5%. Fatty acids yields were increased at treatment 5% and 15%. The highest concentrations of fatty acids were 3.108 mg/g for oleic acid, gamma-linolenic acid (28.401 mg/g), docosahexaenoic acid (41.707 mg/g), palmitic acid (1.305 mg/g), and linoleic acid (0.296 mg/g). Moreover, the range of phycocyanin (0.017-0.084 mg/l), allophycocyanin (0.023-0.095 mg/l), and phycobiliproteins (0.041-0.180 mg/l) were obtained in treatment with 15-100%, respectively. Cultivation with municipal wastewater reduced the values of nitrate, phosphate, and electrical conductivity as well as increased dissolved oxygen. Maximum electrical conductivity was recorded in untreated wastewater with algae, while the highest level of dissolved oxygen was noted at 35% concentration. The use of the household wastewater is more environmentally friendly as an alternative of the traditional cultivation techniques used for long-term for biofuel production.

Elucidating the origins of phycocyanobilin biosynthesis and phycobiliproteins.

Terrestrial ecosystems and human societies depend on oxygenic photosynthesis, which began to reshape our atmosphere approximately 2.5 billion years ago. The earliest known organisms carrying out oxygenic photosynthesis are the cyanobacteria, which use large complexes of phycobiliproteins as light-harvesting antennae. Phycobiliproteins rely on phycocyanobilin (PCB), a linear tetrapyrrole (bilin) chromophore, as the light-harvesting pigment that transfers absorbed light energy from phycobilisomes to the chlorophyll-based photosynthetic apparatus. Cyanobacteria synthesize PCB from heme in two steps: A heme oxygenase converts heme into biliverdin IXα (BV), and the ferredoxin-dependent bilin reductase (FDBR) PcyA then converts BV into PCB. In the current work, we examine the origins of this pathway. We demonstrate that PcyA evolved from pre-PcyA proteins found in nonphotosynthetic bacteria and that pre-PcyA enzymes are active FDBRs that do not yield PCB. Pre-PcyA genes are associated with two gene clusters. Both clusters encode bilin-binding globin proteins, phycobiliprotein paralogs that we designate as BBAGs (bilin biosynthesis-associated globins). Some cyanobacteria also contain one such gene cluster, including a BBAG, two V4R proteins, and an iron-sulfur protein. Phylogenetic analysis shows that this cluster is descended from those associated with pre-PcyA proteins and that light-harvesting phycobiliproteins are also descended from BBAGs found in other bacteria. We propose that PcyA and phycobiliproteins originated in heterotrophic, nonphotosynthetic bacteria and were subsequently acquired by cyanobacteria.

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.

Spirulina- An Edible Cyanobacterium with Potential Therapeutic Health Benefits and Toxicological Consequences.

Spirulina is a blue-green algae which is cultivated not only for its maximum protein content but also due to the presence of other essential nutrients such as carbohydrates and vitamins (A, C and E). It is also a storehouse of minerals including iron, calcium, chromium, copper, magnesium, manganese, phosphorus, potassium, sodium and zinc. Simultaneously, γ- linolenic acid (an essential fatty acid), as well as pigments such as chlorophyll A and phycobiliproteins (C-phycocyanin, allophycocyanin and ß-carotene), is also a major component of its rich nutritional profile. Spirulina is known to have various promising effects on the prevention of cancer, oxidative stress, obesity, diabetes, cardiovascular diseases and anemia. Moreover, it also plays a positive role in treating muscular cramps. The safety recommended dosage of Spirulina is approximately 3-10 g/d for adults and it's biological value (BV) is 75 with a net protein utilization (NPU) of 62. Spirulina does not have pericardium due to which it does not hinder the absorption of iron by chelation with phytates or oxalates. On the contrasting note, it may have some adverse effects due to the toxins (microcystins, ß-methylamino-L-alanine (BMAA)) produced by Spirulina which might contribute to acute poisoning, cancer, liver damage as well as gastrointestinal disturbances. Its long-term consumption may also lead to the pathogenesis of Alzheimer's disease and Parkinson's disease. The current review focuses on the various aspects of spirulina including its cultivation, nutritional composition, extraction techniques, health benefits, adverse effects, industrial scope and market value which could be beneficial for its utilization in the development of value-added products and supplementary foods due to its high content of protein and bioavailability of nutrients.

• Spirulina is a nutrient-dense cyanobacterium which is composed of protein, carbohydrates, vitamins, minerals, essential fatty acids, antioxidants and pigments including chlorophyll A and Phycocyanin.• To avoid the contamination of Spirulina species by other algae, the specific pH maintenance of the media around 9-11 (alkaline) is mandatory.• Positive effects were noticed on the yield and productivity of Spirulina after its biomass was grown in polybags and greenhouse.• Its beneficial effects have been identified in particular reference to obesity, diabetes, hypertension, cardiovascular diseases, anemia, cancer, oxidative stress, arthritis, immunity as well as muscular cramps.• The toxins such as microcystins and hepatotoxins, produced by Spirulina, are accountable to cause acute poisoning, liver damage, gastrointestinal disturbances and cancer.• The safe recommended dosage of Spirulina for adults accounts to approximately 3-10 g/d, with 30 g/d being the maximum limit for consumption.

Mixotrophic Cultivation of a Native Cyanobacterium, Pseudanabaena mucicola GO0704, to Produce Phycobiliprotein and Biodiesel.

Global warming has accelerated in recent decades due to the continuous consumption of petroleum-based fuels. Cyanobacteria-derived biofuels are a promising carbon-neutral alternative to fossil fuels that may help achieve a cleaner environment. Here, we propose an effective strategy based on the large-scale cultivation of a newly isolated cyanobacterial strain to produce phycobiliprotein and biodiesel, thus demonstrating the potential commercial applicability of the isolated microalgal strain. A native cyanobacterium was isolated from Goryeong, Korea, and identified as Pseudanabaena mucicola GO0704 through 16s RNA analysis. The potential exploitation of P. mucicola GO0704 was explored by analyzing several parameters for mixotrophic culture, and optimal growth was achieved through the addition of sodium acetate (1 g/l) to the BG-11 medium. Next, the cultures were scaled up to a stirred-tank bioreactor in mixotrophic conditions to maximize the productivity of biomass and metabolites. The biomass, phycobiliprotein, and fatty acids concentrations in sodium acetate-treated cells were enhanced, and the highest biodiesel productivity (8.1 mg/l/d) was achieved at 96 h. Finally, the properties of the fuel derived from P. mucicola GO0704 were estimated with converted biodiesels according to the composition of fatty acids. Most of the characteristics of the final product, except for the cloud point, were compliant with international biodiesel standards [ASTM 6761 (US) and EN 14214 (Europe)].

New Far-Red and Near-Infrared Fluorescent Phycobiliproteins with Excellent Brightness and Photostability.

Far-red and near-infrared fluorescent proteins can be used as fluorescence biomarkers in the region of maximal transmission of most tissues and facilitate multiplexing. Recently, we reported the generation and properties of far-red and near-infrared fluorescent phycobiliproteins, termed BeiDou Fluorescent Proteins (BDFPs), which can covalently bind the more readily accessible biliverdin. Far-red BDFPs maximally fluoresce at ∼670 nm, while near-infrared BDFPs fluoresce at ∼710 nm. In this work, we molecularly evolved BDFPs as follows: (a) mutations L58Q, S68R and M81K of BDFPs, which can maximally enhance the effective brightness in vivo by 350 %; (b) minimization and monomerization of far-red BDFPs 2.1, 2.2, 2.3, and near-infrared BDFPs 2.4, 2.5 and 2.6. These newly developed BDFPs are remarkably brighter than the formerly reported far-red and near-infrared fluorescent proteins. Their advantages are demonstrated by biolabeling in mammalian cells using super-resolution microscopy.

Probing heavy metal binding to phycobiliproteins.

Blue-green algae, also known as cyanobacteria, contain some of the most efficient light-harvesting complexes known. These large, colourful complexes consist of phycobiliproteins which are extremely valuable in the cosmetics, food, nutraceutical and pharmaceutical industries. Additionally, the colourful and fluorescent properties of phycobiliproteins can be modulated by metal ions, making them highly attractive as heavy metal sensors and heavy metal scavengers. Although the overall quenching ability metal ions have on phycobiliproteins is known, the mechanism of heavy metal binding to phycobiliproteins is not fully understood, limiting their widespread quantitative applications. Here, we show using high-resolution native mass spectrometry that phycobiliprotein complexes bind metal ions in different manners. Through monitoring the binding equilibria and metal-binding stoichiometry, we show in particular copper and silver to have drastic, yet different effects on phycobiliprotein structure, both copper and silver modulate the overall complex properties. Together, the data reveals the mechanisms by which metal ions can modulate phycobiliprotein properties which can be used as a basis for the future design of metal-related phycobiliprotein applications.

A bottom-up perspective on photodynamics and photoprotection in light-harvesting complexes using anti-Brownian trapping.

Single-molecule fluorescence spectroscopy allows direct, real-time observation of dynamic photophysical changes in light harvesting complexes. The Anti-Brownian ELectrokinetic (ABEL) trap is one such single-molecule method with useful advantages. This approach is particularly well-suited to make detailed spectroscopic measurements of pigment-protein complexes in a solution phase because it enables extended-duration single-molecule observation by counteracting Brownian motion. This Perspective summarizes recent contributions by the authors and others that have utilized the unique capabilities of the ABEL trap to advance our understanding of phycobiliproteins and the phycobilisome complex, the primary light-harvesting apparatus of cyanobacteria. Monitoring the rich spectroscopic data from these measurements, which include brightness, fluorescence lifetime, polarization, and emission spectra, among other measurable parameters, has provided direct characterization of pigments and energy transfer pathways in the phycobilisome, spanning scales from single pigments and monomeric phycobiliproteins to higher order oligomers and protein-protein interactions of the phycobilisome complex. Importantly, new photophysical states and photodynamics were observed to modulate the flow of energy through the phycobilisome and suggest a previously unknown complexity in phycobilisome light harvesting and energy transport with a possible link to photoadaptive or photoprotective functions in cyanobacteria. Beyond deepening our collective understanding of natural light-harvesting systems, these and future discoveries may serve as inspiration for engineering improved artificial light-harvesting technologies.

Crystal structure and molecular mechanism of an E/F type bilin lyase-isomerase.

Chromophore attachment of the light-harvesting apparatus represents one of the most important post-translational modifications in photosynthetic cyanobacteria. Extensive pigment diversity of cyanobacteria critically depends on bilin lyases that covalently attach chemically distinct chromophores to phycobiliproteins. However, how bilin lyases catalyze bilin ligation reactions and how some lyases acquire additional isomerase abilities remain elusive at the molecular level. Here, we report the crystal structure of a representative bilin lyase-isomerase MpeQ. This structure has revealed a "question-mark" protein architecture that unambiguously establishes the active site conserved among the E/F-type bilin lyases. Based on structural, mutational, and modeling data, we demonstrate that stereoselectivity of the active site plays a critical role in conferring the isomerase activity of MpeQ. We further advance a tyrosine-mediated reaction scheme unifying different types of bilin lyases. These results suggest that lyases and isomerase actions of bilin lyases arise from two coupled molecular events of distinct origin.

Phycobiliproteins extract from Spirulina protects against single-dose cadmium-induced reproductive toxicity in male mice.

Cadmium (Cd) is known for its many toxic effects on male population such as hypogonadism and fertility difficulties, which are oftenly associated with oxidative stress. As beneficial food, Spirulina(Sp) has been proved efficient against the heavy metal toxicity. This capacity can be associated with its phycobiliproteins (PBP). In this study, the capability of PBP and Sp to treat Cd-induced oxidative damage on the testes and spermatozoa was considered. CD-1 strain mice were orally treated with either Sp or PBP for 10 days prior to single-dose Cd challenge. Sperm quality determinations and testicle histology analysis were performed. Testosterone on serum was measured using enzyme-linked immunosorbent assay (ELISA). Oxidative damage was determined. Antioxidant enzyme activity was analyzed by measuring the activity of super oxide dismutase (SOD), catalase (Cat), and glutathione peroxidase (GpX). The motility and viability of sperm decrease with Cd and improve with PBP and Sp, as the acrosomal reaction (AR) is diminished by PBPs. Testosterone levels decrease due to Cd, and only Sp maintains elevated levels. Cd increases the production of malondialdehyde in the spermatozoa, but not in testes; this production of malondialdehyde in the spermatozoa decreases in the presence of PBP. ROS only decreases with Cd, FBP, and Sp at high concentrations. Advanced oxidative protein products (AOPP) decrease with Cd and PBPs. Cat and GpX increase their activity with Cd and are altered by FBP. Cd produces vascular alterations testes. Within the seminiferous tubule, it produces areas of necrosis and apoptosis, which improve with PBPs and Sp. PBPs have a strong antioxidant activity as they show protective properties against Cd oxidative-induced toxicity on testes and sperm.

The structural basis of far-red light absorbance by allophycocyanins.

Phycobilisomes (PBS), the major light-harvesting antenna in cyanobacteria, are supramolecular complexes of colorless linkers and heterodimeric, pigment-binding phycobiliproteins. Phycocyanin and phycoerythrin commonly comprise peripheral rods, and a multi-cylindrical core is principally assembled from allophycocyanin (AP). Each AP subunit binds one phycocyanobilin (PCB) chromophore, a linear tetrapyrrole that predominantly absorbs in the orange-red region of the visible spectrum (600-700 nm). AP facilitates excitation energy transfer from PBS peripheral rods or from directly absorbed red light to accessory chlorophylls in the photosystems. Paralogous forms of AP that bind PCB and are capable of absorbing far-red light (FRL; 700-800 nm) have recently been identified in organisms performing two types of photoacclimation: FRL photoacclimation (FaRLiP) and low-light photoacclimation (LoLiP). The FRL-absorbing AP (FRL-AP) from the thermophilic LoLiP strain Synechococcus sp. A1463 was chosen as a platform for site-specific mutagenesis to probe the structural differences between APs that absorb in the visible region and FRL-APs and to identify residues essential for the FRL absorbance phenotype. Conversely, red light-absorbing allophycocyanin-B (AP-B; ~ 670 nm) from the same organism was used as a platform for creating a FRL-AP. We demonstrate that the protein environment immediately surrounding pyrrole ring A of PCB on the alpha subunit is mostly responsible for the FRL absorbance of FRL-APs. We also show that interactions between PCBs bound to alpha and beta subunits of adjacent protomers in trimeric AP complexes are responsible for a large bathochromic shift of about ~ 20 nm and notable sharpening of the long-wavelength absorbance band.

Revealing new sponge-associated cyanobacterial diversity: Novel genera and species.

Cyanobacteria are often reported as abundant components of the sponge microbiome; however their diversity below the phylum level is still underestimated. Aiming to broaden our knowledge of sponge-cyanobacteria association, we isolated cyanobacterial strains from Aegean Sea sponges in previous research, which revealed high degree of novel cyanobacterial diversity. Herein, we aim to further characterize sponge-associated cyanobacteria and re-evaluate their classification based on an extensive polyphasic approach, i.e. a combination of molecular, morphological and ecological data. This approach resulted in the description of five new genera (Rhodoploca, Cymatolege, Metis, Aegeococcus, and Thalassoporum) and seven new species (R. sivonenia, C. spiroidea, C. isodiametrica, M. fasciculata, A. anagnostidisi, A. thureti, T. komareki) inside the order Synechococcales, and a new pleurocapsalean species (Xenococcus spongiosum). X. spongiosum is a baeocyte-producing species that shares some morphological features with other Xenococcus species, but has distinct phylogenetic and ecological identity. Rhodoploca, Cymatolege, Metis and Thalassoporum are novel well supported linages of filamentous cyanobacteria that possess distinct characters compared to their sister taxa. Aegeococcus is a novel monophyletic linage of Synechococcus-like cyanobacteria exhibiting a unique ecology, as sponge-dweller. The considerable number of novel taxa characterized in this study highlights the importance of employing polyphasic culture-dependent approaches in order to reveal the true cyanobacterial diversity associated with sponges.

Stability, kinetics, and application study of phycobiliprotein pigments extracted from red algae Gracilaria gracilis.

Phycobiliprotein (PBP) pigments were extracted from red algae Gracilaria gracilis through maceration in phosphate buffer using previously optimized conditions. The stability of PBPs in the extracts was assessed by monitoring the extracts at different pHs and temperatures for 10 days. Since phycoerythrin (PE) is the main PBP present in G. gracilis, PE content was spectroscopically determined and used as a response factor. Kinetic modeling was used to describe PE degradation under different ranges of T and pH. The pigment extracts presented higher stability at pH 6.9 and -20 °C. PE was semipurified by precipitation with ammonium sulphate 65% followed by dialysis against water until a purity index of 0.7. The pigment was successfully applied as colorant in pancakes and yogurts with a pigment concentration of 0.15%. This study highlights the potential of PE pigments extracted from G. gracilis for applications in food products. PRACTICAL APPLICATION: Phycobiliprotein pigments were extracted from red algae Gracilaria gracilis through maceration in phosphate buffer. The stability of the pigment was evaluated at different pHs and temperatures, presenting higher stability at neutral pH and low temperatures. The pigment was successfully applied as colorant in pancakes and yogurts with a low pigment concentration. This study highlights the potential of phycobiliprotein pigments extracted from G. gracilis for applications in food products.

The Arabidopsis CRUMPLED LEAF protein, a homolog of the cyanobacterial bilin lyase, retains the bilin-binding pocket for a yet unknown function.

The photosynthetic bacterial phycobiliprotein lyases, also called CpcT lyases, catalyze the biogenesis of phycobilisome, a light-harvesting antenna complex, through the covalent attachment of chromophores to the antenna proteins. The Arabidopsis CRUMPLED LEAF (CRL) protein is a homolog of the cyanobacterial CpcT lyase. Loss of CRL leads to multiple lesions, including localized foliar cell death, constitutive expression of stress-related nuclear genes, abnormal cell cycle, and impaired plastid division. Notwithstanding the apparent phenotypes, the function of CRL still remains elusive. To gain insight into the function of CRL, we examined whether CRL still retains the capacity to bind with the bacterial chromophore phycocyanobilin (PCB) and its plant analog phytochromobilin (PΦB). The revealed structure of the CpcT domain of CRL is comparable to that of the CpcT lyase, despite the low sequence identity. The subsequent in vitro biochemical assays found, as shown for the CpcT lyase, that PCB/PΦB binds to the CRL dimer. However, some mutant forms of CRL, substantially compromised in their bilin-binding ability, still restore the crl-induced multiple lesions. These results suggest that although CRL retains the bilin-binding pocket, it seems not functionally associated with the crl-induced multiple lesions.

Energy transfer pathways in the CAC light-harvesting complex of Rhodomonas salina.

Photosynthetic organisms had to evolve diverse mechanisms of light-harvesting to supply photosynthetic apparatus with enough energy. Cryptophytes represent one of the groups of photosynthetic organisms combining external and internal antenna systems. They contain one type of immobile phycobiliprotein located at the lumenal side of the thylakoid membrane, together with membrane-bound chlorophyll a/c antenna (CAC). Here we employ femtosecond transient absorption spectroscopy to study energy transfer pathways in the CAC proteins of cryptophyte Rhodomonas salina. The major CAC carotenoid, alloxanthin, is a cryptophyte-specific carotenoid, and it is the only naturally-occurring carotenoid with two triple bonds in its structure. In order to explore the energy transfer pathways within the CAC complex, three excitation wavelengths (505, 590, and 640 nm) were chosen to excite pigments in the CAC antenna. The excitation of Chl c at either 590 or 640 nm proves efficient energy transfer between Chl c and Chl a. The excitation of alloxanthin at 505 nm shows an active pathway from the S2 state with efficiency around 50%, feeding both Chl a and Chl c with approximately 1:1 branching ratio, yet, the S1-route is rather inefficient. The 57 ps energy transfer time to Chl a gives ~25% efficiency of the S1 channel. The low efficiency of the S1 route renders the overall carotenoid-Chl energy transfer efficiency low, pointing to the regulatory role of alloxanthin in the CAC antenna.

Growth, physiological responses and microcystin-production/-release dynamics of Microcystis aeruginosa exposed to various luteolin doses.

By testing time-dependent IC50 of luteolin against Microcystis growth, this study revealed 6.5 mg/L as nearly IC50 value during prolonged stress until day 14, and explored chlorophyll-a (CLA) and phycobiliproteins (PBPs) contents, antioxidant responses and microcystin (MC)-production/-release dynamics at rising luteolin doses (0.5~2-fold IC50). Growth inhibition ratio (GIR) generally rose at rising luteolin dose, while at each dose GIR firstly increased and then leveled off or dropped. In early stage, CLA, allophycocyanin (APC), phycoerythrin (PE) and glutathione (GSH) contents, and superoxide dismutase (SOD) and catalase (CAT) activities, were increasingly stimulated at rising luteolin dose to enhance energy yield and antioxidant defense, but Microcystis was damaged more severely at rising dose, due to stress-repair imbalance. Such more severe damage in early stage, coupled with stronger PBPs-inhibition in mid-late stage, at rising dose could jointly account for rising GIR at rising dose. The CAT/GSH-stimulation persisting until late stage could alleviate cell damage in late stage, which explained for why GIR no longer increased in late stage at each luteolin dose. Besides, more MCs were produced and retained in cell to exert protective roles against luteolin-stress in early stage, but intracellular MCs decreased following inhibited MC-production by prolonged stress to decrease cell protectant. Extracellular MCs detection showed that less MCs amount existed in water phase than control along luteolin-stress, implying luteolin as eco-friendly algaecide with promising potential to remove MPM blooms and MC-risks. This is the first study to reveal the effect of various luteolin doses on MC-production/release and PBP-synthesis dynamics of Microcystis during prolonged stress. The findings shed novel views in anti-algal mechanisms of luteolin, and provided direct evidence for luteolin applied as safe agent to remediate Microcystis-dominant blooms.