Spectral Characteristics, In Silico Perspectives, Density Functional Theory (DFT), and Therapeutic Potential of Green-Extracted Phycocyanin from Spirulina.

Phycocyanin (PC) is a naturally occurring green pigment in Spirulina. It was extracted by ultrasonic extraction using green technology, and its structure was studied using IR- and NMR-spectroscopy. Spectral data confirmed the PC structure. This study also involves an in silico assessment of the diverse applications of green pigment PC. Utilizing QSAR, PreADME/T, SwissADME, and Pro-Tox, this study explores the safety profile, pharmacokinetics, and potential targets of PC. QSAR analysis reveals a favorable safety profile, with the parent structure and most metabolites showing no binding to DNA or proteins. PreADME/T indicates low skin permeability, excellent intestinal absorption, and medium permeability, supporting oral administration. Distribution analysis suggests moderate plasma protein binding and cautious blood-brain barrier permeability, guiding formulation strategies. Metabolism assessments highlight interactions with key cytochrome P450 enzymes, influencing drug interactions. Target prediction analysis unveils potential targets, suggesting diverse therapeutic effects, including cardiovascular benefits, anti-inflammatory activities, neuroprotection, and immune modulation. Based on the in silico analysis, PC holds promise for various applications due to its safety, bioavailability, and potential therapeutic benefits. Experimental validation is crucial to elucidate precise molecular mechanisms, ensuring safe and effective utilization in therapeutic and dietary contexts. DFT calculations, including geometry optimization, MEP analysis, HOMO-LUMO energy surface, and quantum reactivity parameters of the PC compound, were obtained using the B3LYP/6-311G(d,p) level. This integrated approach contributes to a comprehensive understanding of PC's pharmacological profile and informs future research directions.

True-solution-scale utilization of natural chlorophyll a in aqueous media through cooperative aggregation with phycocyanin.

The challenge of applying chlorophyll(Chl) in aqueous media has been a significant obstacle to the diversified development of Chl a-related industries. This study presents the first report on the true-solution-scale utilization of Chl in aqueous media through the construction of chlorophyll a-phycocyanin (Chls-PC) composite nanoparticles. This study determined the optimal conditions for Chls-PC preparation: a composite ratio of 1:25, a solvent ratio of 1:4, and a stirring time of 1 h. Fluorescence spectroscopy, transmission electron microscope, and confocal microscopy confirmed Chl a and PC aggregation. Surface hydrophobicity and contact angle measurements showed that Chls-PC water solubility was similar to PC and much higher than Chl. Infrared spectroscopy, quantum chemical calculations, X-ray photoelectron spectroscopy, and molecular dynamics simulations elucidated the water solubilization mechanism of Chls-PC both experimentally and theoretically. This research provides theoretical guidance for the development and production of water-based products using Chl as a raw material.

Probing the structural stability of R-phycocyanin under pressure.

The red macroalgae Porphyra, commonly known as Nori, is widely used as food around the world due to its high nutrient content, including the significant abundance of colored phycobiliproteins (PBPs). Among these, R-phycocyanin (R-PC) stands out for its vibrant purple color and numerous bioactive properties, making it a valuable protein for the food industry. However, R-PC's limited thermal stability necessitates alternative processing methods to preserve its color and bioactive properties. Our study aimed to investigate the in-situ stability of oligomeric R-PC under high pressure (HP) conditions (up to 4000 bar) using a combination of absorption, fluorescence, and small-angle X-ray scattering (SAXS) techniques. The unfolding of R-PC is a multiphase process. Initially, low pressure induces conformational changes in the R-PC oligomeric form (trimers). As pressure increases above 1600 bar, these trimers dissociate into monomers, and at pressures above 3000 bar, the subunits begin to unfold. When returned to atmospheric pressure, R-PC partially refolds, retaining 50% of its original color absorbance. In contrast, heat treatment causes irreversible and detrimental effects on R-PC color, highlighting the advantages of HP treatment in preserving both the color and bioactive properties of R-PC compared to heat treatment.

C-phycocyanin reinforces autophagy to block pulmonary fibrogenesis by inhibiting lncIAPF biogenesis.

Pulmonary fibrosis is a chronic and irreversible progressive lung disease caused by various factors, such as age and environmental pollution. With countries stepping into an aging society and the seriousness of environmental pollution caused by global industrialization, the incidence of pulmonary fibrosis is annually increasing. However, no effective drug is available for pulmonary fibrosis treatment. C-phycocyanin (C-PC), extracted from blue-green algae, has good water solubility and antioxidation. This study elucidated that C-PC reinforces autophagy to block pulmonary fibrogenesis by inhibiting long noncoding RNA (lncRNA) biogenesis in vivo and in vitro. Cleavage under targets and release using nuclease (CUT & RUN)-PCR, co-immunoprecipitation (Co-IP), and nuclear-cytoplasmic separation experiments clarified that C-PC blocked the nuclear translocation of activating transcription factor 3 (ATF3) to prevent the binding between ATF3 and transcription factor Smad3, thereby hindering lncIAPF transcription. Human antigen R (HuR) truncation experiment and RNA binding protein immunoprecipitation (RIP) were then performed to identify the binding domain with lncIAPF in the 244-322 aa of HuR. lncIAPF exerted its profibrogenic function through the binding protein HuR, a negative regulator of autophagy. In summary, C-PC promoted autophagy via down-regulating the lncIAPF-HuR-mediated signal pathway to alleviate pulmonary fibrosis, showing its potential as a drug for treating pulmonary fibrosis. Exploring how C-PC interacts with biological molecules will help us understand the mechanism of this drug and provide valuable target genes to design new drugs.

Green and sustainable extraction of phycocyanin from Spirulina platensis by temperature-sensitive polymer-based aqueous two-phase system and mechanism study.

In this study, a sustainable and environmentally friendly method was developed for the enrichment and purification of phycocyanin from Spirulina platensis. This was achieved by utilizing a temperature-sensitive polymer, Pluronic F68, in an aqueous two-phase solvent system. The phase behavior of the temperature-sensitive polymer-based biphasic system was evaluated. The extraction conditions were optimized by both single-factor experiments and response surface methodology. Under the optimal conditions, the upper polymer-rich phase was recycled for sustainable phycocyanin extraction, resulting in a grade of 3.23 during the third extraction cycle. Pluronic F68 could be efficiently recovered and reused during the extraction process. The interaction mechanism between Pluronic F68 and phycocyanin was systematically studied using FT-IR and fluorescence analysis. This was further complemented by static and dynamic calculation of molecular motion through molecular docking and molecular dynamics simulation, indicating that hydrophobic segment of Pluronic F68 played a key role in the binding process with phycocyanin.

Red/green cyanobacteriochromes acquire isomerization from phycocyanobilin to phycoviolobilin.

Cyanobacteriochromes (CBCRs) are unique cyanobacteria-specific photoreceptors that share a distant relation with phytochromes. Most CBCRs contain conserved cysteine residues known as canonical Cys, while some CBCRs have additional cysteine residues called second Cys within the DXCF motif, leading to their classification as DXCF CBCRs. They typically undergo a process where they incorporate phycocyanobilin (PCB) and subsequently isomerize it to phycoviolobilin (PVB). Conversely, CBCRs with conserved Trp residues and without the second Cys are called extended red/green (XRG) CBCRs. Typical XRG CBCRs bind PCB without undergoing PCB-to-PVB isomerization, displaying red/green reversible photoconversion, and there are also atypical CBCRs that exhibit diverse photoconversions. We discovered novel XRG CBCRs with Cys residue instead of the conserved Trp residue. These novel XRG CBCRs exhibited the ability to isomerize PCB to PVB, displaying green/teal reversible photoconversion. Through sequence- and structure-based comparisons coupled with mutagenesis experiments, we identified three amino acid residues, including the Cys residue, crucial for facilitating PCB-to-PVB isomerization. This research expands our understanding of the diversity of XRG CBCRs, highlighting the remarkable molecular plasticity of CBCRs.

Turning Waste into Wealth: A Potent Sono-Immune Strategy Based on Microcystis.

Currently, sonodynamic therapy (SDT) has limited therapeutic outcomes and immune responses, highlighting the urgent need for enhanced strategies that can stimulate robust and long-lasting antitumor effects. Microcystis, a notorious microalga, reveals the possibility of mediating SDT owing to the presence of gas vesicles (GVs) and phycocyanin (PC). Herein, a nontoxic strain of Microcystis elabens (labeled Me) is developed as a novel agent for SDT because it generates O2 under red light (RL) illumination, while GVs and PC act as cavitation nuclei and sonosensitizers, respectively. Moreover, algal debris is released after ultrasound (US) irradiation, which primes the Toll-like receptor pathway to initiate a cascade of immune responses. This sono-immune strategy inhibits CT26 colon tumor growth largely by promoting dendritic cell (DC) maturation and cytotoxic T-cell activation. After combination with the immune checkpoint blockade (ICB), the therapeutic outcome is further amplified, accompanied by satisfactory abscopal and immune memory effects; the similar potency is proven in the "cold" 4T1 triple-negative breast tumor. In addition, Me exhibits good biosafety without significant acute or chronic toxicity. Briefly, this study turns waste into wealth by introducing sono-immunotherapy based on Microcystis that achieved encouraging therapeutic effects on cancer, which is expected to be translated into the clinic.

Teal-light absorbing cyanobacterial phytochrome superfamily provides insights into the diverse mechanisms of spectral tuning and facilitates the engineering of photoreceptors for optogenetic tools.

Cyanobacteriochromes (CBCRs) are distinctive tetrapyrrole (bilin)-binding photoreceptors exclusively found in cyanobacteria. Unlike canonical phytochromes, CBCRs require only a GAF (cGMP-phosphodiesterase/adenylate cyclase/FhlA) domain for autolyase activity to form a bilin adduct via a Cys residue and cis-trans photoisomerization. Apart from the canonical Cys, which attaches covalently to C31 in the A-ring of the bilin, some GAF domains of CBCRs contain a second-Cys in the Asp-Xaa-Cys-Phe (DXCF) motif, responsible for isomerization of phycocyanobilin (PCB) to phycoviolobilin (PVB) and/or for the formation of a reversible 2nd thioether linkage to the C10. Unlike green/teal-absorbing GAF proteins lacking ligation activity, the second-Cys in another teal-absorbing lineage (DXCF blue/teal group) exhibits both isomerization and ligation activity due to the presence of the Tyr instead of His next to the canonical Cys. Herein, we discovered an atypical CBCR GAF protein, Tpl7205g1, belonging to the DXCF blue/teal group, but having His instead of Tyr next to the first-Cys. Consistent with its subfamily, the second-Cys of Tpl7205g1 did not form a thioether linkage at C10 of PCB, showing only isomerization activity. Instead of forming 2nd thioether linkage, this novel GAF protein exhibits a pH-dependent photocycle between protonated 15Z and deprotonated 15E. Site-directed mutagenesis to the GAF scaffolds revealed its combined characteristics, including properties of teal-DXCF CBCRs and red/green-absorbing CBCRs (XRG CBCRs), suggesting itself as the evolutionary bridge between the two CBCR groups. Our study thus sheds light on the expanded spectral tuning characteristics of teal-light absorbing CBCRs and enhances feasibility of engineering these photoreceptors.

Phycocyanin-phlorotannin complexes improve the structure and functional properties of yogurt.

Adding natural bioactive ingredients to yogurt can improve the nutritional and physiological benefits. In this study, we used ultrasonic-assisted phlorotannin from Ascophyllum nodosum (A. nodosum) modified phycocyanin (PC) to form a complex (UPP) to produce a fortified fermented yogurt. The effects of PC and UPP on the structure, stability, and function of fermented yogurt within 7 days were assessed using physicochemical properties, texture analysis, rheological testing, 16S rDNA sequencing analysis, and lipidomics analysis. Molecular docking indicated that PC might bind to phlorotannin via ARG-77, ARG-84, LEU-120, ALA-81, CYS-82, and ASP-85 sites.When the mass ratio of the complex is 1:1, the ability of UPP1:1 to remove DPPH· scavenging ability in an acid environment increased by about 50 %. UPP1:1 with more acid stability changed the microstructure of the yogurt, enhanced the stability of the yogurt, improved the antioxidant properties, and inhibited the growth of harmful bacteria within 7 days. This work encouraged the extraction and use of phlorotannin from edible brown algae and offered a straightforward method for making yogurt supplemented with PC.

Optimizing Phycocyanin Extraction from Cyanobacterial Biomass: A Comparative Study of Freeze-Thaw Cycling with Various Solvents.

Cyanobacterial phycocyanin pigment is widely utilized for its properties in various industries, including food, cosmetics, and pharmaceuticals. Despite its potential, challenges exist, such as extraction methods impacting yield, stability, and purity. This study investigates the impact of the number of freeze-thaw (FT) cycles on the extraction of phycocyanin from the wet biomass of four cyanobacteria species (Arthrospira platensis, Chlorogloeopsis fritschii, Phormidium sp., and Synechocystis sp.), along with the impact of five extraction solutions (Tris-HCl buffer, phosphate buffer, CaCl2, deionized water, and tap water) at various pH values. Synechocystis sp. exhibited the highest phycocyanin content among the studied species. For A. platensis, Tris-HCl buffer yielded maximum phycocyanin concentration from the first FT cycle, while phosphate buffer provided satisfactory results from the second cycle. Similarly, Tris-HCl buffer showed promising results for C. fritschii (68.5% of the maximum from the first cycle), with the highest concentration (~12% w/w) achieved during the seventh cycle, using phosphate buffer. Phormidium sp. yielded the maximum pigment concentration from the first cycle using tap water. Among species-specific optimal extraction solutions, Tris-HCl buffer demonstrated sufficient extraction efficacy for all species, from the first cycle. This study represents an initial step toward establishing a universal extraction method for phycocyanin from diverse cyanobacteria species.

Construction of a pectin/sodium alginate composite hydrogel delivery system for improving the bioaccessibility of phycocyanin.

In this study, different concentrations of sodium alginate were compounded with pectin and phycocyanin to co-prepare composite hydrogel spheres (HP-PC-SA 0.2 %, 0.6 %, 1.0 %, 1.4 %) to evaluate the potential of the composite hydrogel spheres for the application as phycocyanin delivery carriers. The hydrogel spheres' physicochemical properties and bioaccessibility were assessed through scanning electron microscopy, textural analysis, drug-carrying properties evaluation, and in vitro and in vivo controlled release analysis in the gastrointestinal environment. Results indicated that higher sodium alginate concentrations led to smaller pore sizes and denser networks on the surface of hydrogel spheres. The textural properties of hydrogel spheres improved, and their water-holding capacity increased from 93.01 % to 97.97 %. The HP-PC-SA (1.0 %) formulation achieved the highest encapsulation rate and drug loading capacity, at 96.87 % and 6.22 %, respectively. Within the gastrointestinal tract, the composite hydrogel's structure significantly enhanced and protected the phycocyanin's digestibility, achieving a bioaccessibility of up to 88.03 %. In conclusion, our findings offer new insights into improving functionality and the effective use of phycocyanin via pectin-based hydrogel spheres.

Chromatographic purification of C-phycocyanin from Spirulina platensis: assessing antioxidant activity and stability.

BACKGROUND: The efficient separation and purification of proteins like C-phycocyanin (C-PC) from Spirulina platensis are essential for their commercialization, yet they remain challenging. This study investigated three chromatographic methods for C-PC purification: weak anion exchange chromatography (DEAE), strong anion exchange chromatography (Q Sepharose), and hydrophobic interaction chromatography (HIC). RESULTS: Weak anion exchange chromatography achieved a recovery of 36.80 mg unit (57.08%) with a purity of 3.23, outperforming Q Sepharose (yield: 23.21 mg unit means that 46.33%, purity: 2.76) and HIC (yield: 22.95 mg unit means that 17.57%, purity: 3.02). The purified C-PC consisted of α and ß subunits with molecular masses of 16 kDa and 17 kDa, respectively. Further assessment revealed its antioxidant capacity through a 2,2'-Azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) assay. The stability of C-phycocyanin was tested at different pH levels and temperatures. Maximum stability was observed at pH 7, and pH 4 showed the lowest stability. Glutaraldehyde-treated C-PC (GC-PC) demonstrated gradual degradation up to 50 °C, retaining 73.25% after 30 min. Notably, GC-PC exhibited stability even at higher temperatures, with degradation rates of 57.32% at 70 °C and 50.96% at 80 °C. CONCLUSION: Weak anion exchange chromatography proved superior for C-PC purification, offering higher yields and purity than Q Sepharose and HIC. The purified C-PC showed promising antioxidant capacity and stability, particularly GC-PC, which exhibited resistance to degradation, even at elevated temperatures. These findings underscore the potential of C-PC as a valuable compound for various applications, with DEAE chromatography being an efficient method for its production and commercialization. © 2024 Society of Chemical Industry.

Characterization of emulgels formulated with phycocyanin and diutan gum as a novel approach for biocompatible delivery systems.

Phycocyanin (PC), a protein derived from algae, is non-toxic and biocompatible. Due to its environmental and sustainable properties, it has been studied as an alternative stabilizer for food emulsions. In this sense, the main objective of this work is to evaluate the effectiveness of PC and its use in combination with diutan gum (DG), a biological macromolecule, to prepare emulgels formulated with avocado oil. Z-potential measurements show that the optimum pH for working with PC is 2.5. Furthermore, the system exhibited a structured interface at this pH. The surface tension did not decrease further above 1.5 wt% PC. Interestingly, emulsions formulated with >1.5 wt% PC showed recoalescence immediately after preparation. Although 1.5 wt% had the smallest droplet size, this emulsion underwent creaming due to the low viscosity of the system. DG was used in combination with PC to increase viscosity and reduce creaming. As little as 0.1 wt% DG was sufficient to form an emulgel when incorporated into the previous emulsion, which exhibited pseudoplastic behaviour and viscoelastic properties with very low creaming rates. However, the use of PC in combination with DG resulted in a non-aggregated and stable emulgel with 1.5 wt% PC and 0.1 wt% DG.

Making a living off the rainbow's edge: How phycobilisomes adapt structurally to absorb far-red light.

Cyanobacteria harvest light by using architecturally complex, soluble, light-harvesting complexes known as phycobilisomes (PBSs). PBS diversity includes specialized subunit paralogs that are tuned to specific regions of the light spectrum; some cyanobacterial lineages can even absorb far-red light. In a recent issue of the Journal of Biological Chemistry, Gisriel et al. reported the cryo-electron microscopic structure of a far-red PBS core, showing how bilin binding in the α-subunits of allophycocyanin paralogs can modify the bilin-binding site to red shift the absorbance spectrum. This work helps explain how cyanobacteria can grow in environments where most of the visible light has been filtered out.

Collagen/chitosan/genipin hydrogel loaded with phycocyanin nanoparticles and ND-336 for diabetic wound healing.

Diabetic wound healing remains a healthcare challenge due to the overexpression of matrix metalloproteinase-9 (MMP-9) and the imbalance between angiogenic factors and vascular inhibitory factors. In this study, we developed a nanocomposite injectable collagen/chitosan hydrogel for the treatment of delayed diabetic wound healing, which can promote cell migration to the wound site (through the addition of phycocyanin) and reduce the expression of MMP-9 (through the use of ND-336) to improve the therapeutic effect of diabetic wound healing. Furthermore, different weight ratios of collagen and chitosan hydrogels were prepared to select the hydrogel with proper mechanical properties. In vitro experiments confirmed that all hydrogels have favorable biocompatibility and hemocompatibility. Notably, Gel 2, with a weight ratio of collagen and chitosan at 25:75, was found to have an excellent capability to facilitate cell migration and in vivo studies further proved that Gel 2 nanocomposite hydrogel had the best ability to improve diabetic wound healing by promoting cell migration and decreasing MMP-9 expression. The collagen/chitosan/genipin hydrogel loaded phycocyanin and ND-336 can be harnessed for non-toxic and efficient treatment of wound healing management of diabetes.

A rapid one-step affinity purification of C-phycocyanin from Spirulina platensis.

The high-purity phycocyanin has a high commercial value. Most current purification methods of C-phycocyanin involve multiple steps, which are complicated and time-consuming. To solve the problem, this research was studied, and an efficient affinity chromatography purification for C-phycocyanin from Spirulina platensis was developed. Through molecular docking simulation, virtual screening of ligands was performed, and ursolic acid was identified as the specific affinity ligand, which coupled to Affi-Gel 102 gel via 1-ethyl (3-dimethylaminopropyl)-3-carbodiimide, hydrochloride as coupling agent. With this customized and synthesized resin, a high-efficiency one-step purification procedure for C-phycocyanin was developed and optimized, the purity was determined to be 4.53, and the yield was 69 %. This one-step purification protocol provides a new approach for purifying other phycobilin proteins.

Assembly of CpcL-phycobilisomes.

CpcL-phycobilisomes (CpcL-PBSs) are a reduced type of phycobilisome (PBS) found in several cyanobacteria. They lack the traditional PBS terminal energy emitters, but still show the characteristic red-shifted fluorescence at ~670 nm. We established a method of assembling in vitro a rod-membrane linker protein, CpcL, with phycocyanin, generating complexes with the red-shifted spectral features of CpcL-PBSs. The red-shift arises from the interaction of a conserved key glutamine, Q57 of CpcL in Synechocystis sp. PCC 6803, with a single phycocyanobilin chromophore of trimeric phycocyanin at one of the three ß82-sites. This chromophore is the terminal energy acceptor of CpcL-PBSs and donor to the photosystem(s). This mechanism also operates in PBSs from Acaryochloris marina MBIC11017. We then generated multichromic complexes harvesting light over nearly the complete visible range via the replacement of phycocyanobilin chromophores at sites α84 and ß153 of phycocyanins by phycoerythrobilin and/or phycourobilin. The results demonstrate the rational design of biliprotein-based light-harvesting elements by engineering CpcL and phycocyanins, which broadens the light-harvesting range and accordingly improves the light-harvesting capacity and may be potentially applied in solar energy harvesting.

Spectral and conformational characteristics of phycocyanin associated with changes of medium pH.

C-phycocyanin (C-PC) is the main component of water-soluble light-harvesting complexes (phycobilisomes, PBS) of cyanobacteria. PBS are involved in the absorption of quantum energy and the transfer of electronic excitation energy to the photosystems. A specific environment of C-PC chromophoric groups is provided by the protein matrix structure including protein-protein contacts between different subunits. Registration of C-PC spectral characteristics and the fluorescence anisotropy decay have revealed a significant pH influence on the chromophore microenvironment: at pH 5.0, a chromophore is more significantly interacts with the solvent, whereas at pH 9.0 the chromophore microenvironment becomes more viscous. Conformations of chromophores and the C-PC protein matrix have been studied by Raman and infrared spectroscopy. A decrease in the medium pH results in changes in the secondary structure either the C-PC apoproteins and chromophores, the last one adopts a more folded conformation.

Hepatoprotective Effect Assessment of C-Phycocyanin on Hepatocellular Carcinoma Rat Model by Using Photoacoustic Spectroscopy.

Hepatocellular carcinoma (HCC) is the most common primary neoplasia of the liver with elevated mortality. Experimental treatment with antioxidants has a beneficial effect on the experimental models of HCC. Arthrospira maxima (spirulina) and its phycocyanin have antitumoral action on different tumoral cells. However, it is unknown whether phycocyanin is the responsible molecule for the antitumoral effect on HCC. Photoacoustic spectroscopy (PAS) stands out among other spectroscopy techniques for its versatility of samples analyzed. This technique makes it possible to obtain the optical absorption spectrum of solid or liquid, dark or transparent samples. Previous studies report that assessing liver damage in rats produced by the modified resistant hepatocyte model (MRHM) is possible by analyzing their blood optical absorption spectrum. This study aimed to investigate, by PAS, the effect of phycocyanin obtained from spirulina on hepatic dysfunction. The optical absorption spectra analysis of the rat blood indicates the damage level induced by the MRHM group, being in concordance with the carried out biological conventional studies results, indicating an increase in the activity of hepatic enzymes, oxidative stress, Bax/Bcl2 ratio, cdk2, and AKT2 expression results, with a reduction in p53 expression. Also, PAS results suggest that phycocyanin decreases induced damage, due to the prevention of the Bax, AKT2, and p53 altered expression and the tumor progression in a HCC rat model.

Cooperative antioxidative defense of the blue-green alga Arthrospira (Spirulina) platensis under oxidative stress imposed by exogenous application of hydrogen peroxide.

Hydrogen peroxide (H2O2) is an environmentally-safe algaecide used to control harmful algal blooms and as a disinfectant in various domestic and industrial applications. It is produced naturally in sunny-water or as a by-product during growth, and metabolism of photosynthetic organisms. To assess the impact of H2O2 on Arthrospira platensis, several biochemical components, and antioxidant enzymes were analysed. The growth and biomass of A. platensis were decreased under the effect of H2O2. Whereas, the concentration up to 40 µM H2O2 non-significantly induced (at P < 0.05) the Chl a, C-phycocyanin (C-PC), total phycobiliprotein (PBP), and the radical scavenging activity of A. platensis. The half-maximal effective concentrations (EC50) for H2O2 were 57, 65, and 74 µM H2O2 with regards to the biomass yield, Chl a, and C-PC content, respectively. While, the total soluble protein, and soluble carbohydrates contents were significantly induced. However, the higher concentrations (60 and 80 µM) were lethal to these components, in parallel to the initiation of the lipid peroxidation process. Surprisingly, the carotenoids content was non-significantly increased by H2O2. Despite the relative consistency of catalase (CAT), the activities of superoxide dismutase (SOD) and peroxidase (POD) enzymes were boosted by all of the tested concentrations of H2O2. The relative transcript abundance of selected regulatory genes was also investigated. Except for the highest dose (80 µM), the tested concentrations had almost inhibitory effect on the relative transcripts of heat shock protein (HSP90), glutamate synthase (GOGAT), delta-9 desaturase (desC), iron-superoxide dismutase (FeSOD) and the Rubisco (the large subunit, rbcL) genes. The results demonstrated the importance of the non-enzymatic and enzymatic antioxidants for the cumulative tolerance of A. platensis.