Chlorella vulgaris, a representative edible algae as integrative and alternative medicine

Abstract

Algae exhibit diverse biological activities, and they are also natural resources with the potential for large-scale production. As a result, research on algae has progressed, and the roles of algae have been revealed. Chlorella vulgaris, one of the species most widely found in East Asian countries such as China, Korea, and Japan, has confirmed diverse functional and pharmacological attributes, leading to extensive consumption of Chlorella vulgaris as a supplement. Seven active compounds extracted from Chlorella vulgaris such as dodecanoic acid and loliolide have been reported to collectively contribute to its overall bioactive and medicinal activities.

1. Introduction

Algae is a generic term that describes photosynthetic organisms. Specifically, algae comprise members from four biological kingdoms: Bacteria, Plantae, Chromista, and Protozoa.¹ Algae can be found anywhere on Earth where photosynthesis is possible, from seas, rivers, lakes, and soil to even walls, animals, and plants.² They can range from unicellular organisms measuring 3–10 μm to massive kelp that can grow to 70 m.³ Therefore, they are roughly classified by size into macroscopic algae, which humans can see with bare eyes, and microscopic algae (phytoplankton), which humans can observe only under a microscope.⁴ Macroscopic algae include red algae, green algae, and brown algae, which are distinguished by the colors of their chloroplast pigments. The microalgae comprise diverse groups: Chlorophyta (green algae), Bacillariophyta (diatoms), Cyanophyta (blue-green algae or cyanobacteria), Rhodophyta (red algae), and Heterokontophyta (brown algae and some golden algae).⁵

Chlorella vulgaris is a unicellular green eukaryotic microalga of the Chlorella genus that has existed since the Precambrian era.⁶ Though it is globally distributed in freshwater, marine water and soil, it can be found easily in the East Asian countries of China, Japan, and Korea.^7,8 It is widely consumed in these countries as a functional supplement with numerous health-promoting properties. Furthermore, it has garnered interest in potential applications in cosmetics and bioplastics to take advantage of its natural origins.

Once, people tended to focus on the negative aspects of algae, such as algal blooms. However, the versatility of algae has renewed interest in researching it. As a result, it has been demonstrated that algae exhibit considerable diversity based on their species, with each one possessing distinct biological activities. This review describes how the bioactivities of an alga’s chemical components affect its biological functions, with a specific emphasis on C. vulgaris—a representative microalga extensively studied in East Asia—to understand how chemical composition confers biological activity.

2. Chemical components of algae

2.1. Proteins

Proteins are some of the most biologically and chemically important molecules in all living organisms, serving as structural supports, biochemical catalysts such as hormones and enzymes, and cellular components. Amino acids are the basic units of all proteins, with 50 to 2000 of them linked to a chain and interacting with one another to construct a structure.⁹ There are 20 amino acids in total, and they are roughly classified as essential and nonessential. The nine essential amino acids are those that cannot be synthesized by animal cells and must therefore be obtained through diet: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. The 11 nonessential amino acids, on the other hand, can be synthesized de novo in animal cells in the amounts required for growth, generation, etc. They include alanine, glycine, and cysteine.

2.1.1. Immune system support

Because amino acids are the basic units of proteins, which are the building blocks of most biological components, amino acids from microalgae support the immune system by providing the materials needed to produce immune-related proteins such as antibodies, cytokines, and complements, which serve as key factors in immunity. In particular, arginine, which is abundant in algae, plays an important role in immune function. Nitric oxide, which is synthesized from arginine, directly removes pathogens as a cytotoxic agent and promotes or inhibits the apoptosis or necrosis of immune cells, depending on the situation.10, 11, 12 It also affects immune cell function by enhancing T cell maturation and increasing the proliferation of CD8 T cells.¹³ In addition, it lowers the levels of pro-inflammatory cytokines (interleukin-(IL)-2, IL-6, and interferon gamma (IFN-γ)) and increases the levels of an anti-inflammatory cytokine (IL-10) to reduce inflammatory responses.¹⁴

2.1.2. Anti-inflammatory and antioxidant properties

Algae contain bioactive compounds that can work as anti-inflammatory and antioxidant agents. These characteristics could be useful for treating chronic diseases such as cardiovascular diseases and diabetes.¹⁵ For example, a Porphyra yezoensis glycoprotein showed anti-inflammatory responses in lipopolysaccharide (LPS)-induced macrophages.¹⁶ Lectin isolated from Pterocladiella capillacea significantly inhibited neutrophil migration by suppressing inflammatory mediators.¹⁷ C-phycocyanin, a biliprotein from Spirulina platensis, repressed inflammation by blocking the generation of pro-inflammatory cytokines and the expression of inducible nitric oxide synthase and cyclooxygenase-2.¹⁸

In addition to its anti-inflammatory traits, C-phycocyanin also displayed antioxidant properties for scavenging radical oxidative species such as alkoxy radicals and hydroxyl radicals.¹⁹ Schizochytrium limacinum protein hydrolysate has 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical and 2,2′-azino-bis-(3-ethylbenzothiazoline-6-sulfonic) acid (ABTS) radical scavenging abilities. It also showed low-density lipoprotein (LDL) oxidization inhibition activity, which means that it could prevent cardiovascular diseases, especially atherosclerotic diseases.^20,21 Protein and hydrolytic extracts from Nitzschia laevis exhibited strong in vitro antioxidant activity, as demonstrated by assays measuring the oxygen radical absorbance capacity and ability to scavenge ABTS radicals, DPPH radicals, and superoxide anion radicals.²²

2.1.3. Pharmaceutical applications

The health-promoting properties are also encouraging studies of algal proteins and amino acids for pharmaceutical uses. For example, phycocyanin, a phycobilin protein sourced from Arthronema africanum, Spirulina platensis, and Porphyra haitanensis, is reported to inhibit the proliferation of human hepatocellular carcinoma, lung/colon cancer, and leukemia cells.²³ Furthermore, microalgae-based protein from 7 microalgal strains showed good reactivity to target proteins. Moreover, the extracellular vesicles in normal and cancer mammalian cell lines in which those proteins were found exhibited no toxicity, highlighting their potential as a promising platform for drug delivery.²⁴

2.2. Carbohydrates

The foremost role of polysaccharides is to supply and store energy in the body. Polysaccharides are hydrolyzed into monosaccharides and oxidized to provide energy for the body, and monosaccharides can be synthesized into polysaccharides to store energy for future use. Polysaccharides are classified as homopolysaccharides and heteropolysaccharides. Homopolysaccharides are composed of single saccharides, and heteropolysaccharides are composed of two or more different saccharides. Most algal polysaccharides are heteropolysaccharides linking galactose, xylose, and glucose in different ratios by glycosidic bonds.²⁵ Other simple sugars, such as arabinose, mannose, fucose, rhamnose, and 2-O-methyl sugars, and acidic components such as glucuronic acid and galacturonic acid also constitute microalgal polysaccharides.²⁶ Microalgal polysaccharides can be categorized instead by their location in the cell: intracellular and structural polysaccharides. Intracellular polysaccharides act energy reservoirs, whereas structural polysaccharides serve as exopolysaccharides secreted into the medium, cell-bound polysaccharides, and cell wall polysaccharides.^26,27 Each microalga has distinctive polysaccharides, and most intracellular polysaccharides accumulate in plastids in the form of starch.²⁸ Meanwhile, the structural polysaccharides, specifically the algal cell wall, are primarily composed of cellulose and hemicellulose.²⁸ Algae thus contain large quantities of carbohydrates that can be transformed into high-value functional foods, animal feed, pharmaceuticals, and health supplements.

2.2.1. Antiviral properties

Among the various types of algal polysaccharides, sulfated polysaccharides are particularly vital to the biological activity of algae. Sulfated polysaccharides are predominantly present in marine algae, rather than terrestrial algae.²⁹ The sulfate group ion cross-links with the backbone of the polysaccharide sugar structure (i.e., the cell wall), imparting a negative charge.³⁰ This characteristic allows the algae to maintain a stable structure under high salinity conditions through mechanical, ionic, and osmotic regulation, supporting their persistence in marine environments.^29,30

As the sulfate content in polysaccharides increases, their antiviral activity also increases, demonstrating that sulfate is an important factor in polysaccharide activity against viruses.³¹ Different types of polysaccharides can block different stages of viral infection. Generally, an early step of the viral replication cycle, especially viral particle adsorption to the host cell surface, is impeded.³² This is the most effective way to interrupt viral invasion.³³ To illustrate, the V3 loop of the viral envelope glycoprotein binds to heparan sulfate, a sulfated polysaccharide on the surface of host cells.³⁴ This link is established through an ionic interaction between anionic (mainly sulfate) groups of polysaccharides and the basic amino acids in the viral envelope glycoprotein.³⁵ Consequently, algae can hinder viral penetration by masking the positively charged V3 loop of the virus with its negatively charged sulfated polysaccharides, thereby thwarting host cell attachment.³⁴ A polysaccharide isolated from Porphyridium sp. effectively prevented herpes simplex virus infection in Vero cells both in vitro and in vivo in newborn rats and rabbits.³⁶ Additionally, all extracellular polymeric substance extracts exhibited potent activity against the V. stomatitis virus, surpassing the effectiveness of all tested chemical compounds.³⁷ It was also found that an extracellular polysaccharide extracted and isolated from Porphyridium cruentum inhibited hepatitis B virus in vitro by reducing HBeAg excretion while showing low toxicity to cells.³⁸ Calcium spirulan (Ca-Sp), a sulfated polysaccharide obtained from Spirulina platensis, prevents the replication of various viruses and demonstrates broad-spectrum antiviral potential against influenza virus, human cytomegalovirus, mumps virus, measles virus, and human immunodeficiency virus type 1 (HIV-1).^39,40 Ca-SP holds significant promise as a potential anti-HIV agent because it does not enhance virus-induced syncytium formation, even at low concentrations, unlike the effects seen in dextran sulfate–treated cultures.⁴⁰

2.2.2. Antioxidant activity

Polysaccharides with high sulfate content scavenge reactive oxygen species more effectively than those with low sulfate content, indicating greater antioxidant activity.^41,42 Sulfated polysaccharides are superior to de-sulfated polysaccharides in radical scavenging and electron-donating. One mechanism underlying their antioxidant activity is the inhibition of free radical generation through the chelation of ions such as iron and copper. These transition metal ions can catalyze the formation of highly reactive hydroxyl radicals from superoxide and hydrogen peroxide via the Fenton reaction.⁴³ The sulfate group enhances the ability of polysaccharides to form stable chelates with metal ions, thereby elevating the antioxidant effects.⁴³ For example, sulfated polysaccharides from Picochlorum sp. demonstrated exceptionally high antioxidant activity, with 85 % DPPH radical scavenging activity at a concentration of 2 mg/ml. Sulfated polysaccharides from three different strains of Chlorella sp. (Chlorella sorokiniana, Chlorella sp. (L2), and Chlorella sp. (D1)), excluding Chlorella sp. (N4), showed >50 % ABTS radical scavenging activity at 1 mg/ml and about 90 % activity at 2 mg/ml.⁴⁴ Moreover, aqueous extracellular polysaccharides derived from the Tunisian thermophilic microalga Graesiella sp. acted as powerful reducing agents and ferrous chelating agents, significantly reducing hydroxyl radicals in a concentration-dependent fashion.⁴⁵

2.2.3. Anticancer activity

Polysaccharides drive the anticancer properties of algae. Ca-Sp, a sulfated polysaccharide isolated from Spirulina platensis, has been found to inhibit pulmonary metastasis and suppress the attachment and growth of tumor cells.⁴⁶ A cell-wall sulfated polysaccharide (PcrPSH) from Porphyridium cruentum has demonstrated strong anti-tumor effects against Graffi myeloid tumors in hamsters. In vivo, PcrPSH reduced transplant-ability, mortality rates, and tumor growth. In vitro, both in the control group and the group of hamsters transplanted with Graffi myeloid tumors, PcrPSH enhanced the expansion and phagocytic ability of peritoneal macrophages. Additionally, in a primary Graffi tumor cell culture, PcrPSH significantly reduced cell viability, suggesting PcrPSH as a potential new antitumor agent.⁴⁷

2.3. Lipids

Lipids are divided into two categories: polar and neutral. Lipids are insoluble to water, but they dissolve in most organic solvents. Polar lipids have phospholipids and glycolipids as components, whereas neutral lipids comprise acylglycerides (including tri-, di-, and monoglycerides) and fatty acids.⁴⁸ Among the diverse bioactive lipids, polyunsaturated fatty acids (PUFAs) are known for their health benefits.^49,50 In fact, microalgae are natural resources abundant in PUFAs.⁵¹ Due to their bountiful enzymes for very long chain PUFA (VLC-PUFA) synthesis, unlike mammals and seed plants, they can be considered a sustainable resource for VLC-PUFA generation.⁵² Two principal types of PUFAs are omega-6 and omega-3 fatty acids. Both of them are crucial for human health, and they are synthesized from linoleic acid and linolenic acid, respectively. Alpha-linolenic acid, eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA) are the main omega-3 fatty acids, and gamma-linolenic acid and arachidonic acid are notable omega-6 fatty acids.⁵³ Among microalgae-produced PUFAs, DHA and EPA are especially significant due to their high bioactivity and nutritional importance. These PUFAs are synthesized in large quantities due to the demand for their health benefits.⁵⁴

2.3.1. Anti-inflammatory activity

It has been confirmed that the glycoglycerolipids monogalactosyldiacylglycerol, digalactosyldiacylglycerol, and sulphoquinovosyldiacylglycerol, which were isolated and purified from the blue-green alga ETS-05, exhibit in vivo anti-inflammatory activity. All three compounds dose-dependently inhibited croton oil–induced ear edema in mice. Similarly, in a separate carrageenan-induced paw edema model, these glycoglycerolipids demonstrated a concentration-dependent suppression of inflammation, confirming their anti-inflammatory effects.⁵⁵

Additionally, lipid extracts from the blue-green algae Nostoc commune var. sphaeroides Kützing and Spirulina platensis (at a concentration of 100 μg/ml) significantly reduced the expression and secretion of pro-inflammatory cytokines such as tumor necrosis factor-alpha (TNF-α), IL-1β, and IL-6 when bone marrow-derived macrophages and splenocytes were pretreated with the extracts and then stimulated with LPS. In RAW264.7 macrophages, lipid extracts from these two algae were found to reduce the nuclear translocation of NF-κB, indicating that their anti-inflammatory activity was mediated by inhibition of the NF-κB pathway.⁵⁶

In clinical studies, EPA and DHA, alongside antirheumatic drugs, alleviated joint pain in rheumatoid arthritis patients, provided beneficial effects for those with ulcerative colitis, and, when used with medications, reduced the toxicity of cyclosporin in patients suffering from psoriasis.⁵⁷ Diets enriched with EPA and DHA could produce anti-inflammatory effects by inhibiting the 5-lipoxygenase pathway in neutrophils and monocytes, as well as by preventing leukotriene B4–mediated neutrophil recruitment into the airways.⁵⁸

2.3.2. Cardiovascular health management

Among the omega-3 fatty acids, EPA and DHA particularly mitigate cardiovascular diseases such as arrhythmia, stroke, and hypertension.⁵⁹ Omega-3 supplementation has been demonstrated to prevent heart rate variability decline, which is an important index of cardiac autonomic regulation.⁶⁰ Research reported that administering PUFAs to patients going through coronary artery bypass graft surgery lowered the occurrence of postoperative atrial fibrillation (54.4 %) and hastened discharge from the hospital.⁶¹ Moreover, DHA supplements improved the high-density lipoprotein (HDL)/LDL ratio and lowered the total cholesterol/HDL ratio at the same time, indicating diminished risks of coronary artery disease. Those findings elucidated some of the mechanisms by which omega-3 PUFAs contribute to cardiovascular disease reduction.⁶²

2.4. Secondary metabolites

2.4.1. Volatile compounds

Volatile organic compounds (VOCs) easily vaporize under normal conditions.⁶³ VOCs include a wide range of compounds such as terpenoids, sulfur compounds, aromatic compounds, amines, alkanes, alkenes, carbonyls, alcohols, esters, ethers, acids, thiols, ketones, and furans.64, 65, 66, 67 These diverse VOCs are emitted by various organisms in both terrestrial and aquatic ecosystems, including algae.⁶⁸ VOCs are involved in a range of processes, enhancing resistance to abiotic stress, transmitting information, contributing to allelopathic interactions, and providing protection against predation.⁶⁹ Nonetheless, VOCs can have both direct and indirect effects on humans, often resulting in harmful impacts ranging from mild irritation to severe health issues such as cancer.⁷⁰ Most VOC emissions stem from the massive emissions of anthropogenic processes; the small quantities produced by natural sources such as algae are not enough to cause adverse effects in humans.⁶³ In a study that exposed 62 subjects to an indoor air pollutant mixture of 22 VOCs at a maximum concentration of 25 mg/m³, everyone remained healthy, and except for a few who experienced mild symptoms of dry mucous membranes in the eyes, nose, or upper airways, no one exhibited symptoms of asthma, allergy, or chronic bronchitis.⁷¹ Therefore, consuming moderate amounts of algae poses no threat of harm. Although research on the role of VOCs in algae is still in the early stages, this review describes some potential uses for algal VOCs.⁷²

2.4.1.1. Antioxidant activity

Zuo Z et al. revealed that under acetic acid stress, Chlamydomonas cells secrete biogenic VOCs that upregulate the activity of the antioxidant enzymes superoxide dismutase, peroxidase, and catalase.⁷² In another instance, limonene, a type of terpene more precisely classified as a monocyclic monoterpene hydrocarbon, was quantified in six algal species: Desertifilum spp., Spirulina sp., Scenedesmus obliquus, C. vulgaris, C. sorokiniana, and Scenedesmus bijuga.^73,74 Limonene not only lowered the H₂O₂ levels in lymphocytes by elevating the activity of catalase and peroxidase, but also protected the cells from oxidative stress caused by the external addition of H₂O₂.⁷⁵ This suggests that limonene has the potential to protect lymphocytes from oxidative stress–related diseases, including cancer.⁷⁵

2.4.1.2. Antimicrobial activity

Terpenes are known to display antimicrobial properties against both antibiotic-susceptible and antibiotic-resistant bacteria. Their effectiveness primarily arises from their ability to induce cell rupture, bacterial homeostasis alteration, and the inhibition of protein and DNA synthesis.⁷⁶ Limonene, a type of terpene, demonstrates synergistic effects when combined with gentamicin antibiotics, improving the inhibition of Gram-positive Staphylococcus aureus, Gram-negative Escherichia coli, and several antibiotic-resistant bacteria.⁷⁷ Moreover, the pairing of limonene and ε-polylysine produced an additive and beneficial synergistic effect against E. coli, S. aureus, Bacillus subtilis, and Saccharomyces cerevisiae.⁷⁸ 1-Octen-3-ol is a volatile alcohol found in Spirulina platensis (blue-green alga), Pyropia haitanensis (marine red alga), Capsosiphon fulvescens (green alga), and C. vulgaris (green alga).79, 80, 81, 82 It exhibits strong antibacterial activity against S. aureus, B. subtilis, S. epidermidis, E. coli, and Pseudomonas aeruginosa, and it inhibits fungal growth and spore germination in Fusarium tricinctum and Fusarium oxysporum.⁸³ Similarly, hexanal, another VOC found in various algae, demonstrates antimicrobial effects. It is a volatile aldehyde found in C. vulgaris (green alga), Isochrysis galbana (Haptophyceae), and Scenedesmus obliquus (green alga).^82,84,85 Hexanal disrupts cell membrane synthesis and mitochondrial function, inducing apoptosis in Aspergillus flavus.⁸⁶ Furthermore, when A. flavus was exposed to hexanal, it displayed abnormal spore morphology and early spore apoptosis, along with increased production of reactive oxygen species, decreased mitochondrial membrane potential, and DNA fragmentation.⁸⁶

2.4.1.3. Flavor and aroma

Aroma is a complex blend of numerous volatile compounds, and its composition is specific to each species and often varies by variety.⁸⁷ Flavor, which is determined by the interplay of taste and odor, can also differ depending on volatile compounds.⁸⁸ Aroma-active volatiles are significant secondary metabolites, commonly used for their flavor and fragrance in the food, pharmaceutical, and perfume industries to enhance product appeal.⁸⁹ Because limonene is a principal component of the oil extracted from many citrus plants, it provides their citrus scent and serves as a flavoring agent in foods, beverages, liqueurs, and confections, as well as in aromatic products such as soap, perfumes, and household items.^90,91 Hexanal has a fresh, grassy odor and can function as a natural additive that imparts a "fresh green" aroma to plant-based foods, vegetables, and fruits.⁹² 1-Octen-3-ol, also known as mushroom alcohol, is a common aroma component found in most mushrooms that contributes to their flavor and scent.⁹³ In the same way, it is used to enhance the taste of plant-based foods and soups.

2.4.2. Pigments

A pigment is a compound that not only plays an important role in the photosynthetic metabolism and pigmentation of algae but also participates in a number of biological activities. Many microalgae commonly contain pigments such as chlorophyll, carotenoids, and phycobilins.⁴⁸ First, chlorophyll, a green pigment found in oxygenic photosynthetic organisms such as plants and algae, enables photosynthesis by absorbing and transferring light energy and then converting it into chemical energy. To date, five types of chlorophyll have been identified—chlorophyll a, b, c, d, and f—and all of them share a similar five-membered ring structure; they are classified by their different side chains, which create variations in their light absorption wavelengths.⁹⁴ Second, carotenoids are fat-soluble compounds that typically appear yellow, orange, or red and are among the most varied and widespread pigments in nature.⁹⁵ In algae, they serve several crucial functions in photosynthesis. They act as “secondary light-harvesting pigments” that assist the primary pigment, chlorophyll, in capturing light.⁹⁶ They absorb electromagnetic energy that chlorophyll cannot absorb in the 400 to 500 nm wavelength range and transfer it to chlorophyll.⁹⁶ Additionally, carotenoids maintain the structure and function of photosynthetic complexes, quench chlorophyll triplet states, neutralize reactive oxygen species, and dissipate excess energy.⁹⁷ Compared with other pigments, carotenoids have a variety of physiological effects that are particularly advantageous to human health. These effects are generally linked to their antioxidant activity, which prevents the accumulation of free radicals and reactive oxidative species.⁹⁸ Major carotenoids such as astaxanthin, β-carotene, lutein, lycopene, and canthaxanthin have significant health benefits.⁹⁹ Third, phycobilins are primarily found in the chloroplast stroma of red algae.⁹⁶ Phycobilins have the unique characteristic of binding to water-soluble proteins to form phycobiliproteins.⁹⁶ Like carotenoids, they act as “secondary light-harvesting pigments” that collect light and transfer the energy to chlorophyll.⁹⁶

2.4.2.1. Antioxidant activities

Carotenoids serve as antioxidants by deactivating electronically activated species, such as singlet oxygen, and reactive chemical species that can cause harmful oxidative reactions, such as peroxyl or alkoxyl radicals.¹⁰⁰ Although β-carotene was once regarded as the most notable antioxidant among carotenoids, astaxanthin has recently been revealed to be a more powerful and effective antioxidant agent.¹⁰¹ Naguib, Y.M. used a newly developed fluorometric assay and demonstrated that astaxanthin exhibits the highest antioxidant activity against peroxyl radicals among carotenoids, including lycopene, α-carotene, β-carotene, and lutein.¹⁰¹ The remarkable efficacy of astaxanthin as an antioxidant against lipid peroxidation can be attributed to its unique chemical structure. Carotenoids have a series of conjugated double bonds that allow electrons to delocalize, transfer to radicals, and form resonance structures that stabilize the radicals, preventing oxidative damage.¹⁰² Among carotenoids, astaxanthin stands out as the most potent antioxidant due to its 13 double bonds.¹⁰³ Furthermore, astaxanthin's ability to trap free radicals both within and on the surface of membranes promotes its potency as an antioxidant.¹⁰⁴

2.4.2.2. Anti-obesity

Obesity, a chronic condition characterized by excessive body fat, is a growing global concern. It significantly increases the risk of coronary heart disease and its associated risk factors such as hypertension, type 2 diabetes, dyslipidemia, and metabolic syndrome.¹⁰⁵ However, obesity is preventable, and pigments from algae offer a promising solution. The green alga Caulerpa racemosa contains seven carotenoids, with β-carotene and canthaxanthin being the most prevalent.¹⁰⁶ The carotenoid extract from C. racemosa has demonstrated potent biological activity, inhibiting enzymes such as α-glucosidase, α-amylase, and lipase and upregulating AMPK, which makes it a potential candidate for anti-obesity drugs.¹⁰⁶ Additionally, the extract exhibits low cytotoxicity in normal cells, enhancing its potential for development into nutraceuticals and pharmaceuticals.¹⁰⁶

In Spirulina platensis, a blue-green alga, both chlorophyll a and a type of phycobilin, C-phycocyanin, have been found to reduce lipid droplet accumulation.^107,108 In 3T3-L1 and C3H10T1/2 cells, they decreased the expression of adipogenic (C/EBPα, PPARγ, and aP2) and lipogenic proteins (LPAATβ, lipin1, and DGAT1).¹⁰⁷

Fucoxanthin, a major carotenoid found in edible seaweeds such as Undaria pinnatifida and Hizikia fusiformis, along with its metabolite fucoxanthinol, inhibits the differentiation of 3T3-L1 preadipocytes into adipocytes by downregulating PPARγ.109, 110, 111 In rats and mice, being fed lipids derived from U. pinnatifida led to a reduction in the weight of abdominal white adipose tissue (WAT).¹¹² These lipids, called Undaria lipids, were primarily composed of glycolipids and the carotenoid fucoxanthin.¹¹² Notably, in mice that were fed fucoxanthin, WAT weight decreased significantly, and that was accompanied by a marked increase in the expression of UCP1 in the WAT.¹¹² In contrast, mice fed only glycolipids showed no change in WAT weight and minimal UCP1 expression.¹¹² This result suggests that fucoxanthin promotes UCP1 expression in WAT, contributing to fat reduction.¹¹² UCP1, which plays a key role in energy expenditure in brown adipose tissue, is essential for maintaining whole-body energy balance, at least in small rodents, and dysfunction in UCP1 expression is linked to obesity.¹¹³ These findings highlight the potential of algal pigments to address obesity through multiple biological pathways.

2.4.3. Phenolic compounds

Phenolic compounds are characterized by the presence of one or more hydroxyl groups attached to an aromatic ring.¹¹⁴ These compounds, which range from simple molecules such as phenolic acids and flavonoids to more complex forms such as tannins and lignins, represent a broad and diverse group of natural substances.¹¹⁵ Among them, flavonoids are the most common and widely distributed subgroup.¹¹⁵ Their beneficial physiological effects include anti-inflammatory, antimicrobial, antioxidant, and cardioprotective properties, as well as anti-thrombotic and vasodilatory actions.^116,117 The antioxidant capacity of phenolic compounds is particularly important, as it underpins many of these health benefits.¹¹⁸

3. Chemical composition of Chlorella vulgaris

3.1. Proteins

Protein composes 40 % to 58 % of the dry weight of C. vulgaris biomass.119, 120, 121, 122 According to the recommended daily intake of essential amino acids published by the WHO (World Health Organization) and FAO (Food and Agriculture Organization) (Table 1), C. vulgaris provides an adequate supply of essential amino acids.⁷

Table 1.

Amino acid profile of C. vulgaris compared with the recommended daily intake of essential amino acids from FAO/WHO (Table modified from⁷).

Amino acids	C. vulgaris	C. vulgaris120	C. vulgaris123	Recommendations from FAO/WHO
Essential amino acids
Threonine	5.30	6.09	5.15	4.00
Valine	7.00	3.09	6.61	5.00
Methionine	1.30	0.65	1.24	N/A1
Isoleucine	3.20	0.09	4.44	4.00
Leucine	9.50	7.49	9.38	7.00
Phenylalanine	5.50	5.81	5.51	N/A
Histidine	2.00	1.25	1.97	N/A
Lysine	6.40	6.83	6.68	5.50
Tryptophan	n.d.2	2.21	2.30	1.00
Non-essential amino acids
Aspartic acid	9.30	10.94	9.80	N/A
Serine	5.80	7.77	4.32	N/A
Glutamic acid	13.70	9.08	12.66	N/A
Glycine	6.30	8.60	6.07	N/A
Alanine	9.40	10.90	8.33	N/A
Cysteine	n.d.	0.19	1.28	3.50
Tyrosine	2.80	8.44	3014	6.00
Arginine	6.90	7.38	6.22	N/A
Ornithine	n.d.	2.21	2.30	N/A
Proline	5.00	2.97	4.90	N/A

N/A: not available; n.d.: not detected.

3.2. Carbohydrates

The dry matter of C. vulgaris contains 12–17 % carbohydrates.¹²⁴ The cell wall features a carbohydrate matrix of polysaccharides, such as cellulose and β−1,3-glucan, and monosaccharides such as rhamnose, galactose, and xylose, which provide both rigidity and elasticity.125, 126, 127, 128 In particular, β−1,3-glucan acts as an immune stimulator, a free-radical scavenger, and a reducer of blood lipids.¹²⁹ Inside, on the other hand, the cell wall contains polysaccharides such as starch and monosaccharides such as glucose, galactose, and arabinose that can be used in metabolism.¹³⁰

3.3. Lipids

Under optimal growth conditions, the lipid content of C. vulgaris is between 5 % and 40 %, but it can increase to 58 % in unfavorable conditions.¹³¹ The lipids mainly consist of triglycerides, which include mono- to polyunsaturated fatty acids, as well as polar fatty acids such as phospholipids and glycolipids (Table 2). Moreover, it contains approximately 0.9 % sterols, including 0.6 % beta-sitosterol, 0.15 % campesterol, and 0.1 % stigmasterol, which, although present in small amounts, are sufficient for their biological functions.¹³² Among the various types of lipids, fatty acids—the biologically active components—are key constituents of the lipid profile of C. vulgaris. Their composition is presented below.¹³³

Table 2.

Fatty acid composition of C. vulgaris (percent of fatty acids).¹³³

Fatty acid	Name	%
∑ Saturated		25.38
∑ Short chain (4:0–18:0)		25.02
4:0	Butyric	0.20
6:0	Caproic	2.77
8:0	Caprylic	0.26
11:0	Undecylenic	1.39
12:0	Lauric	0.87
13:0	Tridecanoic	1.03
14:0	Myristic	0.69
15:0	Pentadecanoic	1.70
16:0	Palmitic	14.42
17:0	Margaric	0.12
18:0	Stearic	1.57
∑ Long chain (20:0–24:0)		0.36
20:0	Arachidic	0.14
23:0	Tricosanoic	Nd 1
24:0	Tetracosanoic	0.22
∑ Unsaturated		66.34
∑ Monodiens		28.04
11:1	Undecylenic	2.17
12:1	Lauroleic	0.41
15:1	Pentadecenoic	3.53
16:1	Palmitoleic	4.04
17:1	Heptadecenoic	0.27
18:1	Oleic	17.62
22:1	Docosaenoic	Nd
24:1	Tetracosenoic	Nd
∑Dienes		17.31
16:2	Hexadecadienoic	5.34
18:2 (ω−6)	Linoleic	11.97
20:2 (ω−6)	Eicosadienoic	Nd
∑Trienes		20.69
16:3	Hexadecatrienoic	4.90
18:3 (ω−6)	Gamma linolenic	Nd
18:3 (ω−3)	Alpha linolenic	15.79
20:3 (ω−8)	Dihomolinolenic	Nd
20:5 (ω−3)	Eicosapentaenoic (EPA)	Nd
22:6 (ω−3)	Docosahexaenoic (DHA)	0.30
∑Others		8.28

¹Nd, Not detected.

3.4. Secondary metabolites

3.4.1. Volatile compounds

Even though volatile compounds (VOCs) are present in much lower quantities than other chemical components, they exhibit significant biological activity. The VOCs consist of hydrocarbons, acids, alcohols, esters, aldehydes, and ketones, accounting for 33.67 %, 23.93 %, 15.62 %, 8.02 %, 3.24 %, and 2.71 % of the total VOCs (Table 3), respectively.¹³⁴ Concrete volatile compounds, identified by chromatography and GC/MS, are shown in the table below.

Table 3.

Composition (%) of C. vulgaris volatile compounds.¹³⁴

Class	Compounds	%
Hydrocarbon	α-pinene	2.11
	β-pinene	7.63
	Dodecane	1.76
	Hexadecene	15.30
	Heptadecane	0.74
	Heptadecene	2.11
	Octadecane	3.21
	Tetracosane	0.35
	Heptacosane	0.55
Acid	Dodecanoic	0.2
	Tetradecanoic	0.9
	Hexadecanoic	8.73
	Octadecanoic	2.2
	Octadec-9-enoic	0.73
	Octa-9,12-dienoic	10.20
	Octadec-9,12,15-trienoic	0.97
Alcohol	Hexadecanol	10.80
	Octadecanol	2.30
	Nonadecanol	2.01
	Phytol	0.51
Ester	Methyl-hexadecanoate	2.20
	Methyl-octadecanoate	3.52
	Methyl-octadec-9-enoate	0.98
	Methyl-octadec-9,12-dienoate	1.32
Aldehyde	Hexanal	0.14
	Nonadecanal	0.90
	Hexadecanal	2.20
Ketone	Decanone	0.7
	Hexadecanone	1.70
	β-ionone	0.13
	α-ionone	0.18
Unidentified		12.72
Total		100

3.4.2. Pigments

Similarly, pigments constitute only a small portion of C. vulgaris. The majority consists of carotenoids, chlorophyll, or chlorophyll derivatives (Table 4).⁷

Table 4.

Potential pigment content of C. vulgaris under different growth conditions.⁷

Pigments	μgg−1 (dw)
β-Carotene	7–12,000
Astaxanthin	550,000
Canthaxanthin	362,000
Lutein	52–3830
Chlorophyll-α	250–9630
Chlorophyll-β	72–5770
Pheophytin-α	2310–5640
Violaxanthin	10–37

3.4.3. Phenolic compounds

C. vulgaris harbors polyphenols and can produce them directly. In dietary supplements containing C. vulgaris, polyphenols such as p-hydroxybenzoic acid, p-coumaric acid, cinnamic acid, kaempferol 7-rhamnoside, epigallocatechin, epigallocatechin gallate, and apigenin have been identified.¹³⁵ Furthermore, it has been confirmed that phenolic compounds, including gallic acid, caffeic acid, p-coumaric acid, and ferulic acid, can be extracted from C. vulgaris using polyol-based deep eutectic solvents.¹³⁶

3.4.4. Other secondary metabolites

The petroleum ether extract of Chlorella demonstrated a significant presence of alkaloids, a moderate amount of terpenoids and flavonoids, and a low level of tannins and polyphenols (Table 5).¹³⁷ Similarly, the methanolic extract exhibited a high content of polyphenols and saponins, along with a measurable quantity of tannins, cardiac glycosides, and quinones (Table 5).¹³⁷

Table 5.

Levels of secondary phytochemicals in petroleum ether and methanolic extracts of C. vulgaris.¹³⁷

Secondary Phytochemicals	C. Vulgaris
	Petroleum ether	Methanol
Alkaloids	+++ 1	ND
Terpenoids	++ 2	ND
Flavonoids	++	ND
Tannins	+ 3	++
Polyphenols	+	+++
Saponins	ND	+++
Cardiac glycosides	ND	++
Quinones	ND	++

¹+++, abundant presence.

²++, moderate presence.

³+, poor presence.

4. Biological activities of Chlorella vulgaris

Variety of biological activities of Chlorellar vulgaris including antioxidative, antimicrobial, anti-cancer, anti-obesity, anti-inflammatory, immune-stimulatory, are summarized in Fig. 1.

Fig. 1.

Biological Activities ofChlorella Vulgaris.

4.1. Antioxidant activity

A methanolic extract of C. vulgaris demonstrated significant antioxidant activity, effectively scavenging various free radicals, as indicated by low IC₅₀ values: 2.82 ± 0.30 μg/ml for DPPH radicals, 2.30 ± 0.25 μg/ml for hydroxyl radicals, 3.24 ± 0.32 μg/ml for hydrogen peroxide (H₂O₂), and 3.15 ± 0.02 μg/ml for superoxide radicals.¹³⁸ Additionally, extracts obtained using supercritical carbon dioxide (SC—CO₂) technology exhibited strong antioxidant properties, showcasing effective radical scavenging, ferric reducing power, and metal chelation capabilities.¹³⁹ Furthermore, the compound (2E,7R,11R)-3,7,11,15-tetramethyl-2-hexadecenol isolated from C. vulgaris revealed high antioxidant potential, with an IC₅₀ of 10.59 μg/ml in the DPPH radical assay.¹⁴⁰ This level of activity is comparable to that of well-known antioxidants such as butylated hydroxy toluene, which has an IC₅₀ of 11.2 μg/ml, and vitamin C, which has an IC₅₀ of 12.9 μg/ml.¹⁴⁰

4.2. Antimicrobial activity

The methanolic extract of C. vulgaris demonstrated potent antibacterial activity, evidenced by a decrease in colony-forming units (cfu × 10⁷/ml) and cell viability percentage.¹³⁸ The chloroform extract of C. vulgaris exhibited substantial antimicrobial effects against two Gram-positive bacteria (Bacillus subtilis and Staphylococcus aureus), two Gram-negative bacteria (E. coli and Pseudomonas aeruginosa), and the fungal strain Candida albicans, as shown by the agar well diffusion assay.¹⁴¹ Additionally, the ethanol extract was found to be effective against Klebsiella sp.¹⁴² Moreover, the ethanolic extract of C. vulgaris (ECV) has been investigated for its antiviral properties against the Newcastle disease virus (NDV), with promising results obtained in both in vitro and in vivo studies.¹⁴³ In vitro, a 100 µL dose of ECV led to a significant reduction in viral titer from 108.5 EID50/ml to 103.4 EID50/ml, achieving complete inhibition of NDV replication and preventing hemagglutination, all while showing no toxicity to embryonated chicken eggs.¹⁴³ In the subsequent in vivo study, 28-day-old Sasso chickens were given a nontoxic dose of 1 g/kg of ECV in their drinking water both before and after exposure to NDV.¹⁴³ Evaluations of clinical signs, post-mortem lesions, and histopathological assessments revealed that the extract effectively hindered viral replication in the treated groups, compared with the controls.¹⁴³ Additionally, a real-time PCR analysis showed a marked decrease in viral shedding on days 3 and 5 post-challenge, with no detectable viral titer by day 7 in the prophylactic group.¹⁴³ These findings highlight the potential of C. vulgaris extract as an effective antiviral agent against NDV in real-world conditions.¹⁴³

4.3. Anti-cancer activity

The extract of C. vulgaris from SC—CO2 technology demonstrated an ability to inhibit the proliferation of H1299, A549, and H1437 human lung cancer cells in a dose-dependent manner.¹³⁹ In addition, treatment with C. vulgaris significantly reduced tumor cell migration, indicating the extract's potential role to prevent lung cancer metastasis.¹³⁹ Furthermore, methanol extracts of C. vulgaris exhibited significant cytotoxic effects against the MCF-7 breast cancer cell line.¹⁴⁴ The pure compound (2E,7R,11R)-3,7,11,15-tetramethyl-2-hexadecenol, isolated from C. vulgaris extract, displayed strong anticancer activity against the HeLa cell line, achieving an IC₅₀ value of 4.38 μg/ml, which is notably lower than that of the synthetic drug doxorubicin, which has an IC₅₀ of 13.3 μg/ml.¹⁴⁰

4.4. Anti-inflammatory activity

The 80 % methanolic extract demonstrated the most potent anti-inflammatory effects by significantly inhibiting the production of nitric oxide, prostaglandin E, TNF-α, and IL-6 in LPS-induced RAW 264.7 cells (P < 0.001).¹⁴⁵ Furthermore, both the hexane and chloroform fractions displayed notable and dose-dependent suppression of inflammatory mediators and cytokines induced by LPS in vitro.¹⁴⁵

4.5. Anti-obesity

A heterotrophic extraction of C. vulgaris using ethanol and ultrasound treatment resulted in a considerable reduction of Nile red fluorescence, which indicates the depletion of neutral lipid reserves.¹⁴⁶ This effect was comparable to that achieved by the positive control, resveratrol.¹⁴⁶

4.6. Immune function

The nutritional parameters and levels of IFN-γ in the blood serum of a group of mice treated with C. vulgaris (0.15 g/kg) showed marked improvements compared with the untreated group on a protein-energy malnutrition diet, which is associated with a weakened immune defense.¹⁴⁷ Furthermore, treatment with C. vulgaris at a concentration of 1 mg/mL led to a notable increase in the production of both IFN-γ and IL-2, compared with the control group.¹⁴⁷ These results indicate that C. vulgaris could play a beneficial role in enhancing immune function.¹⁴⁷

4.7. Pharmacological applications

The antioxidant, antimicrobial, and immunomodulatory properties of C. vulgaris underscore its pharmacological significance, in addition to its role in wound healing. A hydrogel formulation containing C. vulgaris cultivated in mixotrophic conditions has been shown to enhance wound healing in mice.¹⁴⁸ This formulation led to increased collagen deposition, skin appendages, and a reduction of inflammatory cells, as well as clear signs of basal laminae, compared with the control groups.¹⁴⁸ These pro-healing and anti-inflammatory characteristics, coupled with its antioxidant and antibacterial activities, suggest that C. vulgaris could serve as a promising topical treatment for various skin ailments, including both acute and chronic wounds.¹⁴⁸

Moreover, an ointment derived from C. vulgaris containing 15 % extract demonstrated considerable effectiveness in speeding the wound healing process in pig ears, as evidenced by improvements in healing parameters and an increase in the number of fibroblast cells, compared with control groups that did not receive the extract.¹⁴⁹ Fibroblasts play a crucial role in producing collagen and extracellular matrix components, which are essential for replacing blood clots with granulation tissue—a temporary matrix in the healing process that ultimately transforms into scar tissue.¹⁵⁰ The presence of collagen is vital for repairing damage and restoring both anatomical structure and tissue function.^150,151 C. vulgaris contains a variety of beneficial phytochemical compounds such as chlorophyll, carotenoids, phycobilin, Chlorella growth factor, sterols, flavonoids, tannins, phenols, terpenoids, saponins, sulfur compounds, and other nutrients, all of which further facilitate the healing process.¹⁴⁹

4.8. Cardiovascular health

Chlorella microalgae are regarded as promising species for tissue regeneration, particularly in repairing joint injuries and protecting the cardiovascular system. C. vulgaris has been found to reduce arterial collagen degradation, which can help prevent diseases such as atherosclerosis.¹⁵² Furthermore, the daily intake of Chlorella supplements has demonstrated potential health benefits in individuals with mild hypercholesterolemia by lowering serum lipid levels, particularly triglycerides and total cholesterol, which are considered major risk factors for cardiovascular diseases.¹⁵³ Peptidic fractions from peptic digests of C. vulgaris were isolated through ion exchange chromatography and gel filtration.¹⁵⁴ The fractions showed the ability to inhibit angiotensin I-converting enzyme (ACE).¹⁵⁴ When administered orally to spontaneously hypertensive rats (SHRs) at a dose of 200 mg/kg, these peptides were found to reduce hypertension, another critical risk factor for cardiovascular diseases.¹⁵⁴

In addition, a novel ACE inhibitory peptide, Val-His-Trp (VHW), was identified from a peptide library derived from C. vulgaris proteins.¹⁵⁵ When administered to SHRs at a dose of 5 mg/kg, the VHW peptide caused a 50 mmHg reduction in systolic blood pressure (SBP), with a maximum drop from 234 to 184 mmHg within 2 h.¹⁵⁵ Remarkably, VHW reduced SBP by 31 mmHg by the end of the observation period, whereas the control drug, lisinopril, caused only a 10 mmHg reduction.¹⁵⁵ This suggests that C. vulgaris–derived peptides could be effective at lowering blood pressure, even at relatively low doses.¹⁵⁵

5. Active compounds in Chlorella vulgaris

The petroleum etheric extract of Chlorella revealed the presence of three bioactive compounds, dodecanoic acid, 6, 7-dimethoxy-2-tetralone and 3, 7, 11, 15-tetramethyl-2-hexadecen-1-ol, whereas the methanolic extract showed five bioactive compounds, phenol, 2‑methoxy‑5-(1-propenyl)-,(E), cycloheptane, 1, 3, 5-tris (methylene), 4-(6, 6-dimethyl-2-methylenecyclohex-3-enylidene) pentan-2-ol, tetradecanoic acid, and loliolide (Fig. 2).¹³⁷

Fig. 2.

Seven bioactive compounds extracted fromC. vulgaris.156, 157, 158, 159, 160, 161, 162

5.1. Dodecanoic acid (C₁₂H₂₄O₂)

Dodecanoic acid, also called dodecylic acid, N-dodecanoic acid, and lauric acid, is a saturated fatty acid known for its wide range of biological activities.¹³⁷ It has been reported to exhibit antibacterial, antifungal, antitumor, anti-inflammatory, antimycobacterial, and antiviral properties, making it a compound of significant interest in therapeutic and pharmaceutical applications.163, 164, 165, 166, 167, 168

5.2. 6, 7-Dimethoxy-2-tetralone (C₁₂H₁₄O₃)

This compound, a phenolic derivative, is also called 6,7-dimethoxy-3,4-dihydronaphthalen-2(1H)-one, 6,7-dimethoxy-3,4-dihydro-1H-naphthalen-2-one, and 6,7-dimethoxyl-2-tetralone.¹³⁷ It serves as a key precursor in the production of numerous dopaminergic compounds.¹³⁷ Additionally, it plays a crucial role in chemical synthesis, such as forming 2-bromotetralones through bromination.¹³⁷ This compound has also been used to create quinolines with dopaminergic effects, naphthols with anti-inflammatory properties, and benzophenanthridine alkaloids known for their antitumor activities.^137,169

5.3. 3, 7, 11, 15-Tetramethyl-2-hexadecen-1-ol (C₂₀H₄₀O)

This compound, a diterpenoid and a long-chain primary fatty alcohol, is also called phytol, 3,7,11,15-tetramethylhexadec-2-en-1-ol, and 2-hexadecen-1-ol, 3,7,11,15-tetramethyl.¹³⁷ It exhibits a wide array of biological properties, including anxiolytic, cytotoxic, antioxidant, and apoptosis-inducing effects. Additionally, it has been shown to modulate metabolism and demonstrate anti-nociceptive, anti-inflammatory, anticancer, diuretic, and antimicrobial activity. Phytol also exhibits insecticidal, anti-tuberculosis, immune-modulating, and protective properties against oxidative stress.170, 171, 172 These diverse effects highlight its potential in various therapeutic and industrial applications.

5.4. Phenol, 2‑methoxy‑5-(1-propenyl)-(E) (C₁₀H₁₂O₂)

This phenolic compound, also called isoeugenol, isochavibetol, 2‑methoxy‑5-[(E)-1-propenyl] phenol, and 2‑methoxy‑5-[(E)‑prop‑1-enyl] phenol, is notable for its diverse applications.¹³⁷ Isoeugenol, particularly in its acetate ester form, is a precursor in vanillin production.¹³⁷ It is also a non-cannabinoid phenol present in cannabis plants.^137,173 Widely used as both a food flavoring and fragrance agent, it contributes to the characteristic taste of nutmeg and serves as a scent component in soaps, shampoos, perfumes, detergents, and bath tissues. Despite its widespread use, isoeugenol is recognized as a potent contact allergen.^137,174

5.5. 4-(6, 6-Dimethyl-2-methylenecyclohex-3-enylidene) pentan-2-ol (C₁₄H₂₂O)

This terpenoid, also called (4Z)-4-(6,6-dimethyl-2-methylene-3-cyclohexen-1-ylidene)-2-pentanol, serves multiple industrial purposes. It is commonly used as a solvent and an intermediate in the synthesis of various chemicals.¹³⁷ At room temperature, it appears as a colorless liquid with a characteristic fuel-like odor.¹³⁷ Its applications extend to the production of pharmaceuticals, herbicides, antioxidants, extractants, and cosmetics, highlighting its versatility in manufacturing and industrial processes.¹³⁷

5.6. Tetradecanoic acid (C₁₄H₂₈O₂)

This saturated fatty acid is also called myristic acid, n-tetradecanoic acid, and crodacid.¹³⁷ Tetradecanoic acid used as an antioxidant, lubricant, hypercholesterolemic, cancer-preventive, and cosmetic.^137,175, 176, 177

5.7. Loliolide (C₁₁H₁₆O₃)

Loliolide, also called (3S,5R)-loliolide, is a terpenoid with diverse biological activities. It has demonstrated the ability to inhibit cellular senescence in human dermal fibroblasts by reducing senescence-associated β-galactosidase activity, p21 protein levels, and reactive oxygen species in cells subjected to adriamycin treatment.¹³⁷ Functioning as a selective estrogen receptor modulator, it activates ERβ and ERα receptors while exhibiting estrogenic effects.¹³⁷ These properties make loliolide valuable for medicinal and dietary applications aimed at alleviating estrogen deficiency–related conditions, particularly post-menopause.¹³⁷ Moreover, loliolide has shown potential as a hepatitis C virus entry inhibitor, suggesting its role as a candidate antiviral agent.^137,178 Additionally, it is recognized as a promising anti-adipogenic compound for potential use in anti-obesity therapies. Beyond those applications, it exhibits antibacterial, anti-inflammatory, antidepressant, and anticancer effects, showcasing its versatility in pharmaceutical and therapeutic contexts.¹⁷⁹

6. Conclusions and perspectives

This article addresses biological activities of Chlorella vulgaris, the most abundantly and extensively obtained and consumed in East Asian countries. Chlorella vulgaris is a versatile microalga with multifunctional properties ranging from therapeutic biological activities – such as antioxidant, antimicrobial activity, and anti-cancer activity – to attributes with potential for industrial applications including bioplastic and cosmetics. Furthermore, 7 major active compounds of Chlorella vulgaris are elucidated with emphasis on their health-promoting characteristics. In spite of great progress in Chlorella vulgaris research, several challenges still remain. Most of the studies are confined to in vitro experiments, thus, it may be premature to implement in human trials. Moreover, Chlorella vulgaris extracts are collected in different ways by each experiment. Therefore, extraction methods need to be standardized for reproducibility and efficacy. As a final addition, the underlying mechanisms of action of active compounds should be investigated. Therefore, future research should aim to delve into the molecular mechanisms behind these bioactivities and further sequential studies should be conducted, progressing from in vivo to clinical investigations. These studies will help determine how the promising effects observed in preclinical models manifest in humans, providing concrete insights into potential health benefits. By establishing whether these pharmacological effects translate into tangible improvements in human well-being, such clinical investigations will clarify the real-world applicability of Chlorella vulgaris, guide evidence-based usage, and lay the foundation for future interventions aimed at promoting human health.

Funding

This research was supported by the Basic Science Research Program (2017R1A6A1A03015642) through the National Research Foundation of Korea (NRF) funded by the Ministry of Education.

Ethics statement

Not applicable.

Data availability

Data sharing is not applicable to this review article.

CRediT authorship contribution statement

Yein Heo: Conceptualization, Methodology, Formal analysis, Investigation, Data curation, Writing – original draft. Mi-Yeon Kim: Software, Validation, Formal analysis, Writing – review & editing, Visualization. Jae Youl Cho: Conceptualization, Supervision, Project administration, Funding acquisition.

Declaration of competing interest

The authors declare that they have no conflicts of interest.