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Microalgal biotechnology has received considerable interest among research organizations and industries in the last few years. One of the most important characteristics of microalgae is their capability to synthesize bioactive compounds, including polyunsaturated fatty acids, carotenoids, essential amino acids, vitamins, etc., which are known to have a wide range of health benefits. Microalgae have demonstrated potential as a source of adequate nutrition and a tool for combating problems related to inadequate diet or food scarcity. The use of microalgae as a functional ingredient enhances the nutritional value of food, improves human health, and reduces the incidence of disease and illness. Microalgae synthesize various bioactive compounds with effects on cancer and neurodegenerative and infectious diseases. This chapter discusses the current knowledge about the value-added products from microalgae, the significance of utilizing microalgae as an ingredient of innovative food products, and the purpose of using bioactive metabolites from microalgae in the pharmaceutical industry as therapeutic agents.

Around 3.5 million deaths are reported yearly due to malnutrition and food shortage.1,2  Innovative food sources with adequate nutrients are essential to avoid problems related to malnutrition and inadequate diet. Nowadays, scientific communities are exploring microalgae as a natural and effective source of nutrients to combat malnutrition.1,3  Microalgae are unicellular, microscopic, polyphyletic, photosynthetic microorganisms that inhabit a wide range of habitats and have the capacity to produce various biologically active substances.4,5  Microalgae account for about half of the global photosynthetic activity and form the source of the food chain for more than 70% of the world's biomass.6  Humans started consuming microalgae as food thousands of years before to survive under extreme food shortages.7  However, the efforts to develop commercial products from microalgae began in the mid-20th century. During the 1940s to 1960s the earliest proposals to use microalgae as a source of lipids and carotenoids were put forth by Harder and Von Witsch (for lipids) and Katherin (for carotenoids).8  In the 1970s, extensive research work was done on microalgae as a single-cell protein source.8 Chlorella and Spirulina were the first microalgae to be commercialized as a “health food” which was followed by the commercialization of carotenoids from Dunaliella salina (β-carotene) and Haematococcus pluvialis (astaxanthin) and docosahexaenoic acid from Crypthecodinium cohnii.8 

The advantages of commercial microalgae cultivation include (1) rapid growth, (2) utilization of CO2 coupled with the production of O2, (3) consumption of relatively less amount of water, and (4) simple nutritional requirements.5  Microalgae are produced commercially at about 5000 tonnes per year of dry matter, and there are more than a hundred commercial producers in the Asia-Pacific region with a production range of 3 to 500 tonnes per year.9  Almost nine-tenths of microalgal production is situated in Asia, with the majority of commercial producers based in China, Taiwan, and India.9 

Microalgae are cultivated on a large scale for different nutraceuticals and health food applications.1  Microalgae are a rich source of bioactive substances, like proteins, polysaccharides, lipids, polyunsaturated fatty acids, pigments, etc., which have a wide range of health benefits, including antioxidant, antibacterial, antiviral, antitumor, neuroprotective, and immunostimulatory effects.5,10  These bioactive substances from microalgae are being explored by various industries like pharmaceutical, medical, cosmetic, chemical, fish farming, energy, food and feed, etc.5  As microalgae have a rich profile of essential nutrients, they serve as an important source of food throughout the world, mainly in Asian countries like China, Japan, and Korea.11 Spirulina (Arthrospira) platensis is a rich source of protein with a balanced amino acid content, and the protein levels are comparable to those found in meat and soybean.12 Spirulina is also a natural source of vitamins (vitamin A, B1, B2, B12), pigments, and essential fatty acids.9  High protein content and an attractive nutrient value made Spirulina popular as a food supplement. Spirulina production for human consumption in the world exceeds 1000 metric tonnes annually.9  Microalgae like Porphyridium, Dunaliella, Isochrysis, Nannochloropsis, Tetraselmis, Phaeodactylum, Chlorella, and Schizochytrium accumulate a high amount of lipids (20 to 50%).13  The microalgal lipids contain a good amount of omega-3 polyunsaturated fatty acids (PUFAs), which have a broad range of health benefits, including cardioprotective effects, neuroprotective effects, pre- and postnatal brain development, retinal stimulation, and neurotransmission.13,14 Schizochytrium sp. contains a DHA content of about 40% of total lipids, whereas Porphyridium cruentum and Nannochloropsis salina can accumulate EPA at more than 25% of total lipids.13  The antioxidant activity of carotenoids reduces the harmful effects of free radicals in the body.1  It is well known that the intake of carotenoid-rich foods helps to reduce the risk of various degenerative diseases, including cancer,15  and the microalgal cultures like Dunaliella salina, Haematococcus pluvialis, Chlorella vulgaris, Chlorella pyrenoidosa, and Scenedesmus sp. accumulate β-carotene, astaxanthin, canthaxanthin, lutein, and other carotenoids.16  Generally, microalgae can produce 0.1 to 2% of carotenoids, and while under stress, they accumulate a higher amount of carotenoids. For example, Dunaliella sp. can accumulate up to 14% of β-carotene under stress.1 

Most of the microalgal biomass is an attractive source for the production of high-value metabolites.9  Microalgae are distributed widely in the environment, and a large number of microalgal species are still unknown and uncharacterized. Therefore, numerous potentially advantageous microalgal species with novel characteristics and bioactive substances are expected to be discovered.4  This chapter presents an overview of the importance of microalgae as a renewable and sustainable source of high-value metabolites, their industrial applications, and possible areas for further research and development.

Microalgae can produce different classes of lipids, and the major lipids found in microalgae are neutral lipids, including triacylglycerol (TAG) and free fatty acids (FFA), and polar lipids like glycolipids and phospholipids.5,17  Glycolipids like monogalactosyldiacylglycerol (MGDG), digalactosyldiacylglycerol (DGDG), and sulfoquinovosyl diacylglycerol (SQDG), and phospholipids like phosphatidylglycerol (PG) are the major components of chloroplast lipids.18  Microalgal lipids serve various functions, including (1) cellular membrane development, (2) as a storage form of energy, (3) metabolic activities like signal transduction and transcriptional and translational regulation, (4) intracellular interactions, and (5) vesicle transfer.5  The polar lipids and sterols develop a selectively permeable barrier that safeguards the cells from the outside environment and separates different intracellular organelles.19  TAG serves mainly as a storage form of energy and is involved in various cellular functions. TAG participates in membrane restructuring under fluctuating environmental conditions. By providing a special acyl group, TAG helps in the synthesis of polar lipids for immediate structural changes of the membrane.19–21  The lipid classes present in the microalgae depend on the microalgal species and the cultivation conditions.5  The lipid content in microalgae generally ranges from 1.5 to 75%.17  A given class of microalgae may contain variable lipid contents based on the cultivating medium and the method. For example, Chlorella vulgaris accumulates lipids in the range of 12 to 26% and Botryococcus braunii accumulates 14 to 75% of lipids.17  In other words, the cultivation condition determines the lipid content of microalgae. Increased light intensity dramatically increased the lipid content of Isochrysis galbana, Nannochloropsis oculata, and Dunaliella salina.22  Reducing the cultivation temperature from 25 °C to 5 °C enhanced the lipid content by 1.4 to 1.8 fold in Scenedesmus dimorphus, Desmodesmus sp., Chlorella sp., and Oocystis pusilla.23  Low temperature cultivation leads to the generation of reactive oxygen species and oxidative stress causing an increased lipid accumulation in microalgae.24  Nutrient limitation causes undesirable situations inside the cell and reduces cell growth and protein synthesis, which leads to an increased accumulation of lipids.25  Nitrogen and phosphorus starvation led to 1.6 to 1.8 fold increased accumulation of lipids in different microalgal cultures.23,26  Increasing the CO2 concentration increased the lipid content of Scenedesmus bajacalifornicus, with a maximum observed at 25% CO2.27  Recently, various cultivation strategies have been developed by different research groups for improving the lipid yields from microalgae.28 

The significance of microalgal lipids is their commercial application as a source of PUFAs. PUFAs of the omega-3 series, especially α-linolenic acid (ALA), eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA), have a broad range of health benefits. These PUFAs are associated with reduced risk of cardiovascular diseases, neuroprotective effects, pre and postnatal brain development, retinal stimulation, and neurotransmission.29  As vertebrates are devoid of Δ12 and Δ15 fatty acid desaturases, which convert oleic acid into linoleic acid and α-linolenic acid, they are unable to synthesize PUFAs by a de novo pathway and hence these have to be included in the diet.1,30  Therefore, PUFAs are a vital part of the human diet, and an optimal proportion of omega-3 to omega-6 fatty acids (1 : 1 to 1 : 4) is important for the prevention of various diseases.3  ALA is found mainly in vegetable oils and nuts,31  whereas EPA and DHA are found mainly in fish oil.3,32  There is growing interest in the exploitation of microalgae as a source of lipids for food, feed, and nutraceutical applications due to their capacity to produce a high amount of PUFAs.

The de novo production of fatty acids in microalgae is assumed to take place in the plastids of microalgal cells like in higher plants and requires acetyl-CoA as a carbon source, ATP as an energy source, and reducing power in the form of NADPH as well as a set of nuclear-encoded proteins.20  There is minimal knowledge about the synthesis of acetyl-CoA in microalgae, but it is assumed that, like in plants, the chloroplastic pyruvate dehydrogenase complex is the major source of acetyl CoA biosynthesis. Acetyl-CoA carboxylase, a biotin-containing enzyme, catalyzes the conversion of acetyl-CoA to malonyl-CoA, which is the first and most important regulatory step in fatty acid biosynthesis.20  Malonyl-CoA is considered to play a central role in fatty acid biosynthesis as it functions as the initial precursor of the de novo fatty acid biosynthetic pathway, and at the same time, it is also used for the elongation steps in the later stage, which occurs in the endoplasmic reticulum (ER).33  Generally, fatty acid biosynthesis in plants and microalgae is divided into three stages. Initially, palmitic acid (16 : 0) and other saturated fatty acids will be synthesized from acetyl-CoA, by the fatty acid synthase (FAS) pathway. Chain elongation steps will synthesize the longer chain fatty acids, and the desaturation steps of n-3 and n-6 pathways will produce more complex PUFAs.33  The termination of fatty acid biosynthesis is catalyzed by acyl-ACP thioesterases (TE), producing ACP and free fatty acids from the acyl-ACP complexes.20  In microalgae, generally, the fatty acids range from C14 to C22. The most common fatty acids in microalgae are C16 : 0, C18 : 0, C16 : 1, C18 : 1, C18 : 2, and C18 : 3.17  The polyunsaturated fatty acids (PUFA) commonly observed in microalgae are α-linolenic acid (ALA), arachidonic acid (ARA), docosahexaenoic acid (DHA), eicosapentaenoic acid (EPA), and γ-linolenic acid (GLA).3  The fatty acid composition of microalgae varies substantially in different phylogenetic groups.20 Isochrysis galbana, Phaeodactylum tricornutum, and Porphyridium cruentum contain an EPA content of about 22 to 29% of total fatty acids. The DHA content in Schizochytrium sp. and Aurantiochytrium sp. ranges from 23 to 64% of total fatty acids. Porphyridium cruentum has an ARA content of 23% of total fatty acids. The Desmodesmus sp. accumulates about 25% of ALA fraction of total fatty acids.34  ARA and EPA are the precursors of eicosanoids and prostaglandins, which are involved in the immune responses and cell signaling pathways.1,30  An adequate consumption of EPA and DHA is required for the healthy development of the fetal brain.35  ARA and DHA are vital for the normal growth and development of infants.35  Immuno-modulatory effects were observed when omega-3 fatty acids are used in the treatment of various inflammatory conditions like rheumatoid arthritis, psoriasis, asthma, etc.35  ALA can be converted into DHA and EPA inside the human body at a lower rate by enzymes elongases, Δ5 desaturase, and Δ6 desaturase. However, the conversion of ALA is observed to be 5% for EPA and <0.5% for DHA.3  Various cultivation methods like low-temperature cultivation, UV irradiation, nutrient stress, media optimization, etc. are developed by different research groups to enhance the PUFA content in microalgae.36  A decrease in the cultivation temperature from 25 °C to 5 °C increased the ALA fraction of total fatty acids by 1.3 to 2.4 fold in Desmodesmus sp., Chlorella sp., and Oocystis pusilla.23  UV A exposure led to an increase in PUFA content in Porphyridium cruentum.37  Increasing the concentration of nitrate from 0.25 g L−1 to 1.5 g L−1 in the medium resulted in a 2.4 fold increase in EPA production by Phaeodactylum tricornutum.38  The salinity stress (9 g L−1 NaCl) enhanced the DHA fraction of total fatty acids by about 2 fold in Crypthecodinium cohnii.39  The recent advances in microalgal research, such as synthetic toolboxes, availability of the whole genomic database, and multi-omics data, have helped the detailed understanding of gene manipulation and production of high-value metabolites like PUFAs.40  A toolkit has been developed for genetic modification of Nannochloropsis oceanica by overexpression of different fatty acid desaturase genes, and the overexpressed Δ5 and Δ12 fatty acid desaturase encoding sequences resulted in higher production of long-chain PUFAs in the form of EPA.40,41  However, the increase in the yields of high-value metabolites like PUFAs should be coupled with improved microalgal biomass production for making the platform commercially competitive.

The demand for omega-3 PUFAs has been increasing in recent years and is accompanied by increased consumption of fish and other marine products. These increasing trends will lead to a challenge in their supply.33  Microalgae-based polyunsaturated fatty acids are an alternative for fish oil with a number of benefits considering the taste, reduced or no toxin accumulation, and veganism nature when compared to animal-based oils.3  Apart from this, microalgae can generate a larger quantity of omega-3 fatty acids when compared to animal-based oils. Isochrysis galbana and Phaeodactylum tricornutum produce about two-fold higher EPA, and Crypthecodinium cohnii produces about six-fold higher DHA than cod liver oil. In the case of terrestrial plant-based PUFAs, they are devoid of EPA and DHA. Therefore, microalgae-based PUFAs are a more suitable and sustainable alternative for food and feed production than fish- and plant-based PUFAs.3 

Microalgal lipids are used as a source of omega-3 fatty acids in various food and feed industries. The major portion (75%) of biomass produced annually from microalgae is used in the health food market.33,42  The omega-3 PUFA rich powders are produced from microalgal biomass and are used in the form of capsules or tablets.33,43  PureOne™ is a supplement capsule composed of EPA and DHA.33,44  Microalgal omega-3 fatty acids are also formulated as oils. Omega Tech (USA) uses Schizochytrium for the production of a low-cost oil named “DHA Gold”, which is supplemented as an adult dietary supplement in food and beverages.14,33,45  Similarly, Nutrinova (Germany) and Martek (USA) produce DHA rich oils from Ulkenia sp. and Crypthecodinium cohnii, respectively.33,46  Microalgae serve as food for various freshwater and marine consumers in the ecosystem and play an essential role in aquaculture and feed industries.33  Microalgae, including Dunaliella, Chlorella, Arthrospira, Nannochloropsis, and Tetraselmis, are widely used in aquaculture.33,47  The microalga Nannochloropsis sp. is a rich source of PUFAs, whereas the chlorophytes like Chlorella contain a less amount of PUFAs in their biomass which is used as feed by mixing with other species.33 

Microalgal pigments play an essential functional role in photosynthesis and photo-protection. Chlorophyll is the primary photosynthetic pigment in microalgae, and carotenoids (xanthophylls and carotenes) and phycobiliproteins act as accessory or secondary pigments. The secondary pigments are characterized by their beneficial biological functions such as anti-inflammatory, anti-oxidant, anti-cancer, anti-obesity, anti-angiogenic, and neuroprotective functions.16,48  Microalgae will surpass their synthetic equivalents due to their sustainability and renewable nature. They are recognized as a source of nutraceuticals and as well as an excellent source of natural food colorants.

The fat-soluble green chlorophyll pigments are structurally characterized by the presence of a porphyrin ring containing magnesium and a phytol chain.49  These pigments play a vital role in converting solar energy into chemical energy during photosynthesis as primary pigments. Microalgal species of Cyanobacteria (blue-green algae) and Rhodophyta contain only chlorophyll-a. Chlorophyte microalgae contain chlorophyll-a and chlorophyll-b, whereas chlorophyll-c, -d, and -e are found in several marine algae and freshwater diatoms. Chlorophylls constitute about 0.5 to 1.5% dry weight of microalgal biomass50–52  and they exhibit anti-oxidant, anti-carcinogenic, anti-genotoxic, and anti-mutagenic properties.53  As a phyco-nutrient, chlorophyll has been reported to stimulate bile secretion and liver recovery; it improves the metabolism of carbohydrates, lipids, and proteins in humans with positive effects in human reproduction as well.54,55  Chlorophyllin, a chlorophyll derivative, has the magnesium being replaced by either sodium or copper, and the phytol chains are lost.56  Studies have shown that incorporation of chlorophyllin in the diet inhibits cancer progression by targeting carcinogenic cells and invading their cell cycle.57  The chlorophyllin molecule has been shown to cross the blood–brain barrier in mice which makes it a valuable metabolite for human medicinal applications.58 

Phycobiliproteins are a class of water-soluble proteins which are accessory pigments for harnessing light during photosynthesis. These deeply colored pigments with fluorescence are the most important components of photosynthetic light-harvesting antenna pigments, namely the phycobilisomes.59  Phycobiliproteins are further classified on the basis of color into phycocyanin (blue), phycoerythrin (purple), phycoerythrocyanin (orange), and allophycocyanin (green-blue). This group of pigments offer a wide range of applications such as for labeling of antibodies and as highly sensitive fluorescence markers in clinical diagnosis, and they are also used in multi-color fluorescence activated cell-sorter analysis.60 Spirulina (Arthrospira) containing phycocyanin and Porphyridium of Rhodophyta consisting of phycoerythrin are the primary sources of natural phycobiliproteins. Phycocyanin is used as a food additive in chewing gum, candy, jelly, ice cream, dairy products, and soft drinks and also as a colorant in cosmetics. Structurally, phycobiliproteins consist of a protein as a backbone which is linked to a tetrapyrrole chromophore prosthetic group covalently, known as phycocyanobilin (PCB). Many investigations have reported that phycocyanin has significant anti-oxidant, anti-inflammatory, hepatoprotective, and free radical scavenging properties.61  The anti-inflammatory effect of phycocyanin is due to its ability to scavenge oxygen free radicals and inhibit enzymes that are responsible for the formation of inflammatory prostaglandins.62  PCB has a chemical structure which is similar to that of bilirubin, a bile pigment, which is known to scavenge reactive oxygen species. Microalgae-derived phycocyanin has been shown to be helpful in the treatment of neurodegenerative diseases such as Alzheimer's and Parkinson's disease which are caused by oxidative stress-induced neuronal injury.63  The aqueous extract from microalgae containing phycocyanin has also been shown to possess anti-cancer and cytotoxic properties.63,64 

Carotenoids are naturally occurring pigments which impart red, orange, and yellow color to fruits, vegetables, and flowers.65  Carotenoids possess anti-oxidant properties and therefore have a therapeutic effect on humans and animals.9  Carotenoids constitute 0.1 to 0.2% of total dry weight of microalgae.66  The classification of carotenoids is based on their functional groups. Carotenes contain only one hydrocarbon group and the ones with hydroxyl, epoxy, and oxo groups are the xanthophylls. Carotenoids play a major role in protecting the photosynthetic apparatus against solar radiation as they are accessory light-harvesting pigments.67  Microalgae undergo a carotenogenesis process in response to various environmental and nutritional stresses where the cells stop growing and start accumulating carotenoids as secondary metabolites as an adaptation mechanism to severe environments. Carotenoids are consumed as health supplements and are used as colorants in cheese, butter, and margarine.68  The consumption of carotenoids in diet reduces the risk of cardiovascular disease,1  lowers the prevalence of metabolic syndrome,69  diminishes the prevalence of certain types of cancer, atherosclerosis, cataracts, and age-related macular degeneration,70  and enhances immune resistance to bacterial, viral, parasitic, and fungal infections.71 

There are about 400 carotenoids identified among which mainly β-carotene (provitamin A) and astaxanthin are commercially produced from Dunaliella salina, and Haematococcus pluvialis, respectively.72  Provitamin A through the catalytic action of β-carotene-15,15′-monooxygenase, an intestinal enzyme, is converted to retinal in vivo. The β-carotene uptake reduces the risk of age-related macular degeneration (AMD) and cancer due to its free radical scavenging activity. Astaxanthin is a red pigment which occurs naturally as an esterified isomer in cis and trans forms.73  Astaxanthin is reported as the most potent antioxidant among all carotenoids74  and thus exhibits exceptional anti-tumor activity.16  Studies have revealed the role of dietary astaxanthin in the production of antibodies, anti-aging, and prevention of lipid peroxidation, oral cancer, and colon cancer.75–77  Microalgae are one of the primary sources of naturally occurring lutein and zeaxanthin which are obtained from Dunaliella salina, Chlorella protothecoides, and Spirulina.78,79  Lutein and zeaxanthin are structurally similar80,81  and both play a vital role in maintaining a normal visual function.82  Lutein is also used as a yellow colorant in pharmaceuticals and animal products such as egg yolk and chicken skin.83  Zeaxanthin, when administered orally, has been found to be effective in the management of acute inflammation caused by UV-B irradiation induced responses. Lutein obtained from C. vulgaris showed anti-cancer activity against the human colon cancer cell line (HCT-116).84 

Violaxanthin is an orange-colored xanthophyll found in D. tertiolecta and C. ellipsoidea. Violaxanthin extracted from C. ellipsoidea reported anti-inflammatory property by the suppression of NF-κβ and MAPK pathways. Another study demonstrated the inhibition of nitric oxide and prostaglandin E2 (NO and PGE2) in a dose-dependent manner in RAW 264.7 cell lines.85  Violaxanthin was also shown to exert an anti-proliferative effect on MCF-7 breast cancer cells and induce early apoptosis.86  Fucoxanthin is a golden-brown colored carotenoid pigment which is one of the major carotenoids obtained from marine microalgae.87  It is mainly found in diverse classes of microalgae such as bacilophytes, pinguiophytes, bolidophytes, silicoflagellates, chrysophytes, and phaehytes. Fucoxanthin has received much attention due to its anti-inflammatory, anti-cancer, anti-oxidant, and anti-obesity properties and preventive effect against cerebrovascular diseases.88  Dietary fucoxanthin increases the serum level of both non-HDL and HDL cholesterol, and fucoxanthinol has demonstrated anti-oxidant activity as a free radical scavenger by quenching singlet oxygen under in vitro conditions. Fucoxanthin was reported to be more effective against cell viability of colon cancer cell lines (Caco-2DLD-1, and HT-29) in comparison with other carotenoids.84 

Microalgae are used to produce three kinds of carbohydrate polymers: polysaccharides (PS), exopolysaccharides (EPS), and sulfated exopolysaccharides (sEPS). Microalgal polysaccharides are heteropolymers of various sugars such as glucose, xylose, galactose, mannose, fucose, rhamnose, and fructose. However, the sulfated exopolysaccharide obtained from Gyrodinium impudicum is a homopolymer of galactose.89  Many studies have indicated that the marine microalgal polysaccharides exhibit antiviral bioactivity against various viruses. The intracellular polysaccharide calcium-spirulan, produced by Arthrospira platensis, has been shown to inhibit the replication of many viruses by preventing the penetration of the virus into the host cells.90,91  Sulfated exopolysaccharides (sEPS) released by marine microalgae can prevent the accumulation of reactive chemical species and free radicals, thus acting as protective metabolites. Among these, the polysaccharides secreted by Porphyridium, Phaeodactylum tricornutum, and Chlorella stigmatophora have been widely studied towards inhibition of oxidative damage, auto-oxidation, and inflammation.92  One of the most potential functions of polysaccharides is prevention of cell growth and tumor. The homopolysaccharide of G. impudicum can suppress tumor cell growth, both in vivo and in vitro,93  by stimulating the innate immune system. The sulfated polysaccharides from C. stigmatophora exhibited immunosuppressant effects.94 

Proteins are essential macronutrients and building blocks of the human body required for growth and development. Proteins consist of long chains of amino acids, categorized as non-essential and essential, linked by peptide bonds.95  The essential amino acids (EAAs) are the ones which cannot be synthesized by humans and thus need to be supplemented externally through the diet. The most common sources of these EAAs are meat, poultry, eggs, red meat, fish, soy, and dairy products. However, for vegetarian, lactose intolerant individuals, and for people allergic to eggs, there are limited options to obtain EAAs as most of the proteins from plants do not have complete EAAs. Recommendations by the WHO suggest that microalgae mainly Chlorella sp. and Spirulina sp. consist of a well balanced EAA content which is essential for human consumption.96  Microalgae are now being considered as an alternative protein source to meet the requirements of malnourished population. Microalgal proteins are rich in all the essential amino acids including the branched chain amino acids and leucine, valine, lysine, and isoleucine which aid in muscle protein health.97  The amino acid content in some microalgae has been found to be comparable to that of protein-rich sources such as eggs and soybean.66  Microalgal biomass or proteins derived from biomass have been successfully incorporated in biscuits, yogurt, sweets, noodles, bread, and beverages.43,98 Spirulina, a genus of filamentous cyanobacteria, is the most popular commercially available protein rich microalgae which is widely consumed as a dietary supplement in the form of powder, pills, paste, or tablets. Spirulina platensis has a protein content of 55 to 65% dry weight which is higher than that of dried skimmed milk (24%), chicken (36%), soy flour (37%), fish (24%), beef (22%), and peanuts (26%).99,100  Some other microalgae known for their protein content are Dunaliella tertiolecta (11.4%), Dunaliella salina (57%), Chlorella vulgaris (58%), Scenedesmus sp. (41%), and Tetraselmis sp. (36%).83,101,102  Microalgae, in particular marine microalgae, are considered as potential candidates for protein which is used for the generation of protein isolates and hydrolysates suitable for the food processing industry. The bioactive peptides in addition play a vital role in human health and nutrition. A few species of marine and freshwater microalgae like Chlorella vulgaris, Navicula incerta, Spirulina platensis, and Pavlova lutheri can be used to generate bioactive peptides. To date, several antioxidant and antimicrobial peptides have been isolated including the antioxidant peptide obtained from hydrolyzed Pavlova lutheri biomass and from the microbial hydrolysis of Chlamydomonas sp. which was reported to suppress Helicobacter pylori induced carcinogenesis.103  Hence, antioxidant peptides obtained from marine food sources are being considered as new antioxidant alternatives in the past few years.104–106 

The human body requires several micronutrients for survival along with macronutrients such as carbohydrates, proteins, and lipids. These micronutrients act as either co-enzymes or active electron/proton carriers during the breakdown of macronutrients. Hence deficiency of vitamins in humans results in various diseases like scurvy, beriberi, rickets, methylmalonic acidemia, etc. Vitamins B6, B9, and B12 play a vital role in the energy metabolism of humans by regulating the enzymes in mitochondria.107  Microalgae are known as non-conventional sources of vitamins and they contain both water- and fat-soluble vitamins. Some vitamins are found to be in higher concentrations in the microalgae than in traditional foods such as β-carotene (provitamin A), vitamin E (tocopherol), C (ascorbic acid), and B12.108,109 Chlorella and Dunaliella are rich in fat-soluble and B-group vitamins, D. tertiolecta is rich in vitamin C, B2, B12, folic, nicotinic acid, and vitamin E, and C. stigmatophora has a higher content of biotin, vitamin E, vitamin C, nicotinic acid, and pantothenic acid. The role of vitamins D3 and D2 in the cellular functions of microalgae is sparsely understood. There are reports hypothesizing that provitamin D acts as a UV-B receptor in plants. Vitamin D might be the end product of biological membrane degradation or damage due to UV radiation. Vitamin E acts as an antioxidant and also as a protective agent against photo-oxidative stress.110,111  Due to the inhibition of tocopherol synthesis in algae that are exposed to high light along with PS II inactivation, there is a rapid depletion of the tocopherol pool thus proving its role in protection against photo-oxidative stress.112–115  Vitamin K1 functions as a redox cofactor in green algae, plants, and some cyanobacteria. In particular, vitamin K1 acts as a secondary electron acceptor of photosystem I (PS-I) and it is partially bound to PS-I.116  Similarly, menaquinone (vitamin K2) acts as a secondary electron acceptor of PS-I in diatoms, red algae, archaeal species, and cyanobacteria.117  Vitamin C is involved in hormone biosynthesis, photosynthesis, and regeneration of anti-oxidants, and it also acts as a cofactor of many enzymes. Ascorbic acid acts as a cofactor for the de-epoxidase enzyme, which plays an important role in the xanthophyll cycle (violaxanthin–antheraxanthin–zeaxanthin or diadinoxanthin–diatoxanthin) resulting in the synthesis of the photoprotective xanthophyll, thus providing photo-protection for the algae.118  Even though in plants many processes like cell division and cell expansion are controlled by ascorbic acid, its role is not well established in microalgae, except for in red algae.119  Vitamin B1 has a major role in acetyl-CoA synthesis, pentose phosphate pathway (PPP), tricarboxylic acid cycle (TCA), isoprenoid biosynthesis pathway, and Calvin–Benson cycle.118  This vitamin is known for its defence mechanism against abiotic and biotic stressors in plants and in several microalgae. Vitamin B2 is an important precursor for flavo-coenzymes which are involved in various physiological processes like the circadian clock and as chromophores in blue light photoreceptors of fungi and plants. Flavo-coenzymes can catalyze redox processes involving electron transitions as well as a range of reactions such as photo-repair of thymidine dimers in photo-damaged DNA. Vitamin B3 is required for assimilatory nitrate reductase activity in photo-autotrophs120,121  besides its physiological role in the form of NADH.122 

Phytosterols are a family of triterpenoids which are the derivatives of squalene. In higher plants and microalgae, the first compound to be formed in the cyclization of squalene in the cyclic biosynthetic pathway is cycloartenol.123  A few microalgal species have been reported to contain cholesterol such as Chaetoceros sp. and Skeletonema sp. The presence of sterols makes microalgae an attractive aqua feed for promoting the growth of fish and oysters. Other sterols such as brassicasterol, campesterol, sitosterol, and stigmasterol have been reported in Thalassiosira and Pavlova species. It is well known that high levels of cholesterol and LDL are of high risk for heart and coronary diseases. Microalgae consist of different types of sterols that are typical of each species. Besides, studies on bivalve production have suggested that the type and quantity of sterols present in dietary microalgae are directly related to the bivalve growth rate.124  Hence, studies on sterol compositions of unexplored microalgae, including new and rare species, may be of great value. The major sterols in red microalgae are C-27 compounds; cholesterol occurs in substantial amounts and is usually the primary sterol. Desmosterol and dehydrocholesterol are also present in high concentrations and may even be the major sterol in the Gigartinales class.125  On the other hand, in brown algae, the dominant sterol is fucosterol, and cholesterol is present only in low amounts, such as in Bifurcaria bifurcate, Cladostephus hirsitus, Dictyota dichotoma, and Cystoseira sedoides. In green algae, there is no single major sterol, and the dominant sterol seems to vary within orders and families.125  Moreover, in green marine microalgae such as Isochrysis galbana and Diacronema vlkianum, sitosterol was the main sterol identified. Sterols are not absorbed by the body but inhibit the absorption of cholesterol. This feature is primarily due to the presence of double bonds in stigmasterol and an alkyl group in campesterol, stigmasterol, and b-sitosterol.126  Sitostanol is a product that is obtained by chemical reduction of sitosterol and seems to have a superior effect in the reduction of blood cholesterol levels. Since phytosterols reduce the level of cholesterol in the blood, they are valuable in the development of functional foods.

Polyphenols are a family of polar biomolecules comprised of phenolic acids, flavonoids, isoflavonoids, stilbenes, and lignans.42  The basic structure of polyphenols contains one or more hydroxyl groups bound to an aromatic ring. These biomolecules exhibit various biological properties including antioxidant, anti-inflammatory, anti-cancer, anti-allergic, anti-diabetes, anti-aging, and antimicrobial activities along with prevention of cardiovascular disorders.42,127,128  Polyphenols are antioxidant compounds with health benefits reported to range from improved cardiovascular health to protection against certain cancers and Alzheimer's disease.129  Due to their characteristic bitter flavor, the polyphenols can be incorporated in small amounts in processed foods. The addition of other compounds such as sucrose, sucralose, polydextrose, and milk can reduce the bitterness, astringency, and characteristic flavor of polyphenol extracts. It has been reported that the total phenolic content of microalgae such as Arthrospira platensis and Chlorella vulgaris is similar to that of several fruits and vegetables.130,131  The important bioactive metabolites from microalgae and their applications are summarized in Table 1.

Table 1.1

Bioactive metabolites extracted from microalgae and their food, feed, and therapeutic applications.a

Bioactive metabolitesMicroalgal class/speciesApplicationsRef.
  • Chlorophyll a and b

 
  • Chlorophyceae

  • Cyanophyceae

 
  • Natural colorant, anti-oxidant, anti-carcinogenic, anti-genotoxic, anti-inflammatory, and anti-mutagenic properties, stimulate liver function, increase bile secretion

 
 
 
  • β-Carotene

 
  • Arthrospira platensis

  • Dunaliella salina

  • Nannochloropsis gaditana

  • Synechococcus spp.

 
  • Natural colorant, food additive, anti-oxidant, provitamin A source, anti-cancer and anti-inflammatory properties, preventive against liver fibrosis

 
 
 
  • Lutein

  • Astaxanthin

  • Zeaxanthin Violaxanthin Fucoxanthin

 
  • Chlorella protothecoids

  • Chlorella zofingiensis

  • Dunaliella tertiolecta

  • Haematococcus pluvialis

  • Phaeodactylum tricornutum

 
  • Natural colorant, anti-tumoral, anti-oxidant, anti-inflammatory, preventive against atheroscelerosis, retinal neural damage, maculopathy, and cataracts

 
 
 
  • Phycocyanin

  • Phycoerythrin

  • Allophycocyanin

 
  • Anabaena cylindrical

  • Arthrospira platensis

  • Porphyridium aerugineum

  • Rhodella spp.

 
  • Natural colorant, food additive, pharmaceutical dye, anti-oxidant, anti-inflammatory, hepatoprotective, radical scavenging properties, immunological markers, and fluorescence properties

 
 
 
  • PUFAs –

  • ALA (ω-3)

  • DHA (ω-3)

  • EPA (ω-3)

  • GLA (ω-6)

  • ARA (ω-9)

 
  • Arthrospira spp.

  • Isochrysis spp.

  • Nannochloropsis spp. Phaeodactylum spp. Nitzschia spp.

  • Porphyridium spp.

  • Schizochytrium spp.

 
  • Dietary significance, essential fatty acids, cholesterol management, fluidity of cell membranes, regulation of blood pressure and glucose level, anti-inflammatory, preventive against arthritis, cardiovascular disease, and Alzheimer's disease

 
 
 
  • Proteins and peptides

 
  • Arthrospira platensis

  • Dunaliella salina

  • Chlorella vulgaris

  • Scenedesmus spp.

  • Tetraselmis spp.

 
  • Edible protein, essential amino acids, poultry and aquaculture feed, anti-microbial, anti-cancer, anti-oxidant, hypo-glycemic, and anti-hypertensive properties

 
 
 
  • Exopolysaccharides

 
  • Arthrospira platensis

  • Chlorella stigmatophora Gyrodinium impudicum Porphyridium spp. Phaeodactylum tricornutum

 
  • Food additives, anti-microbial, anti-cancer, anti-oxidant, anti-inflammatory, anti-coagulant properties, bio-lubricants, plant growth bio-stimulants

 
 
 
  • Vitamins

 
  • Arthrospira platensis

  • Chlorella spp.

  • Dunaliella spp.

 
  • Dietary supplements, anti-oxidants, anti-inflammatory, flavo-coenzymes, and cosmetics, cardiovascular health, skin health, and neurological health

 
 
 
  • Phytosterols

 
  • Pavlova lutheri

  • Thalassiosira sp.

  • Isochrysis galbana

  • Chlamydomonas reinhardtii

  • Diatoms

  • Dinoflagellates

 
  • Functional foods, blood cholesterol management, cardiovascular health, anti-cancer, cytotoxic, and anti-inflammatory properties

 
 
 
  • Polyphenols

 
  • Arthrospira platensis

  • Chlorella vulgaris

  • Phaeodactylum tricornutum

  • Tetraselmis suecica

  • Haematococcus spp.

 
  • Anti-oxidant, anti-inflammatory, anti-cancer, anti-allergic, anti-diabetic, anti-aging, and anti-microbial activity, prevention of cardiovascular disorders

 
 
Bioactive metabolitesMicroalgal class/speciesApplicationsRef.
  • Chlorophyll a and b

 
  • Chlorophyceae

  • Cyanophyceae

 
  • Natural colorant, anti-oxidant, anti-carcinogenic, anti-genotoxic, anti-inflammatory, and anti-mutagenic properties, stimulate liver function, increase bile secretion

 
 
 
  • β-Carotene

 
  • Arthrospira platensis

  • Dunaliella salina

  • Nannochloropsis gaditana

  • Synechococcus spp.

 
  • Natural colorant, food additive, anti-oxidant, provitamin A source, anti-cancer and anti-inflammatory properties, preventive against liver fibrosis

 
 
 
  • Lutein

  • Astaxanthin

  • Zeaxanthin Violaxanthin Fucoxanthin

 
  • Chlorella protothecoids

  • Chlorella zofingiensis

  • Dunaliella tertiolecta

  • Haematococcus pluvialis

  • Phaeodactylum tricornutum

 
  • Natural colorant, anti-tumoral, anti-oxidant, anti-inflammatory, preventive against atheroscelerosis, retinal neural damage, maculopathy, and cataracts

 
 
 
  • Phycocyanin

  • Phycoerythrin

  • Allophycocyanin

 
  • Anabaena cylindrical

  • Arthrospira platensis

  • Porphyridium aerugineum

  • Rhodella spp.

 
  • Natural colorant, food additive, pharmaceutical dye, anti-oxidant, anti-inflammatory, hepatoprotective, radical scavenging properties, immunological markers, and fluorescence properties

 
 
 
  • PUFAs –

  • ALA (ω-3)

  • DHA (ω-3)

  • EPA (ω-3)

  • GLA (ω-6)

  • ARA (ω-9)

 
  • Arthrospira spp.

  • Isochrysis spp.

  • Nannochloropsis spp. Phaeodactylum spp. Nitzschia spp.

  • Porphyridium spp.

  • Schizochytrium spp.

 
  • Dietary significance, essential fatty acids, cholesterol management, fluidity of cell membranes, regulation of blood pressure and glucose level, anti-inflammatory, preventive against arthritis, cardiovascular disease, and Alzheimer's disease

 
 
 
  • Proteins and peptides

 
  • Arthrospira platensis

  • Dunaliella salina

  • Chlorella vulgaris

  • Scenedesmus spp.

  • Tetraselmis spp.

 
  • Edible protein, essential amino acids, poultry and aquaculture feed, anti-microbial, anti-cancer, anti-oxidant, hypo-glycemic, and anti-hypertensive properties

 
 
 
  • Exopolysaccharides

 
  • Arthrospira platensis

  • Chlorella stigmatophora Gyrodinium impudicum Porphyridium spp. Phaeodactylum tricornutum

 
  • Food additives, anti-microbial, anti-cancer, anti-oxidant, anti-inflammatory, anti-coagulant properties, bio-lubricants, plant growth bio-stimulants

 
 
 
  • Vitamins

 
  • Arthrospira platensis

  • Chlorella spp.

  • Dunaliella spp.

 
  • Dietary supplements, anti-oxidants, anti-inflammatory, flavo-coenzymes, and cosmetics, cardiovascular health, skin health, and neurological health

 
 
 
  • Phytosterols

 
  • Pavlova lutheri

  • Thalassiosira sp.

  • Isochrysis galbana

  • Chlamydomonas reinhardtii

  • Diatoms

  • Dinoflagellates

 
  • Functional foods, blood cholesterol management, cardiovascular health, anti-cancer, cytotoxic, and anti-inflammatory properties

 
 
 
  • Polyphenols

 
  • Arthrospira platensis

  • Chlorella vulgaris

  • Phaeodactylum tricornutum

  • Tetraselmis suecica

  • Haematococcus spp.

 
  • Anti-oxidant, anti-inflammatory, anti-cancer, anti-allergic, anti-diabetic, anti-aging, and anti-microbial activity, prevention of cardiovascular disorders

 
 
a

Abbreviations: ALA – α-linolenic acid; ARA – arachidonic acid; DHA – docosahexaenoic acid; EPA – eicosapentaenoic acid; GLA – γ-linolenic acid.

The food and animal feed industries are reportedly the main markets for bulk microalgal biomass. Although the market for microalgal nutraceuticals and functional foods is continuously growing,149,150  recent studies have shown that commercial production of high-value bio-products is not yet cost-effective.151  The major bottlenecks in large scale microalgal biomass production are high capital costs associated with closed photobioreactors, dewatering large volumes of dilute microalgal cultures, and lack of appropriate technologies to extract the different biomass fractions such as proteins, carbohydrates, and lipids.152,153  Thus, the focus of research groups and industries in recent years has been towards developing economical downstream processing methods and innovative green technologies for complete valorization of biomass.154,155  This has led to the concept of biorefinery which is analogous to the fossil fuel industry, wherein the microalgae-derived biomass could serve as a potential feedstock for obtaining fine chemicals and bioenergy. In the case of petroleum, the production of valuable petrochemicals such as plastics, paints, lubricants, etc. makes the overall refining process a profitable endeavor.156  Similarly, the cost of microalgal cultivation and biomass harvesting can be covered through the production of high market value bioactive metabolites resulting from fractional processing of biomass. An ideal microalgae-based biorefinery would integrate different unit operations such as dewatering, extraction, and fractionation of biomass to produce a variety of bio-based products and bio-energy.157 

The high value biomolecules produced from microalgae such as polyunsaturated fatty acids, proteins, pigments, etc. have become popular sources of nutraceuticals. Their demand is driven by two factors: the rising consumer need for healthy food and the increasing need for high value metabolites in pharmaceutical, cosmetic, and industrial applications to achieve specific technical properties (like texture, dyes, and antioxidants). Most recent additions are phytosterols, peptides, vitamins, and polyphenols which are in the race to meet their market and achieve economic competitiveness. Further research and technical innovations are required to make the overall process of microalgal cultivation, harvesting, strain improvement, and extraction and purification of selective metabolites an economical and environmentally friendly process. In this context, the development of a biorefinery approach has come up as an emerging methodology to valorize microalgal biomass for food, feed, and fuel.

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