**4.2 Heterotrophic metabolism**

*Biotechnological Applications of Biomass*

**Metabolic mode Energy** 

Mixotrophic Light &

*Microalgal metabolic requirements.*

**source**

organic

Photo-autotrophic Light Inorganic Obligatory Fixed

Heterotrophic Organic Organic Not required Switch between

Photoheterotrophic Light Organic Obligatory Switch between

Inorganic & organic

or chemolithotrophic [31].

ATP synthesis by oxidative phosphorylation.

organisms can harness light energy with the help of photosynthetic pigments and convert it to chemical bond energy in the form of ATP (photophosphorylation). Autotrophy is the ability of PMOs to use inorganic carbon in the form of CO2 as the sole source of carbon to synthesize organic compounds necessary to build cell components. This is also referred to as carbon-autotrophy to distinguish the ability of some organisms to use molecular nitrogen as the sole source of nitrogen. Such organisms are referred to as nitrogen autotrophs. However, autotrophy as used in this chapter is carbon autotrophy. This is a property that is present primarily, in

**Carbon source Light** 

**availability**

Not obligatory **Metabolism availability**

sources

sources

Simultaneous utilization

Aside from these organisms, all of which are photosynthetic, several groups of non-photosynthetic bacteria can grow using CO2 as the sole source of carbon by their ability to oxidize inorganic compounds. Such organisms are chemoautotrophic

CO2 is the end-product of aerobic respiration, a process that releases the energy of respiratory substrates. Carbon dioxide is, therefore, poor in energy content. In autotrophic metabolism, this energy-poor compound is used to build organic molecules which are much richer in energy content. Therefore, It is noted that the conversion of CO2 to organic compounds requires the input of energy from an external source. The ultimate source in the case of photosynthesis is radiant energy and in the case of chemolithotrophy is the oxidation energy of inorganic chemical compounds. In either case, the immediate source of energy for driving the endergonic reaction involved in the conversion of CO2 to organic compounds is

In photosynthesis, ATP is generated with the help of photosynthetic pigments through a process known as photophosphorylation. In chemoautotrophy, the energy of oxidation of inorganic compounds is channelized into the respiratory chain for

Thus, autotrophic metabolism consists of two sets of reactions viz. (1) the ATP and the reducing force are generated and, (2) they are used for the reduction of CO2

The reactions in (1) are different in phototrophic and non-phototrophic autotrophs. But the reactions in (2) are common between the two groups. In the majority of autotrophs, the reactions involved in the reduction of CO2 proceed via a cyclic pathway, known as the reductive pentose phosphate pathway or, more commonly, as the Calvin-Benson cycle, or simply the Calvin cycle, although other pathways are also known to operate in some organisms, both in the phototrophic green plants and bacteria. The reduction of CO2 to yield organic compounds is commonly known as

plants, algae, and phototrophic bacteria including cyanobacteria [30].

**370**

ATP [32].

**Table 3.**

to organic compounds.

CO2-fixation [32, 33].

The supply of sufficient light for massive growth is the main goal and a limiting factor for microalgal cultivation. To ignore the requirement for illumination and present the possibility of high cell concentration, points at heterotrophic cultivation as a promising, efficient, and sustainable strategy for certain microalgae to produce metabolites of value by using carbon substances as the sole carbon and energy source. The optimized preliminary cell culturing of microalgae species is an important stage in culturing microalgae biomass at the commercial scale. The growth environment during the culturing process can be [32] either autotrophic (inorganic carbon) or heterotrophic (organic carbon) depending upon the nature of cells and their growth tendencies. Heterotrophic and mixotrophic microalgae are more capable of growing much faster with higher cellular oil accumulation as compared to autotrophic microalgae species. However, heterotrophic microalgae require organic carbon sources like glycerol, glucose, or acetate as a sole source of carbon for growth, which is responsible for about 80% of the costs of culture media [33]. The metabolism of respiration is applied to produce energy. The respiration rates, intimately geared to the growth and division, are determined by the oxidization of organic substrates of the given microalgae [32]. Glucose provides the organic carbon needed and it is preferred because of its high energy density compared to other sources. The oxidative assimilation of glucose employs either the Embden–Meyerhof–Parnas (EMP) pathway or the pentose phosphate (PP) pathway depending on the cycle position. During the dark cycle, PMOs assimilate and metabolize glucose via the PP pathway. However, during the daytime cycle, glycolysis in the cytosol is via the EMP pathway [34]. The growth rate, lipid content, and the ATP of microalgae under the heterotrophic metabolic strategy are higher compared to those under the photoautotrophic metabolic strategy but depend mainly on the PMO's species and strain used. The PMO's growth is steady and rapid in a nutrient-rich culture media using a high level of system control, to achieve biomass production of 50–100 g L−1 in heterotrophy which is higher than that achieved in photoautotrophy [35].

Heterotrophic metabolism eliminates the two main problems associated with autotrophic metabolism viz. (i) it allows the use of practically any vessel as a bioreactor, and (ii) low energy and high yield, as major outcomes, giving a significant reduction in costs for the process. Cost-effectiveness and relative simplicity of operations and daily maintenance are the main attractions of the heterotrophic growth approach. A significant benefit is that it is possible to obtain, heterotrophically, high densities of microalgae cells that provides an economically feasible method for large scale, mass production cultivation [34].

Heterotrophy has its drawbacks viz. (1) The microalgae species and strains that can grow by the heterotrophic strategy are limited; (2) Increasing energy expenses and costs by adding organic carbon substrate; (3) Contamination and competition with local microorganisms; (4) Inhibition of growth by excess organic substrate; and (5) Inability to produce light-induced metabolites [35]. Nonetheless, heterotrophic cultures are gaining increasing application for producing a wide variety of microalgal metabolites from bench experiments to commercial scale.

#### **4.3 Mixotrophic metabolism**

Mixotrophic cultivation of microalgae strategies provides both carbon dioxide and organic carbon simultaneously and both chemoheterotrophic and photoautotrophic metabolisms operate concurrently. Microalgae biomass produced by this approach has high density and contains high-value lipids, proteins, carbohydrates, and

pigments; and the product range is very versatile [7–10]**.** These products range from high-value nutraceuticals, food supplements, and cosmetics to the lower value commodities biofuels, food, fertilizer, and application in wastewater treatment [10–12].

### **4.4 Microalgal metabolites**

Microalgal biomass contains considerable amounts of bioactive molecules such as carotenoids (astaxanthins, β-carotenes, and xanthophylls), omega-3 fatty acids, polysaccharides, and proteins, which can be used in several applications as colorants, pharmaceuticals, food, food additives, and feed and as bioplastics.

#### *4.4.1 Carotenoids*

Microalgae produce carotenoids and all known xanthophylls found in terrestrial plants (e.g., zeaxanthin, lutein, antheraxanthin). Astaxanthin is a carotenoid pigment that occurs in microalgae, trout, yeast, and shrimp, among other sea creatures. It is found in abundance in Pacific salmon and the fish appears pinkish due to the presence of astaxanthin. Astaxanthin is an antioxidant; it is said to have many health benefits. Carotenoids as accessory pigments, capture light energy during photosynthesis and promote photoprotection. Stains of *Nannochloropsis sp., Rhodotorula glutinis,* and *Neochloris oleoabundans* have high contents of carotenoids. The red ketocarotenoid, astaxanthin (3, 30-dihydroxy-b,β-carotene 4,40-dione) is an antioxidant and the green microalga *Haematococcus pluvialis* is said to be a good natural source of astaxanthin [36–40].

#### *4.4.2 Lutein*

Lutein, a xanthophyll, is one of the many known naturally occurring carotenoids. Lutein is synthesized only by plants and is found in large quantities in green leafy vegetables like kale, spinach, yellow carrots, and in dietary supplements. The lutein-rich microalgae *Scenedesmus almeriensis* and *Desmodesmus sp.* could be considered as promising sources of lutein for their tolerance to harsh environmental growth conditions. It is a food colorant with the potential for preventing cancer. It is used for maintaining eye health and to reduce the risk of retinal macular degeneration. The performance of three *Chlorella* species on the production of biomass, lipid, and lutein showed high productivities, presenting the microalgae as a promising resource for these products [41].

### *4.4.3 Poly-unsaturated fatty acids*

Microalgae are the dominant sources of polyunsaturated fatty acids in the marine food chain. *Schizochytrium sp.* is a type of marine microalgae with the natural capacity to produce oil extremely rich in docosahexaenoic acid (DHA) omega-3 fatty acids [42]. DHA-rich extracts from *Schizochytrium sp*. are presented as a feed supplement to swine for their muscle tissue development and as a raw material for the production of aquafeed. *N. oculata* and *P. tricornotum* have a favorable omega-3: omega-6 ratio that is adequate to enrich food [43]. The growth conditions are deliberately manipulated to achieve the desired fatty acid composition of the biomass. At low nutrient concentrations, the microalgal lipids accumulated are rich in triglycerides and are more suitable for biodiesel production; and high nutrients supply to the growth medium leads to the accumulation of long-chain unsaturated fatty acids [44]. DHA applications include healthcare, pharmaceutical, and food & beverage sectors. Within this segment, the pharmaceutical application holds a larger share of it [45].

**373**

**Figure 14.**

*The structure of triglyceride showing the simple and mixed types.*

*Microalgae: The Multifaceted Biomass of the 21st Century*

The acetyl-CoA condensation to fatty acyls is one of the methods by which biohydrocarbons are produced in-situ biotic organisms. The second biohydrocarbon production pathway is the isopentenyl pyrophosphate (IPP) condensation to higher isoprenoids, which is responsible for the diverse isoprene derivatives, many of which are suitable for fuels or fuel additives due to their desirable cetane and pour point and other fuel properties [5]. The low-to-zero-oxygen content of isoprenoids results in energy densities similar to the alkanes in current diesel fuels and diversity of ring structures affords lower cloud points [46, 47]. Additionally, it has been found that slight modifications to enzymes involved in the final steps of higher isoprenoid synthesis can result in subtle product variants with distinct thermochemical and thermophysical properties [47]. The precursors for the majority of these compounds are metabolic intermediates in photosynthetic microorganisms (PMOs). Genetic engineering of microalgae and

cyanobacteria would be required to enhance the productivity of PMOs [5].

Triglycerides are lipids or waxes, formed by biochemically combining glycerol and fatty acids in the ratio of 1: 3 respectively. This combination may be a simple type or a mixed type. Triglycerides in which the glycerol backbone is attached to three molecules of the same fatty acid are referred to as simple triglycerides. Typical in this category is tripalmitin, C3H5(OCOC15H31)3. Only a few of the glycerides occurring in nature are of the simple type; most are mixed triglycerides (see **Figure 14**) [48]. Based on saturation and unsaturation of the attached fatty acids, triglycerides can be classified as saturated, monounsaturated, and polyunsaturated. In saturated triglycerides, all the fatty acids are saturated. Saturated fats abound in

*DOI: http://dx.doi.org/10.5772/intechopen.94090*

*4.4.4 Microalgal triglycerides*

*Microalgae: The Multifaceted Biomass of the 21st Century DOI: http://dx.doi.org/10.5772/intechopen.94090*

The acetyl-CoA condensation to fatty acyls is one of the methods by which biohydrocarbons are produced in-situ biotic organisms. The second biohydrocarbon production pathway is the isopentenyl pyrophosphate (IPP) condensation to higher isoprenoids, which is responsible for the diverse isoprene derivatives, many of which are suitable for fuels or fuel additives due to their desirable cetane and pour point and other fuel properties [5]. The low-to-zero-oxygen content of isoprenoids results in energy densities similar to the alkanes in current diesel fuels and diversity of ring structures affords lower cloud points [46, 47]. Additionally, it has been found that slight modifications to enzymes involved in the final steps of higher isoprenoid synthesis can result in subtle product variants with distinct thermochemical and thermophysical properties [47]. The precursors for the majority of these compounds are metabolic intermediates in photosynthetic microorganisms (PMOs). Genetic engineering of microalgae and cyanobacteria would be required to enhance the productivity of PMOs [5].

#### *4.4.4 Microalgal triglycerides*

*Biotechnological Applications of Biomass*

**4.4 Microalgal metabolites**

natural source of astaxanthin [36–40].

ing resource for these products [41].

*4.4.3 Poly-unsaturated fatty acids*

*4.4.1 Carotenoids*

*4.4.2 Lutein*

pigments; and the product range is very versatile [7–10]**.** These products range from high-value nutraceuticals, food supplements, and cosmetics to the lower value commodities biofuels, food, fertilizer, and application in wastewater treatment [10–12].

Microalgal biomass contains considerable amounts of bioactive molecules such as carotenoids (astaxanthins, β-carotenes, and xanthophylls), omega-3 fatty acids, polysaccharides, and proteins, which can be used in several applications as colorants, pharmaceuticals, food, food additives, and feed and as bioplastics.

Microalgae produce carotenoids and all known xanthophylls found in terrestrial

plants (e.g., zeaxanthin, lutein, antheraxanthin). Astaxanthin is a carotenoid pigment that occurs in microalgae, trout, yeast, and shrimp, among other sea creatures. It is found in abundance in Pacific salmon and the fish appears pinkish due to the presence of astaxanthin. Astaxanthin is an antioxidant; it is said to have many health benefits. Carotenoids as accessory pigments, capture light energy during photosynthesis and promote photoprotection. Stains of *Nannochloropsis sp., Rhodotorula glutinis,* and *Neochloris oleoabundans* have high contents of carotenoids. The red ketocarotenoid, astaxanthin (3, 30-dihydroxy-b,β-carotene 4,40-dione) is an antioxidant and the green microalga *Haematococcus pluvialis* is said to be a good

Lutein, a xanthophyll, is one of the many known naturally occurring carotenoids. Lutein is synthesized only by plants and is found in large quantities in green leafy vegetables like kale, spinach, yellow carrots, and in dietary supplements. The lutein-rich microalgae *Scenedesmus almeriensis* and *Desmodesmus sp.* could be considered as promising sources of lutein for their tolerance to harsh environmental growth conditions. It is a food colorant with the potential for preventing cancer. It is used for maintaining eye health and to reduce the risk of retinal macular degeneration. The performance of three *Chlorella* species on the production of biomass, lipid, and lutein showed high productivities, presenting the microalgae as a promis-

Microalgae are the dominant sources of polyunsaturated fatty acids in the marine food chain. *Schizochytrium sp.* is a type of marine microalgae with the natural capacity to produce oil extremely rich in docosahexaenoic acid (DHA) omega-3 fatty acids [42]. DHA-rich extracts from *Schizochytrium sp*. are presented as a feed supplement to swine for their muscle tissue development and as a raw material for the production of aquafeed. *N. oculata* and *P. tricornotum* have a favorable omega-3: omega-6 ratio that is adequate to enrich food [43]. The growth conditions are deliberately manipulated to achieve the desired fatty acid composition of the biomass. At low nutrient concentrations, the microalgal lipids accumulated are rich in triglycerides and are more suitable for biodiesel production; and high nutrients supply to the growth medium leads to the accumulation of long-chain unsaturated fatty acids [44]. DHA applications include healthcare, pharmaceutical, and food & beverage sectors. Within this segment, the pharmaceutical application holds a larger share of it [45].

**372**

Triglycerides are lipids or waxes, formed by biochemically combining glycerol and fatty acids in the ratio of 1: 3 respectively. This combination may be a simple type or a mixed type. Triglycerides in which the glycerol backbone is attached to three molecules of the same fatty acid are referred to as simple triglycerides. Typical in this category is tripalmitin, C3H5(OCOC15H31)3. Only a few of the glycerides occurring in nature are of the simple type; most are mixed triglycerides (see **Figure 14**) [48]. Based on saturation and unsaturation of the attached fatty acids, triglycerides can be classified as saturated, monounsaturated, and polyunsaturated. In saturated triglycerides, all the fatty acids are saturated. Saturated fats abound in

**Figure 14.** *The structure of triglyceride showing the simple and mixed types.*

many animal products such as butter, cheese, cream, and fatty meats, ice cream, and whole milk. In monounsaturated triglycerides most of the fatty acids are monounsaturated. Vegetable oils such as canola oil, olive oil, peanut oil, and sesame oil have high levels of monounsaturated fats and polyunsaturated triglycerides. Omega-3 and omega-6 fatty acids are polyunsaturated.

Microalgae are a promising renewable resource for green production of triacylglycerols (TAGs), which can be used as a biofuel feedstock. Nitrogen starvation is the most effective strategy to induce TAG biosynthesis in microalgae [48]. One of the best microalgae for lipid production is *Botryococcus braunii* Kutzing, above 70% of lipid in its cell content. Whereas other microalgae like *Scenedesmus sp., Chlorella sp*., and *Nanochloropsis sp*. also produce lipid up to 40% [49–51].

#### *4.4.5 Microalgal phospholipids*

Phospholipids are made up of four components viz. fatty acids, a platform to which the fatty acids are attached, phosphate, and an alcohol attached to the phosphate. Phospholipids may be built on either glycerol or sphingosine framework. Phospholipids built on glycerol framework are called phosphoglycerides (or glycerophospholipids). A phosphoglyceride consists of a glycerol molecule, two fatty acids, a phosphate, and choline, which is an alcohol. Phosphoglycerides are the most abundant phospholipid molecules found in cell membranes. The phospholipids built on sphingosine framework are referred to as sphingolipids or glycolipids, depending on the number of glucose or galactose molecules they contain; and lipoproteins, which are complexes of cholesterol, triglycerides, and proteins that transport lipids in the aqueous environment of the bloodstream. These are complex lipids. The algae contain three major phospholipids, phosphatidylglycerol (PG), phosphatidylethanolamine (PE), and phosphatidylcholine (PC). Phospholipids are synthesized by both prokaryotic and eukaryotic organisms. They are the major component of most eukaryotic cell membranes, which play a fundamental role in compartmentalizing the biochemistry of life [52]. The hydroxyl groups at positions C-1 and C-2 in phosphoglycerides are esterified to the carboxyl groups of the two fatty acid chains. The hydroxyl group at position C-3 hydroxyl group of the glycerol backbone is esterified to phosphoric acid. At this extent of conversion, the product is phosphatidic acid, which is the simplest phosphoglyceride. Phosphatidic acid now serves as the backbone on which most phosphoglycerides are derived having moieties such as serine, ethanolamine, choline, glycerol, and the inositol. Consequently, we have phosphatidylserine, phosphatidylethanolamine, phosphatidylcholine, phosphatidylglycerol, and phosphatidylinositol respectively (see **Figure 15**) [52].

#### *4.4.6 Vitamins and fine chemicals from microalgal biomass*

Metabolites from both microalgae and cyanobacteria have attended to both human and animal health and food needs and these microorganisms have become attractive resources for bioactive natural products that have wide applications in pharmaceutical, food, and chemical industries. Algae-derived bioactive substrates are employed for drug screening, given their tremendous structural diversity and biological availability. Microalgae biomass has a wide range of physiological and biochemical characteristics and contains 50–70% protein compared to 50% in meat, and 15–17% in wheat, with 30% lipids, more than 40% glycerol, 8–14% carotene, and a reasonably high levels of vitamins B1, B2, B3, B6, B12, E, K, D, and others [54–56].

Microalgae that have been cultivated on commercial scales and are available include *Chlorella, Dunaliella, Nannochloris, Nitzschia, Crypthecodinium, Schizochytrium, Tetraselmis, Skeletonema,* etc. and the cyanobacterium, *Spirulina,* and

**375**

**Figure 15.**

antifungal, and antiviral activity [55, 56].

**4.5 Microalgal biomass production limiting factors**

*Microalgae: The Multifaceted Biomass of the 21st Century*

a host of others. Most of the commercially produced microalgal biomass is presented to the market as a food supplement, and they are presented as tablets and capsules. Breakfast cereals, noodles, beverages, wines, and cosmetics now contain microalgae and their extracts. More than 75% of pharmaceutical product development is carried out by the microalgal food supplement production outfits. In the recent several years, microalgal and cyanobacterial research has explored diverse cultivation protocols aimed at improving growth rates, biomass yields, and accumulating metabolites for high nutritional value, and high-value chemicals (pigments and vitamins) [55]. Many more bioactive metabolites have been reported in microalgae. Dried microalgae biomass could be used as high-protein feeds for animals such as shrimp and fish, and microalgal biomass is a significant resource for cytotoxic agents with applications in cancer chemotherapy. The blooms of *Phaeocystis sp*., a marine microalga have antibiotic substances listed therein. *Phaeocystis pouchetii* produces acrylic acid, which makes up to 7.0% of its dry weight. The antibiotic metabolites so produced migrate in the food chain through the digestive system of some Antarctic marine animal species. Also, the alga *Dunaliella sp.* produces β-carotene and certain vitamins, which have boosted the Mariculture activities. Some cyanobacteria and microalgae such as *Ochromonas* sp. and *Prymnesium parvum* produce toxins, which may have the potential for pharmaceutical applications. These marine cyanobacteria produce bioactive metabolites such as acetogenins, bromophenols, fatty acids, terpenes, sterols, alkaloids, etc. with antibiotics, and antifungal activities. Diverse strains of cyanobacteria produce intracellular and extracellular metabolites with bioactive functions such as antitumor, anti-inflammatory, antialgal, antibacterial,

*Sphingolipids and phospholipids: The classification of sphingolipids is based on the group attached to the sphingosine (LCB) backbone (a). Sphingomyelin (b) and ceramides (c-e) differ in fatty acid length, unsaturation, and in the type of attached head group and hydroxylation. Phospholipids with glycerol* 

*framework: (f) phosphatidylethanolamine, (g) phosphatidylcholine [53].*

Abiotic, Biotic, and process-related factors influence the growth of algae. Some of the abiotic factors are illumination and luminous intensity, daytime to night-time ratio, the temperature of the culture medium, nutrient availability, O2, and CO2 mass transfer, pH value, the hydraulic retention time (HRT), salinity, and presence

*DOI: http://dx.doi.org/10.5772/intechopen.94090*

*Microalgae: The Multifaceted Biomass of the 21st Century DOI: http://dx.doi.org/10.5772/intechopen.94090*

#### **Figure 15.**

*Biotechnological Applications of Biomass*

*4.4.5 Microalgal phospholipids*

Omega-3 and omega-6 fatty acids are polyunsaturated.

and *Nanochloropsis sp*. also produce lipid up to 40% [49–51].

and phosphatidylinositol respectively (see **Figure 15**) [52].

*4.4.6 Vitamins and fine chemicals from microalgal biomass*

Metabolites from both microalgae and cyanobacteria have attended to both human and animal health and food needs and these microorganisms have become attractive resources for bioactive natural products that have wide applications in pharmaceutical, food, and chemical industries. Algae-derived bioactive substrates are employed for drug screening, given their tremendous structural diversity and biological availability. Microalgae biomass has a wide range of physiological and biochemical characteristics and contains 50–70% protein compared to 50% in meat, and 15–17% in wheat, with 30% lipids, more than 40% glycerol, 8–14% carotene, and a reasonably high levels of vitamins B1, B2, B3, B6, B12, E, K, D, and others [54–56]. Microalgae that have been cultivated on commercial scales and are available include *Chlorella, Dunaliella, Nannochloris, Nitzschia, Crypthecodinium,* 

*Schizochytrium, Tetraselmis, Skeletonema,* etc. and the cyanobacterium, *Spirulina,* and

many animal products such as butter, cheese, cream, and fatty meats, ice cream, and whole milk. In monounsaturated triglycerides most of the fatty acids are monounsaturated. Vegetable oils such as canola oil, olive oil, peanut oil, and sesame oil have high levels of monounsaturated fats and polyunsaturated triglycerides.

Microalgae are a promising renewable resource for green production of triacylglycerols (TAGs), which can be used as a biofuel feedstock. Nitrogen starvation is the most effective strategy to induce TAG biosynthesis in microalgae [48]. One of the best microalgae for lipid production is *Botryococcus braunii* Kutzing, above 70% of lipid in its cell content. Whereas other microalgae like *Scenedesmus sp., Chlorella sp*.,

Phospholipids are made up of four components viz. fatty acids, a platform to which the fatty acids are attached, phosphate, and an alcohol attached to the phosphate. Phospholipids may be built on either glycerol or sphingosine framework. Phospholipids built on glycerol framework are called phosphoglycerides (or glycerophospholipids). A phosphoglyceride consists of a glycerol molecule, two fatty acids, a phosphate, and choline, which is an alcohol. Phosphoglycerides are the most abundant phospholipid molecules found in cell membranes. The phospholipids built on sphingosine framework are referred to as sphingolipids or glycolipids, depending on the number of glucose or galactose molecules they contain; and lipoproteins, which are complexes of cholesterol, triglycerides, and proteins that transport lipids in the aqueous environment of the bloodstream. These are complex lipids. The algae contain three major phospholipids, phosphatidylglycerol (PG), phosphatidylethanolamine (PE), and phosphatidylcholine (PC). Phospholipids are synthesized by both prokaryotic and eukaryotic organisms. They are the major component of most eukaryotic cell membranes, which play a fundamental role in compartmentalizing the biochemistry of life [52]. The hydroxyl groups at positions C-1 and C-2 in phosphoglycerides are esterified to the carboxyl groups of the two fatty acid chains. The hydroxyl group at position C-3 hydroxyl group of the glycerol backbone is esterified to phosphoric acid. At this extent of conversion, the product is phosphatidic acid, which is the simplest phosphoglyceride. Phosphatidic acid now serves as the backbone on which most phosphoglycerides are derived having moieties such as serine, ethanolamine, choline, glycerol, and the inositol. Consequently, we have phosphatidylserine, phosphatidylethanolamine, phosphatidylcholine, phosphatidylglycerol,

**374**

*Sphingolipids and phospholipids: The classification of sphingolipids is based on the group attached to the sphingosine (LCB) backbone (a). Sphingomyelin (b) and ceramides (c-e) differ in fatty acid length, unsaturation, and in the type of attached head group and hydroxylation. Phospholipids with glycerol framework: (f) phosphatidylethanolamine, (g) phosphatidylcholine [53].*

a host of others. Most of the commercially produced microalgal biomass is presented to the market as a food supplement, and they are presented as tablets and capsules. Breakfast cereals, noodles, beverages, wines, and cosmetics now contain microalgae and their extracts. More than 75% of pharmaceutical product development is carried out by the microalgal food supplement production outfits. In the recent several years, microalgal and cyanobacterial research has explored diverse cultivation protocols aimed at improving growth rates, biomass yields, and accumulating metabolites for high nutritional value, and high-value chemicals (pigments and vitamins) [55]. Many more bioactive metabolites have been reported in microalgae. Dried microalgae biomass could be used as high-protein feeds for animals such as shrimp and fish, and microalgal biomass is a significant resource for cytotoxic agents with applications in cancer chemotherapy. The blooms of *Phaeocystis sp*., a marine microalga have antibiotic substances listed therein. *Phaeocystis pouchetii* produces acrylic acid, which makes up to 7.0% of its dry weight. The antibiotic metabolites so produced migrate in the food chain through the digestive system of some Antarctic marine animal species. Also, the alga *Dunaliella sp.* produces β-carotene and certain vitamins, which have boosted the Mariculture activities. Some cyanobacteria and microalgae such as *Ochromonas* sp. and *Prymnesium parvum* produce toxins, which may have the potential for pharmaceutical applications. These marine cyanobacteria produce bioactive metabolites such as acetogenins, bromophenols, fatty acids, terpenes, sterols, alkaloids, etc. with antibiotics, and antifungal activities. Diverse strains of cyanobacteria produce intracellular and extracellular metabolites with bioactive functions such as antitumor, anti-inflammatory, antialgal, antibacterial, antifungal, and antiviral activity [55, 56].

#### **4.5 Microalgal biomass production limiting factors**

Abiotic, Biotic, and process-related factors influence the growth of algae. Some of the abiotic factors are illumination and luminous intensity, daytime to night-time ratio, the temperature of the culture medium, nutrient availability, O2, and CO2 mass transfer, pH value, the hydraulic retention time (HRT), salinity, and presence

of growth-inhibiting chemical agents [30]. Some of the biotic factors are the presence of pathogens (bacteria, fungi, viruses) and the presence of more than one algae strains. Each algae strain has a different capacity to assimilate nutrients, and in mixed cultures, there is competition for the available nutrients in the media, which may afferent the growth of some strains [36]. Process related factors that may influence algal growth are hydrodynamics of the culture broth, which is influenced by the choice of the bioreactor, the initial algal cell concentration in the reactor, and the related frequency of harvesting algal biomass [57, 58].
