**3. High valued products**

Apart from lipids for biodiesel, microalgae synthesize other products of value such as astaxanthin, canthaxanthin, lutein, β-carotene, phycocyanin, chlorophyll *a*, and polyunsaturated fatty acids (PUFAs) such as γ-linolenic acid, docosahexaenoic acid, and eicosapentaenoic acid. A subset of these, astaxanthin, canthaxanthin, lutein, and β-carotene, have antioxidant properties and are commercially high valuable pharmaceutical products. Phycocyanin is a high value natural food dye, and products such as γ-linolenic acid and eicosapentaenoic acid are considered animalfree based sources of essential fatty acids. Some of these products are also considered to boost the immune system. **Table 2** summarizes the common high value products, their use, the species of microalgae most commonly grown to harvest the products, and the global market values.


**217**

be discovered.

*Physiological Limitations and Solutions to Various Applications of Microalgae*

a better understanding of the physiological roles of astaxanthin.

The major fraction of studies on microalgal PUFA have primarily focused on screening for microalgal species and optimization of conditions that lead to enhanced PUFA products. However, we have no clear understanding on the physiological role of PUFAs in microalgae. It is hypothesized that PUFAs might play a role in homeoviscous adaptation [47], i.e., the enhanced fluidity provided by increased unsaturation membrane fatty acids. Additionally, the fatty acid composition and therefore the PUFA concentrations appear to have some level species specificity, suggesting varying roles [48]. However, the physiological roles of PUFAs are not yet confirmed in microalgae, and therefore strategic optimization of their synthesis in microalgae has not been realized. Nevertheless, PUFAs make a significant portion of the neutral lipid content synthesized in microalgae; therefore optimization strategies discussed above to enhance biomass and lipid synthesis should also increase the

Astaxanthin is another promising product synthesized by microalgae that has already achieved a profit of \$200 million per year [49]. Astaxanthin is a red ketocarotenoid pigment ubiquitous in nature and has antioxidative, anti-inflammatory, and anti-apoptotic properties. It is also proposed as a potential therapeutic agent for cardiovascular and neurological diseases [50]. Apart from its pharmaceutical use, this product is also used as a pigment source in aquaculture of salmon and trout [51]. Despite the relatively vast amount of research into its properties and applications, the mass production of astaxanthin is still unable to meet its huge market demand [49]. This problem can be primarily attributed to the lack of knowledge on the underlying mechanisms of why these algae accumulate astaxanthin similar to PUFAs and hence the lack of better strategies to optimally produce the pigment. The most prominent hypothesis regarding its production includes a multifunctional photoprotective response to stress induced by exposure to unfavorable conditions (excess light, UV-B radiation, nutrient deprivation) leading to ROS formation [52–54]. However, this hypothesis does not explain the synthesis of astaxanthin under heterotrophic conditions [55, 56]. Therefore, more studies are needed to have

Nutrient limitation such as nitrogen, phosphorus or sulfur are widely used strategies for inducing astaxanthin accumulation in *Haematococcus pluvialis* [57, 58]. However, nutrient limitation reduces the maximum amount of biomass one can achieve, thereby reducing the total amount of astaxanthin that can be produced. Additional strategies for inducing astaxanthin production include high salt stress and high light exposure, but these external stressors also lower the biomass yield and therefore the associated pigment production. Using stress as a mechanism to induce astaxanthin synthesis has the fundamental problem of stopping cell cycle and therefore reducing biomass and astaxanthin production. Hence, new strategies to boost astaxanthin and biomass production with the use of stressors need to

Phycocyanin is a phycobiliprotein exclusively produced by cyanobacteria and commercially important as a high value natural blue coloring agent for food. Being a part of light-harvesting pigment-protein complex in cyanobacteria, phycocyanin is very critical in light capture and therefore is an indispensable element in the growth and survival of cyanobacteria. Phycocyanin has been shown to constitute up to 60% of the total cellular protein content [59]. As it is involved in light capture, its synthesis is tightly regulated by the wavelength of light the cells are exposed to. Green light has been shown to stimulate the synthesis of phycocyanin, whereas red light has the opposite effect [60]. In addition, low light levels have been also shown to induce accumulation of phycocyanin [61, 62]. Other factors such as glucose and salt have also been shown to enhance the synthesis of phycocyanin [61]. Despite knowing their physiological roles, the commercial success of phycocyanin from

*DOI: http://dx.doi.org/10.5772/intechopen.90206*

PUFA content in microalgae.

#### **Table 2.**

*Summary of the main high value products derived from microalgae.*

#### *Physiological Limitations and Solutions to Various Applications of Microalgae DOI: http://dx.doi.org/10.5772/intechopen.90206*

*Microalgae - From Physiology to Application*

**3. High valued products**

products, and the global market values.

β-carotene *Antioxidant,* 

Astaxanthin Feed additive

**Phycobilins** Food coloring,

C-Phycocyanin *Antioxidant,* 

Sterols Pharmaceutical

Polyhydroxyalkanoates Production of

*Summary of the main high value products derived from microalgae.*

*\*(from Borowitzka - 2013, Chew et al. 2017) [45, 46].*

fed-batch mode has resulted in further increase in biomass as much as two- to fivefold [43, 44]. Therefore, by growing a hybrid strain of lipid-secreting *Botryococcus* sp. and fast-growing *Scenedesmus* sp., in a fed-batch heterotrophic or mixotrophic cultivation system, one can possibly overcome the physiological limitations of the maximum amount of lipids that can be synthesized in a microalgal system.

Apart from lipids for biodiesel, microalgae synthesize other products of value such as astaxanthin, canthaxanthin, lutein, β-carotene, phycocyanin, chlorophyll *a*, and polyunsaturated fatty acids (PUFAs) such as γ-linolenic acid, docosahexaenoic acid, and eicosapentaenoic acid. A subset of these, astaxanthin, canthaxanthin, lutein, and β-carotene, have antioxidant properties and are commercially high valuable pharmaceutical products. Phycocyanin is a high value natural food dye, and products such as γ-linolenic acid and eicosapentaenoic acid are considered animalfree based sources of essential fatty acids. Some of these products are also considered to boost the immune system. **Table 2** summarizes the common high value products, their use, the species of microalgae most commonly grown to harvest the

**Product Use Species (dominant) Global market** 

**Carotenoids** Total US\$1.2 billion

*Dunaliella salina, Dunaliella bardawil*

*Spirulina*, *Porphyridium*, *Rhodella, Galdieria*

*Nannochloropsis, Tetraselmis, Isochrysis, Thalassiosira, and Chaetoceros*

> *Spirulina, Synechocystis*

*Haematococcus pluvialis* US\$

*anti-inflammatory*

for farmed fish; pigmenter of the fish flesh

cosmetics coloring

*anti-inflammatory*

*Health food supplements*

applications or in functional foods

biodegradable plastics

Docosahexaenoic acid *Crypthecodinium cohnii* US\$140 kg<sup>−</sup><sup>1</sup> Omega-3 oils US\$1.5 billion

**value\***

in 2010

US\$ 300–1500 kg<sup>−</sup><sup>1</sup>

2500–7000 kg<sup>−</sup><sup>1</sup>

US\$ 60 million

US\$ 500 to 100,000 kg<sup>−</sup><sup>1</sup>

or US\$80–160 kg<sup>−</sup><sup>1</sup>

US\$ 300 million

**216**

**Table 2.**

**Fatty acids** *DHA EPA PUFA*

The major fraction of studies on microalgal PUFA have primarily focused on screening for microalgal species and optimization of conditions that lead to enhanced PUFA products. However, we have no clear understanding on the physiological role of PUFAs in microalgae. It is hypothesized that PUFAs might play a role in homeoviscous adaptation [47], i.e., the enhanced fluidity provided by increased unsaturation membrane fatty acids. Additionally, the fatty acid composition and therefore the PUFA concentrations appear to have some level species specificity, suggesting varying roles [48]. However, the physiological roles of PUFAs are not yet confirmed in microalgae, and therefore strategic optimization of their synthesis in microalgae has not been realized. Nevertheless, PUFAs make a significant portion of the neutral lipid content synthesized in microalgae; therefore optimization strategies discussed above to enhance biomass and lipid synthesis should also increase the PUFA content in microalgae.

Astaxanthin is another promising product synthesized by microalgae that has already achieved a profit of \$200 million per year [49]. Astaxanthin is a red ketocarotenoid pigment ubiquitous in nature and has antioxidative, anti-inflammatory, and anti-apoptotic properties. It is also proposed as a potential therapeutic agent for cardiovascular and neurological diseases [50]. Apart from its pharmaceutical use, this product is also used as a pigment source in aquaculture of salmon and trout [51]. Despite the relatively vast amount of research into its properties and applications, the mass production of astaxanthin is still unable to meet its huge market demand [49]. This problem can be primarily attributed to the lack of knowledge on the underlying mechanisms of why these algae accumulate astaxanthin similar to PUFAs and hence the lack of better strategies to optimally produce the pigment. The most prominent hypothesis regarding its production includes a multifunctional photoprotective response to stress induced by exposure to unfavorable conditions (excess light, UV-B radiation, nutrient deprivation) leading to ROS formation [52–54]. However, this hypothesis does not explain the synthesis of astaxanthin under heterotrophic conditions [55, 56]. Therefore, more studies are needed to have a better understanding of the physiological roles of astaxanthin.

Nutrient limitation such as nitrogen, phosphorus or sulfur are widely used strategies for inducing astaxanthin accumulation in *Haematococcus pluvialis* [57, 58]. However, nutrient limitation reduces the maximum amount of biomass one can achieve, thereby reducing the total amount of astaxanthin that can be produced. Additional strategies for inducing astaxanthin production include high salt stress and high light exposure, but these external stressors also lower the biomass yield and therefore the associated pigment production. Using stress as a mechanism to induce astaxanthin synthesis has the fundamental problem of stopping cell cycle and therefore reducing biomass and astaxanthin production. Hence, new strategies to boost astaxanthin and biomass production with the use of stressors need to be discovered.

Phycocyanin is a phycobiliprotein exclusively produced by cyanobacteria and commercially important as a high value natural blue coloring agent for food. Being a part of light-harvesting pigment-protein complex in cyanobacteria, phycocyanin is very critical in light capture and therefore is an indispensable element in the growth and survival of cyanobacteria. Phycocyanin has been shown to constitute up to 60% of the total cellular protein content [59]. As it is involved in light capture, its synthesis is tightly regulated by the wavelength of light the cells are exposed to. Green light has been shown to stimulate the synthesis of phycocyanin, whereas red light has the opposite effect [60]. In addition, low light levels have been also shown to induce accumulation of phycocyanin [61, 62]. Other factors such as glucose and salt have also been shown to enhance the synthesis of phycocyanin [61]. Despite knowing their physiological roles, the commercial success of phycocyanin from

microalgae is limited as their production is strictly dependent on the maximum amount of biomass that can be generated. A suggested final concentration of higher than 10% of cell dry weight of phycocyanin is required to make a profit over the cost of pigment downstream separation [63]. With factors such as blue light and low light levels required to induce maximum cellular synthesis of phycocyanin, the growth is significantly attenuated under these low energy light conditions, thereby limiting the maximum biomass and hence the maximum quantity of phycocyanin that can be synthesized. Therefore, a dual phase production approach to maximize the biomass production under mixotrophic conditions with cheaper organic carbon source such as molasses [41] followed by the second phase of low levels of blue light to stimulate the synthesis phycocyanin can significantly maximize the total amount of phycocyanin that be produced.
