**3. Application of microalgae**

In recent years, several researches have been carried out seeking to develop technologies for the elaboration and diversification of products based on microalgae.

The growing expansion of these products is part of a wide range of utilities inserted in the most different commercial niches, expanding the possibilities of use and adding value to the market. According to Hu et al. [30], the global algae market is expected to be worth of about \$ 1.1 billion by 2024.

Microalgae are inserted in a wide variety of species, distinguishing one from the other due to their biological structure. In this sense, these microorganisms offer potential possibilities for CO2 biofixation, remediation, effluent treatment, production of biofuels, high-value products including pharmaceuticals, food and neutraceutics [31].

According to Rizwan et al. [32], microalgae can be a source of antioxidant compounds, carotenoids, enzyme polymer, lipid, natural dye, polyunsaturated fatty acid, peptide, toxin and sterols, which are widely used in industry. In addition, they are used for the synthesis of antiviral, antimicrobial, antiviral, antibacterial and anticancer drugs [33].

Commercial microalgae cultivation systems are operated to produce mainly pigments and metabolites for nutritional supplements [34]. The algae that have technical and economic viability of production are Spirulina (Arthrospira) for supplements with a high protein content, Haematococcus as a source of astaxanthin and *Dunaliella salina* for the production of pro-vitamin A [31]. Spirulina represents 60% of all biomass produced on a large scale [35]. This species can be easily grown in tropical regions and is well adapted to extreme environments, being relatively less susceptible to contamination than other microalgae, making it the most favorable choice for large-scale production [36].

Spirulina consists mainly of proteins (50–70%), being widely used in human nutrition to combat malnutrition [37]. This species is rich in essential amino acids, beta-carotene, minerals, essential fatty acids, vitamins, polysaccharides, among others. Chlorella accumulate high concentrations of carotenoids (astaxanthin, lutein, β-carotene, violaxanthin and zeaxanthin), antioxidants, vitamins, polysaccharides, proteins, peptides and fatty acids [5, 38]. In addition to all the benefits mentioned, the bioactive compounds of microalgae can have a biological, immune, antiviral and anti-cancer properties, being highly active [39].

Global warming has been worrying environmentalists across the planet. Although there are different ways of capturing CO2, the biological method stands out as a potentially attractive alternative. The requirements for producing and obtaining biomass from microalgae are basically CO2 and a source of light, be it natural or artificial [2]. Carbon dioxide can be converted into organic matter by performing photosynthesis using sunlight as an energy source [40–42]. Microalgae are more efficient for fixing CO2 and have a higher productivity rate (ton/ha/year) when compared to terrestrial plants. In addition, CO2 biofixation can be combined with other processes, such as the treatment of organic waste, being advantageous in terms of economic viability and environmental sustainability. Microalgae can also be grown in nutrient-rich organic effluents, salt and brackish water, reducing the use of fertile land and fresh drinking water [43].

Studies involving the mixotrophic cultivation of microalgae using industrial residues from agro-industry as a source of organic carbon have been carried out to minimize the cost of biomass production, treat the effluent and promote CO2 biofixation [7]. In this sense, expanding the ways in which these residues are used, avoiding their incorrect disposal, minimizes the effects of environmental pollution and adds value to industrial processes, encouraging a cleaner and more sustainable bioeconomy.

Currently, fossil fuels represent the main source of energy in the world, but unsustainable and directly related to the pollution of air, land, water and climate change. The burning of fossil fuels consolidated to increase the atmospheric

**405**

*Microalgae Growth under Mixotrophic Condition Using Agro-Industrial Waste: A Review*

concentration of CO2 being directly associated with global warming. Allied to this, the future oil scarcity is a major challenge for scientists, motivating a constant search for technologies capable of producing clean and sustainable fuels [44]. Among many biomasses, microalgae represent a promising source for the production of clean renewable energy, as they are capable of fixing CO2 by performing photosynthesis with efficiency and productivity superior to that of conventional oilseeds and terrestrial plants used in the production of biodiesel and bioethanol. Among the available biomass sources, microalgae have been evaluated and investigated as generation third biomass, being researched to produce different types of biofuels, among which are biodiesel, bioethanol, bio-oil, char, hydrogen and synthesis gas [45]. Recent research involving the production of biofuels has been focused on third generation biomass, since the first and second raw materials are based on terrestrial cultures that compete with food production and can lead to food crises [46]. Algae biofuels are not yet obtained on a large scale due to the high cost of the process justifying the development of new technologies that can bring

Bioremediation and biofuel production from waste resources by microalgae platform is mainly important to utilize abundantly available solar energy biofixing CO2 and treat effluents through the mixotrophic growth of microalgae [7]. Algal bioremediation is a good strategy to produce biomass for biofuels production while remediating wastes, also improving carbon-footprint through carbon capturing and

A microalgae biorefinery enables to integrate fractionation and conversion processes to transform biomass into bioproducts such as food, feed, chemicals, and bio-energy as optimization of the use of the microalgae for reducing waste production, and maximizes process profit. After lipid transesterification for biodiesel, the residual biomass can be used to produce other biofuels such as methane, bio-oil and

There are currently four cultivation technologies in use for the production of commercial microalgae including open ponds and raceway ponds (open systems), photobioreactors and fermenters [49]. In open systems, microalgae are grown in open areas, including tanks, lakes, and ponds, deep channels, among others. In closed systems, crops are grown in transparent bioreactors, exposed to sunlight or

Natural and artificial lakes and ponds, where most of the systems commonly used are large, shallow ponds and tanks, represent open pond systems. The main advantages of these systems are the ease of construction and operation when compared to photobioreactors and the possibility of operating hybrid processes involving the cultivation of algae associated with the treatment of wastewater. However, the disadvantages are inefficient light distribution, losses through evaporation, diffusion of CO2 into the atmosphere, contamination and the requirement for large areas of land [50]. Open ponds are currently in use for wastewater treatment and production of *Dunaliella salina*, characterized as a hybrid process. These systems are used by Ognis Australia Pty Ltd. to produce β-carotene from *Dunaliella salina* in Hutt Lagoon and Whyalla. In terms of surface area used, these are among the largest algae produc-

ethanol or biocompounds for food and pharmaceutical industry [48].

*DOI: http://dx.doi.org/10.5772/intechopen.93964*

economic viability [47].

utilization technology.

**4.1 Opens systems**

tion systems in the world.

**4. Cultivation systems of microalgae**

artificial radiation for photosynthesis and fermenters.

#### *Microalgae Growth under Mixotrophic Condition Using Agro-Industrial Waste: A Review DOI: http://dx.doi.org/10.5772/intechopen.93964*

concentration of CO2 being directly associated with global warming. Allied to this, the future oil scarcity is a major challenge for scientists, motivating a constant search for technologies capable of producing clean and sustainable fuels [44]. Among many biomasses, microalgae represent a promising source for the production of clean renewable energy, as they are capable of fixing CO2 by performing photosynthesis with efficiency and productivity superior to that of conventional oilseeds and terrestrial plants used in the production of biodiesel and bioethanol. Among the available biomass sources, microalgae have been evaluated and investigated as generation third biomass, being researched to produce different types of biofuels, among which are biodiesel, bioethanol, bio-oil, char, hydrogen and synthesis gas [45]. Recent research involving the production of biofuels has been focused on third generation biomass, since the first and second raw materials are based on terrestrial cultures that compete with food production and can lead to food crises [46]. Algae biofuels are not yet obtained on a large scale due to the high cost of the process justifying the development of new technologies that can bring economic viability [47].

Bioremediation and biofuel production from waste resources by microalgae platform is mainly important to utilize abundantly available solar energy biofixing CO2 and treat effluents through the mixotrophic growth of microalgae [7]. Algal bioremediation is a good strategy to produce biomass for biofuels production while remediating wastes, also improving carbon-footprint through carbon capturing and utilization technology.

A microalgae biorefinery enables to integrate fractionation and conversion processes to transform biomass into bioproducts such as food, feed, chemicals, and bio-energy as optimization of the use of the microalgae for reducing waste production, and maximizes process profit. After lipid transesterification for biodiesel, the residual biomass can be used to produce other biofuels such as methane, bio-oil and ethanol or biocompounds for food and pharmaceutical industry [48].
