**1. Introduction**

Microalgae are a variety of autotrophic, prokaryotic or eukaryotic organisms, where their single-cell structure allows solar energy to be easily converted into chemical energy. This biochemical conversion is being used commercially to obtain the biomass, consequently, in the insertion in products with commercial application. The most used microalgae cultivation techniques are opens aerated lagoons and closed photobioreactors [1–4].

Due to the advantages that microalgae offer over many other species, researchers and entrepreneurs have shown great interest in the development of production processes for biofuels, functional foods and bio-products from different species. Compared to terrestrial crops, these microorganisms have photosynthetic efficiency, growth rate and higher biomass production, consequently mass cultivation for commercial microalgae production can be carried out efficiently [5]. In addition, the cultivation of microalgae does not require arable soil, and can be grown in saline, brackish and wastewater and in harsh conditions, not competing with the production of food that is currently a major challenge for the production of first and second biofuels generation [6]. Therefore, competition for arable land with other crops, especially for human consumption, is greatly reduced.

Although most microalgae grow exclusively through photosynthesis, some species are mixotrophic and use extracellular organic carbon when a light source is not available [7]. Microalgae can be a source of several important compounds, including hydrogen and hydrocarbons, pigments and dyes, food and feed, biopolymers, biofertilizers, insecticides, neutraceuticals (foods capable of providing health benefits) and pharmacological compounds, in addition to being a potential biomass for production of biofuels [8].

Although the production of microalgae does not directly compete with food production and can be grown in harsh conditions, economic viability does not yet exist in many of the processes of industrial interest. However, the improvement and mastery of technologies capable of making inserted industrial processes viable become essential. Despite of the microalgae have a wide potential for production and applications, there are many obstacles to the biodiversity of these algae, such as mastery of technologies for production, genetic improvement research of strains more resistant to pathogens and economic viability in large-scale production [9–10]. According to Georgianna and Mayfield [11], although promising, the success of inserting microalgae in the production of various products depends mainly on two important factors: high productivity and quality of biomass, as well as cost-effective production.

One of the viable solutions to reduce the costs of microalgae biomass production is to explore different forms of energy metabolism, highlighting the photoautotrophic, heterotrophic and mixotrophic for commercial production. Understanding these forms of metabolism allows the application of efficient crop strategies aimed at increasing the production of biomass and bioproducts on a large scale with cost optimization to couple the agroindustry waste treatment [7]. Microalgae are able to eliminate a variety of pollutants in wastewater mainly nitrogenated, phosphates and organic carbons [12].

Mixotrophic cultivation is a preferable microalgae growth mode for biomass production [13]. Compared to photoautotrophic and heterotrophic metabolism, mixotrophic cultures have been demonstrated many advantages, such as less risk of contamination, reduced cost and high biomass productivity. Even susceptible to contaminations, the use of photobioreactors minimizes this risk, but increases the cost of the process, which can be offset by the high biomass yield that can reach 5–15 g/L, being 3–30 times higher than those produced under autotrophic growth conditions [14, 15].

The use of waste for microalgae mixotrophic growth has been researched, mainly with the objective of expanding and diversifying in an alternative way the control and combating the inappropriate disposal of these in the respective industries, combined with the perspective of minimizing the operational costs of producing microalgae in large scale that are still considered high. The waste generated by the agribusiness has a high load of organic matter with high concentrations of Biochemical Oxygen Demand (BOD), Chemical Oxygen Demand (COD), ammonia, phosphates, suspended solids harmful to the environment, in addition to dissolved components such as sugars, fat and proteins originating from food, contributing to environmental pollution [16].

According to Patel et al. [7], research involving the mixotrophic cultivation of microalgae using organic matter as a source of carbon points to the production of high yields of biomass and biocomposites of industrial interest when compared to systems involving photoautotrophic and heterotrophic metabolisms. In this sense, recent studies have been carried out using agroindustrial waste to grow microalgae in a mixotrophic regime in order to minimize the cost of the biomass production process and treat the effluent adding value to the process, suggesting a microalgae biorefinery system [17].

**403**

biorefinery.

**2. Microalgae**

used for any purpose.

**3. Application of microalgae**

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

inorganic pollutants and produce biomass and lipids fractions.

phate in mixed wastewater were effectively removed by microalgae.

lysate was at high quality in terms of biodiesel properties.

Patel et al. [18] cultivated *C. protothecoides* UTEX-256 under mixotrophic conditions using dairy waste as source of carbon. The high CO2-emitting dairy industry obligated to treat waste and improve its carbon-footprints. In general, biochemical treatment was effective to remove respectively 99.7 and 91–100% of organic and

Xio-Bo Tan et al. [19] demonstrated that *Chlorella pyrenoidosa* (FACHB-9) cultivated under mixotrophic conditions using anaerobic digestate of sludge with an optimal addition of acidified starch wastewater improved biomass and lipids production by 0.5-fold (to 2.59 g·L−1) and 3.2-fold (87.3 mg·L−1 ·d−1), respectively. In addition, 62% of total organic carbon, 99% of ammonium and 95% of orthophos-

Wang et al. [20] utilized glucose recovered from enzymatic hydrolysis of food

Due to the success of mixotrophic microalgae growth, the use of agro-industrial by-products stands out, adding value to production processes and reducing costs. The nutritional characteristics, availability and low cost of obtaining evidence the possibility of using the by-products in the cultivation of microalgae. This work reviews the mixotrophic cultivation system of microalgae using waste from agribusiness as a source of organic carbon, pointing out the benefits of this strategy as a solution to the environmental problems caused by these effluents, adding value to an industrial process for the production of biomass and biocompounds as

Microalgae is a generic term used to refer a widely diverse group of photosynthetic microorganisms [21]. There are several species of microalgae, which are found in aquatic environments of fresh water, brackish and saline [22]. Microalgae in general, have varying microscopic sizes, perform photosynthesis, use carbon dioxide as a nutrient source for growth, in addition to playing a fundamental role in ecosystems [23–25]. It is estimated that there are about 800 thousand species of microalgae, of which about 40 to 50 thousand are of scientific knowledge, which makes it an almost unexplored resource, demonstrating the great biodiversity of these algae [26–27]. In addition, most species are not yet known and very few are

The basic composition of microalgae is based on carbohydrates, lipids, proteins, ash and nucleic acids, in addition to chlorophyll and other protective pigments and light capture that provide high photosynthetic capacity, allowing conversion of up to 10% of energy in biomass [28]. In conventional plants, this percentage is higher when compared to other conventional plants, whose conversion is limited to a maximum of 5% [29]. The predominant elements in the biomass of microalgae are carbon, nitrogen

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

and phosphorus and some metals such as iron, cobalt, zinc is also found [28].

waste as culture medium in mixotrophic cultivation of *Chlorella sp*. to obtain high levels of lipid and lutein. The algal biomass was 6.9 g L−1 with 1.8 g L−1 lipid and 63.0 mg L−1 lutein using hydrolysate with an initial glucose concentration of 20 g L − 1. Furthermore, lipid derived from microalgae biomass using food hydro-

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

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

Patel et al. [18] cultivated *C. protothecoides* UTEX-256 under mixotrophic conditions using dairy waste as source of carbon. The high CO2-emitting dairy industry obligated to treat waste and improve its carbon-footprints. In general, biochemical treatment was effective to remove respectively 99.7 and 91–100% of organic and inorganic pollutants and produce biomass and lipids fractions.

Xio-Bo Tan et al. [19] demonstrated that *Chlorella pyrenoidosa* (FACHB-9) cultivated under mixotrophic conditions using anaerobic digestate of sludge with an optimal addition of acidified starch wastewater improved biomass and lipids production by 0.5-fold (to 2.59 g·L−1) and 3.2-fold (87.3 mg·L−1 ·d−1), respectively. In addition, 62% of total organic carbon, 99% of ammonium and 95% of orthophosphate in mixed wastewater were effectively removed by microalgae.

Wang et al. [20] utilized glucose recovered from enzymatic hydrolysis of food waste as culture medium in mixotrophic cultivation of *Chlorella sp*. to obtain high levels of lipid and lutein. The algal biomass was 6.9 g L−1 with 1.8 g L−1 lipid and 63.0 mg L−1 lutein using hydrolysate with an initial glucose concentration of 20 g L − 1. Furthermore, lipid derived from microalgae biomass using food hydrolysate was at high quality in terms of biodiesel properties.

Due to the success of mixotrophic microalgae growth, the use of agro-industrial by-products stands out, adding value to production processes and reducing costs. The nutritional characteristics, availability and low cost of obtaining evidence the possibility of using the by-products in the cultivation of microalgae. This work reviews the mixotrophic cultivation system of microalgae using waste from agribusiness as a source of organic carbon, pointing out the benefits of this strategy as a solution to the environmental problems caused by these effluents, adding value to an industrial process for the production of biomass and biocompounds as biorefinery.
