**2. Microalgae**

*Biotechnological Applications of Biomass*

biomass for production of biofuels [8].

as cost-effective production.

and organic carbons [12].

conditions [14, 15].

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

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

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

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

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

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

from food, contributing to environmental pollution [16].

**402**

biorefinery system [17].

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 used for any purpose.

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