**3. Microalgae**

*Biotechnological Applications of Biomass*

stimulating [9, 12].

**2. Biofuels**

significantly [1].

to evaluate their effects [21].

such as microalgae, which are attractive options [7].

Biofuels are classified into three generations, depending on their sources. The first generation is derived from plant sources; the second ones comes from agroindustrial waste and wood waste, while the third one comes from microalgae. The fact that first- and second-generation raw material are limited by competition with food production and arable land emphasizes the importance of using alternatives,

Microalgae, which are photosynthetic microorganisms with simple growth needs, adapt to a wide range of variations in the culture medium. Depending on the conditions, they can produce large amounts of compounds, such as carbohydrates and lipids that can be processed into bioethanol and biodiesel, respectively [8]. Magnetic Fields (MF) are capable of causing effects on biological systems and, therefore, application of this technique has attracted attention in biotechnology and bioenergy to increase production of biomolecules of interest [9]. MF correspond to the region in which magnetic forces act [10]. MF can be generated either by magnets composed of conductive material with characteristic magnetic intensity, such as ferrite and neodymium or by an electric current that results from straight conductors, circular loop, flat circular coil, electromagnets and solenoids [11]. Since magnetic forces can act differently on microorganisms due to distinct biochemical and physiological constitutions, their biological effects can be inhibitory, null or

Studies have reported that some changes that can affect production of compounds are electro activation of some enzymatic systems and metabolic routes [13], oxidative stress, changes in enzyme and protein activity, gene expression, electron and ion movements [14, 15], cell growth [16–19] and high activity of photosystem II [20]. However, different strains of microalgae, application time and MF intensity can give different responses. Therefore, previous studies of MF, applied at different exposure times and intensities during cultivation, should be investigated

The world population has currently faced a major challenge, i. e., to associate economic development with sustainable practices [22]. The amount of fossil fuel consumed by the population and, consequently the number of environmental problems, such as excess of CO2 in the atmosphere and global warming, increased

Biofuels are promising for the replacement of fossil fuels since they can reduce environmental impacts and meet the global demand for energy consumption [23]. The first generation biofuels, such as biodiesel is produced from oleaginous crops. It has currently been questioned due to the large amount of water it consumes, the use

Third generation bioethanol and biodiesel are biofuels that use microalgal biomass as raw material which has become an alternative for this generation of sustainable and renewable biofuels. Adequate cultivation conditions are necessary to obtain high biomass yields, desirable carbohydrate accumulation for bioethanol production [25, 26] and essential lipid levels for biodiesel production [27, 28].

Global interest in renewable energy sources, such as biofuel production has been continuously growing. Thus, microalgal biomass is an excellent alternative for bioethanol production, not only because it decreases the use of traditional energy sources, but also because of the large carbohydrate accumulation in biomass. Regarding the third-generation bioethanol production, three countries, i. e., the USA, Brazil and China, produced 14,806, 7093 and 813 million gallons, respectively, in 2015 [26, 29].

of agricultural land and its competition with food production [24].

**420**

Microalgae are photoautotrophic microorganisms that grow fast under relatively simple nutritional conditions. Therefore, they are considered promising organisms for biomass production due to their high-value biomolecules for commercial application [32, 33]. Due to the diversity of biomolecules, several studies have investigated the use of microalgae for biofuel production. Both genera *Chlorella* and *Spirulina* have great potential for this purpose, since they have high concentrations of composts of interest, such as carbohydrates and lipids [33, 34].

Microalgae can be cultivated in three forms: photoautotrophic, heterotrophic and mixotrophic cultivation. The mixotrophic one is a variant of the heterotrophic culture, where CO2 and organic carbon are assimilated by the respiratory and photosynthetic metabolism with high growth rate and biomass productivity [35]. In this type of cultivation, an organic source of carbon, such as molasses, glycerol and glucose is added [36, 37]. The capacity for assimilating high concentrations of available carbon by microalgae tends to accumulate more carbohydrates and lipids [36], macromolecules of interest in biofuel production.

Production of metabolites by microalgae is determined by several factors, such as species, agitation, pH, nutrient composition, CO2 concentration, light intensity and temperature [38, 39]. According to Khan et al. [26], light intensity and temperature are the main limiting factors in microalgal cultivation, since these physical stress factors directly influence biochemical processes, such as photosynthesis and biomass production yield.
