*3.1.4 Biomass by the air-lift method*

This is an outdoor microalgal cultivation technique for the production of biomass and metabolites under a highly controlled environment. By this technique, the air is moved within the system to circulate the medium in which microalgae is growing. The culture is grown in transparent tubes that lie horizontally on the ground and are connected by a network of pipes (see **Figure 13**). Air is passed through the tube such that air escapes from the end that rests inside the reactor that contains the culture and creates an effect like stirring [28]. Other configurations of the airlift reactor are an improvement over this design. The external-loop ALR is a promising configuration for breakthrough scale-up *Scenedesmus sp.* biomass production [26].

### **4. Metabolic platforms for microalgal growth**

Different microalgae strains acclimate in different environments, evolving their metabolic pathways to stimulate and propagate growth. However, the extent of growth depends on the composition of the culture media which can be enhanced by either inorganic or organic carbon metabolism or both. Other co-factors such as nutrient availability, pH, chemical oxygen demand (COD), and temperature also influence growth, and the accumulation of metabolites in microalgae (see **Table 3**) [29].

#### **4.1 Autotrophic metabolism**

The photosynthetic CO2-fixation in microalgae suffices to possess a greater ability to fix CO2. Photo trophy refers to an autotrophic mode of metabolism in which


#### **Table 3.**

*Microalgal metabolic requirements.*

organisms can harness light energy with the help of photosynthetic pigments and convert it to chemical bond energy in the form of ATP (photophosphorylation).

Autotrophy is the ability of PMOs to use inorganic carbon in the form of CO2 as the sole source of carbon to synthesize organic compounds necessary to build cell components. This is also referred to as carbon-autotrophy to distinguish the ability of some organisms to use molecular nitrogen as the sole source of nitrogen. Such organisms are referred to as nitrogen autotrophs. However, autotrophy as used in this chapter is carbon autotrophy. This is a property that is present primarily, in plants, algae, and phototrophic bacteria including cyanobacteria [30].

Aside from these organisms, all of which are photosynthetic, several groups of non-photosynthetic bacteria can grow using CO2 as the sole source of carbon by their ability to oxidize inorganic compounds. Such organisms are chemoautotrophic or chemolithotrophic [31].

CO2 is the end-product of aerobic respiration, a process that releases the energy of respiratory substrates. Carbon dioxide is, therefore, poor in energy content. In autotrophic metabolism, this energy-poor compound is used to build organic molecules which are much richer in energy content. Therefore, It is noted that the conversion of CO2 to organic compounds requires the input of energy from an external source. The ultimate source in the case of photosynthesis is radiant energy and in the case of chemolithotrophy is the oxidation energy of inorganic chemical compounds. In either case, the immediate source of energy for driving the endergonic reaction involved in the conversion of CO2 to organic compounds is ATP [32].

In photosynthesis, ATP is generated with the help of photosynthetic pigments through a process known as photophosphorylation. In chemoautotrophy, the energy of oxidation of inorganic compounds is channelized into the respiratory chain for ATP synthesis by oxidative phosphorylation.

Thus, autotrophic metabolism consists of two sets of reactions viz. (1) the ATP and the reducing force are generated and, (2) they are used for the reduction of CO2 to organic compounds.

The reactions in (1) are different in phototrophic and non-phototrophic autotrophs. But the reactions in (2) are common between the two groups. In the majority of autotrophs, the reactions involved in the reduction of CO2 proceed via a cyclic pathway, known as the reductive pentose phosphate pathway or, more commonly, as the Calvin-Benson cycle, or simply the Calvin cycle, although other pathways are also known to operate in some organisms, both in the phototrophic green plants and bacteria. The reduction of CO2 to yield organic compounds is commonly known as CO2-fixation [32, 33].

**371**

**4.3 Mixotrophic metabolism**

*Microalgae: The Multifaceted Biomass of the 21st Century*

The supply of sufficient light for massive growth is the main goal and a limiting factor for microalgal cultivation. To ignore the requirement for illumination and present the possibility of high cell concentration, points at heterotrophic cultivation as a promising, efficient, and sustainable strategy for certain microalgae to produce metabolites of value by using carbon substances as the sole carbon and energy source. The optimized preliminary cell culturing of microalgae species is an important stage in culturing microalgae biomass at the commercial scale. The growth environment during the culturing process can be [32] either autotrophic (inorganic carbon) or heterotrophic (organic carbon) depending upon the nature of cells and their growth tendencies. Heterotrophic and mixotrophic microalgae are more capable of growing much faster with higher cellular oil accumulation as compared to autotrophic microalgae species. However, heterotrophic microalgae require organic carbon sources like glycerol, glucose, or acetate as a sole source of carbon for growth, which is responsible for about 80% of the costs of culture media [33]. The metabolism of respiration is applied to produce energy. The respiration rates, intimately geared to the growth and division, are determined by the oxidization of organic substrates of the given microalgae [32]. Glucose provides the organic carbon needed and it is preferred because of its high energy density compared to other sources. The oxidative assimilation of glucose employs either the Embden–Meyerhof–Parnas (EMP) pathway or the pentose phosphate (PP) pathway depending on the cycle position. During the dark cycle, PMOs assimilate and metabolize glucose via the PP pathway. However, during the daytime cycle, glycolysis in the cytosol is via the EMP pathway [34]. The growth rate, lipid content, and the ATP of microalgae under the heterotrophic metabolic strategy are higher compared to those under the photoautotrophic metabolic strategy but depend mainly on the PMO's species and strain used. The PMO's growth is steady and rapid in a nutrient-rich culture media using a high level of system control, to achieve biomass production of 50–100 g L−1 in heterotrophy which is higher than that achieved in photoautotrophy [35].

Heterotrophic metabolism eliminates the two main problems associated with autotrophic metabolism viz. (i) it allows the use of practically any vessel as a bioreactor, and (ii) low energy and high yield, as major outcomes, giving a significant reduction in costs for the process. Cost-effectiveness and relative simplicity of operations and daily maintenance are the main attractions of the heterotrophic growth approach. A significant benefit is that it is possible to obtain, heterotrophically, high densities of microalgae cells that provides an economically feasible

Heterotrophy has its drawbacks viz. (1) The microalgae species and strains that can grow by the heterotrophic strategy are limited; (2) Increasing energy expenses and costs by adding organic carbon substrate; (3) Contamination and competition with local microorganisms; (4) Inhibition of growth by excess organic substrate; and (5) Inability to produce light-induced metabolites [35]. Nonetheless, heterotrophic cultures are gaining increasing application for producing a wide variety of

Mixotrophic cultivation of microalgae strategies provides both carbon dioxide and organic carbon simultaneously and both chemoheterotrophic and photoautotrophic metabolisms operate concurrently. Microalgae biomass produced by this approach has high density and contains high-value lipids, proteins, carbohydrates, and

method for large scale, mass production cultivation [34].

microalgal metabolites from bench experiments to commercial scale.

*DOI: http://dx.doi.org/10.5772/intechopen.94090*

**4.2 Heterotrophic metabolism**
