**Abstract**

Microalgae are unicellular, eukaryotic organisms which possess unique qualities of replication, producing biomass as a precursor for biofuels, nutraceuticals, biofertilizer, and fine chemicals including hydrocarbons. Microalgae access nitrates and phosphates in wastewater from municipalities, industries, and agricultural processes to grow. Wastewater is, therefore, culture media for microalgae, and provides the needed nutrients, micronutrients, inorganic and organic pollutants to produce microalgae biomass. Suitable strains of microalgae cultivated under mesophilic conditions in wastewater with optimized hydrodynamics, hydraulic retention time (HRT), luminous intensity, and other co-factors produce biomass of high specific growth rate, high productivity, and with high density. The hydrodynamics are determined using a range of bioreactors from raceway ponds, photobioreactors to hybrid reactors. Carbon dioxide is used in the photosynthetic process, which offers different growth stimuli in the daytime and the night-time as the microalgae cultivation technique is navigated between autotrophy, heterotrophy, and mixotrophy resulting in microalgal lipids of different compositions.

**Keywords:** biomass production, autotrophy, heterotrophy, mixotrophy, wastewater treatment, pollutant sequestration, microalgal lipid production, biofuels, nutraceuticals, biofertilizer, photobioreactors, hybrid reactors

#### **1. Introduction**

Algae represent a highly diverse consortium of polyphyletic, thallophytic, photosynthetic, and cryptogamic organisms. *They* are morphologically simple, chlorophyll-containing, non-flowering, and typically aquatic plants of a large family with members including seaweeds and a range of microscopic and unicellular to very large multicellular organisms [1]. Algae are either prokaryotes or eukaryotes and lack vascular tissue, leaves, true stems, and roots. The prokaryotic algae are the blue-green algae, which are also referred to as Cyanobacteria or Cyanoprokaryota and belong to the kingdom Eubacteria. The eukaryotic algae belong to the kingdom Protoctista. Cyanobacteria also derive their energy through photosynthesis but do not have a nucleus and membrane-bound organelles, like chloroplasts (see **Figures 1** and **2**) and their prokaryotic nature describes the single-stranded deoxyribonucleic acid (DNA) in their formation, which confers the bacterial identity. On the other hand, eukaryotic algae have double-stranded DNA in their makeup and are equipped with a nucleus and chloroplast. The term "algae" is therefore exclusively reserved for the eukaryotic organisms; and this chapter considers and treats the prokaryotic cyanobacteria as bacteria [1, 2].

**Figure 1.** *The microalga* Chlamydomonas reinhardtii's *cell structure [3].*

**Figure 2.**

*Schematic of a prokaryotic cell with an indication of some of the methods used to probe cellular activity or growth [4].*

Algae have six types of life cycles viz. haplontic, diplontic, isomorphic, heteromorphic, haplobiontic, and diplobiontic cycles; the exposition of these algal life cycles is discussed elsewhere [5]. The microscopic algae are the microphytes or microalgae and are typically found in freshwater and marine ecosystems at the benthic depths and in the water column. They are reported to be the chief converters of water and carbon dioxide to biomass and oxygen (see Eq. (1)) as they receive radiation from sunlight, and are therefore referred to as primary producers. Microalgae

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

exist either individually, or in chains or groups; and depending on the species, their sizes are typically 3–30 μm, while the cyanobacteria are as small as 0.2–2 μm [2].

H O CO CH O O *hv*

Pigments are chemical compounds that reflect and transmit only certain wavelengths of visible light. This makes them appear as the colors perceived. More important than their reflection of light is the ability of pigments to absorb light of certain wavelengths. A photosynthetic pigment (accessory pigment; chloroplast pigment; antenna pigment) is a pigment that is present in chloroplasts of algae and other photosynthetic organisms and captures the light energy necessary for photosynthesis. The reaction of each pigment is associated with only a narrow range of the spectrum, and it is necessary to produce several kinds of pigments with different colors to capture more of the sun's energy. Five important pigments found in algae are (i) chlorophyll (ii) xanthophyll (iii) fucoxanthin (iv) phycocyanin and

Algae and plants have chloroplasts in which the light-capturing chlorophyll is located, while in cyanobacteria the main light-capturing complex protein molecular assemblies are the phycobilisomes, which are located on the surface of thylakoid membranes [7]. Both chlorophyll and phycobilisomes absorb light most strongly between the high-frequency, high-energy wavelengths of 450 and 495 nm, which happen to be the blue region of the electromagnetic spectrum. Also, the photosynthetic pigments absorb the low-frequency, low-energy wavelengths between 620 and 750 nm, which is the red region of the electromagnetic spectrum. The chlorophyll pigment comes in different forms, and the structure of each type of Chlorophyll pigment is anchored on a chlorin ring with a magnesium ion at the centre. The side chain of each chlorophyll pigment type is different and they are so

Chlorophyll a with the molecular formula C55H72O5N4Mg is the most common type of Chlorophyll. It is a green pigment with a chlorin ring having magnesium at the centre (see **Figure 3**). Chlorin is a tetrapyrrole pigment, which is partially hydrogenated porphyrin. The ring-shaped molecule is stable with electrons freely migrating around it to establish resonance structures [9]. It also has side chains and a hydrocarbon trail and contains only –CH3 groups as side chains. The long hydrophobic tail anchors the molecule to other hydrophobic proteins on the surface of the thylakoid membrane. The chemical structural layout of chlorophyll shows a porphyrin ring attached to a protein backbone (see **Figure 3**). By substituting functional groups at positions C2, C3, C7, C8, and the C17-C18 bond, one can identify the structure of the desired chlorophyll (see **Tables 1** and **2**). Chlorophyll captures and absorbs blue, violet, and red light from the spectrum to transmit or reflect green, which is the color that the green algae exhibit [9, 10]. Oxygenic photosynthesis uses chlorophyll a to furnish electrons in the electron-transport chain. Photosystems I and II harbor many pigments that help to capture light energy.

Aside from producing oxygen and availing themselves as food for a large number of aquatic animals, algae are a good resource base for fine chemicals, crude oil, food supplement for humans, and some pharmaceutical products and finished goods [5].

2 2 22 +→ + (1)

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

**2. Photosynthetic pigments**

(v) phycoerythrin [6].

identified (see **Figure 3** and **Tables 1** and **2**) [7, 8].

**2.1 Chlorophyll**

exist either individually, or in chains or groups; and depending on the species, their sizes are typically 3–30 μm, while the cyanobacteria are as small as 0.2–2 μm [2].

$$\text{CH}\_{\text{z}}\text{O} + \text{CO}\_{\text{z}} \xrightarrow{hv} \text{CH}\_{\text{z}}\text{O} + \text{O}\_{\text{z}}\tag{1}$$

Aside from producing oxygen and availing themselves as food for a large number of aquatic animals, algae are a good resource base for fine chemicals, crude oil, food supplement for humans, and some pharmaceutical products and finished goods [5].
