*4.1.2 Nutrients availability*

*Biotechnological Applications of Biomass*

from 14.71 to 5.90%.

*4.1.1 Inorganic carbon*

(NADPH) as shown in Eq. (1).

The release of H<sup>+</sup>

influences the composition of microalgae biomass. For instance, Varshney and coworkers [37] reported that an increase in temperature from 20 to 25°C doubled the lipid content of *Nannochloropsis oculata* (from 7.90 to 14.92%), whilst an increase from 25 to 30°C brought about a decrease of the lipid content of *Chlorella vulgaris*

Inorganic carbon that is accessible to microalgae is mostly CO2 gas. This gas is available in dilute concentrations in the atmosphere at 0.035 mole percent (dry basis) [38]. Microalgae absorb CO2 from the atmosphere to produce sugars by the physiochemical process of photosynthesis. The biological conversion of CO2 results in products of the photosynthetic metabolism such as cells, oxygen biopolymers which are soluble in the culture medium and volatile organic compounds (VOC's). Zhao and Su [39] described photosynthesis as a two-stage process. The first stage is the light-dependent reaction which captures the energy of light for oxidative phosphorylation in the metabolic cycle that produces the energy-storage molecules, adenosine triphosphate (ATP) and nicotinamide adenine dinucleotide phosphate

2i 2 2H O 2NADP 3ADP 3P 2NADPH 2H 3ATP O *hv* + + + + +→ + + + (1)

(2)

Eq. (2) shows the second-stage reaction, which is carbon dioxide fixation; and it is not directly light-dependent. This photosynthetic dark-reaction captures and reduces carbon dioxide to carbohydrates and releases molecular oxygen [40].

CO 2H O CH O O 22 2 2 +→ + [ ] *photons*

With a solubility of 0.1449 g CO2/100 mL H2O at 25°C and 101.325 kPa vapor pressure, carbon dioxide gas dissolves in surface water and slowly reacts with water

H CO H HCO 2 3 <sup>3</sup>

medium. However, the ability to thrive in a wide pH range has given microalgae the privilege to access nutrients from municipal and industrial wastewaters. In the

<sup>2</sup> OH HCO H O CO 32 3

When the dynamic ionization equilibrium is attained, dissolved inorganic carbon

− , CO3

are accessible to microalgae and both species are utilized simultaneously

presence of hydroxide ions, carbonate ions are also released as in Eq. (5).

ion in Eq. (4) causes the reduction in pH of the culture

CO H O H CO 2 2 23 + (3)

− − <sup>−</sup> + + (5)

2−, and H2CO3. However, only CO2

+ − + (4)

to alter its chemistry as shown in Eqs. (3) and (4).

(DIC) is available in the form of CO2, HCO3

**436**

and HCO3

−

Standard microalgae culture media have been developed and produced out of the need to produce desired products and are available in the market for freshwater microalgae growth management. Some of the media are the (i) Blue-Green medium, BG-11 (ii) Bold's Basal medium, BBM (iii) Bold's Basal medium modified, BBM-3 N (iv) CHU 13 and (v) Jaworski's Medium, JM. Both municipal and industrial wastewater have the basic nutrients common to all the artificial media designed for microalgae cultivation; and microalgae access these nutrients as nitrates and reactive phosphates from wastewaters to produce biomass and bioproducts [42, 43].
