# Sustainable Microalgae and Cyanobacteria Biotechnology
## 2 Applications of Biotechnology on Microalgae and Cyanobacteria Utilization
### Biofuels from Microalgae
Implementation of an Efficient Production System: Mixotrophic Cultivation of Microalgae for Biodiesel Production  
The main cultivation systems for microalgae are open (such as raceway) and closed photobioreactors. The selection of a cultivation system depends on several factors, which can be chosen in order of application and economic objectives. Combining the selection criteria of the cultivation system in order of importance the selection can be as: the algal species, the desired final product, water supply, availability of sunlight, the cost of facilities, availability of nutrients, climatic conditions, and CO\({}_{2}\) supply [86]. The amount of nutrients and certain metals (i.e., iron and magnesium) must be optimal because they are important for the growth of microalgae and CO\({}_{2}\) fixation efficiency. A well-designed cultivation system can improve the efficiency of CO\({}_{2}\) capture by microalgae [86]. Another important factor when selecting a cultivation system is the likelihood of contamination by other microorganisms. For example, algal species like _Chlorella_, _Dunaliella_, and _Spirulina_ can grow only in specific environments, hence are not likely to be contaminated by other microalgae when \begin{table}
\begin{tabular}{l l l l} \hline \hline
**Microalgae Strain** & **Pretreatment Condition** & **Bioethanol** & **References** \\ \hline _Chlorella_ & Dilute acid 5\% sulphuric acid & 0.28 g/g biomass & [70] \\ \hline Mixed microalgae & Dilute sulphuric acid 0.5 N H\({}_{2}\)SO\({}_{4}\) at 120 \({}^{\circ}\)C for 4 h & 0.18 g/g biomass & [71] \\ \hline _Chlorella_ sp. & Dilute alkaline treatment at 2\% NaOH for 2 h at 120 \({}^{\circ}\)C & 0.081 g/g biomass & [72] \\ \hline _Chlorococccum infusionum_ & 0.74\% NaOH at 120 \({}^{\circ}\)C for 30 min & 0.26 g/g biomass & [73] \\ \hline _Nammochloropsis gaditana_ & 1 M NaOH, 120 \({}^{\circ}\)C for 30 min & 0.094 g/g biomass & [74] \\ \hline Defatted _Nammochloropsis colulata_ & 4\% H\({}_{2}\)SO\({}_{4}\) & 0.062 g/g sugar & [75] \\ \hline _Dunaliella tertiolecta_ & Pretreatment using 1\% (\(\
icefrac{{p}}{{v}}\)) sulfuric acid at 120 \({}^{\circ}\)C for & 0.62 g/g biomass & [76] \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Bioethanol production from different microalgal biomass feedstock. cultivated in an open-air system, and low possibility of bacterial contamination. In contrast, some marine microalgae such as _Tetraselmis_, _Skeletonem_, and _Isochrysis_ are susceptible to foreign invasion and some additional manipulation should be considered [87]. Light as a source of energy for photosynthetic organisms is the main limiting factor in the development of a cultivation system for microalgae. The photosynthesis rate is directly proportional to the light intensity above the light compensation point (not photosynthesis inhibition and not overshadowed by other cells). Mostly in microalgae, the photosynthetic system becomes saturated to a radiation close to 30% of the total solar irradiance, i.e., between 1700-2000 \(\mu\)Em\({}^{-2}\)s\({}^{-1}\). Some species of phytoplankton grow to optimal intensities of 50 \(\mu\)Em\({}^{-2}\)s\({}^{-1}\) and are photoinhibited to 130 \(\mu\)Em\({}^{-2}\)s\({}^{-1}\). The most significant issues for commercial cultivation of photosynthetic cells are photoinhibition and light limitation. One possible solution is to use photosynthetic cell heterotrophic metabolism to replace or supplement energy and carbon material from organic sources. According to some research, heterotrophic and autotrophic metabolic activities coexist, resulting in mixotrophic growth [88]. The relative contribution of photoautotrophic metabolism to biomass production increases with an increase in the coefficient of absorption of light or an increase in the supply of CO\({}_{2}\) and a decrease in the supply of the organic carbon source. Certain cellular components can accumulate during heterotrophic growth [89; 90].