# Sustainable Microalgae and Cyanobacteria Biotechnology
## 2 Applications of Biotechnology on Microalgae and Cyanobacteria Utilization
### Biofuels from Microalgae  
Microalgal biomass contained several major biomolecules, such as lipid and carbohydrate, which could be converted into third-generation biofuels. The amount of lipids in microalgal biomass varies by species. Several studies have investigated the lipid content range of 20-50 percent in various microalgal species. Microalgal lipid extracted from biomass can then be converted into biodiesel via a catalytic chemical reaction using either a chemical or an enzyme as a catalyst [66]. Microalgal-based biodiesel is thought to be competitive with terrestrial crops due to the high growth rate, high lipid intracellular accumulation, and non-seasonal production. Furthermore, microalgal growth can be performed on bioremediation processes, wastewater treatment, or CO2 sequestration [67], or in limited growth conditions for other microorganisms [68]. Other liquid biofuels, such as bioethanol and biobutanol, can also be produced by utilizing microalgal biomass. The production of these biofuels involves the fermentation of microalgal carbohydrate polymer using microorganisms such as yeast and bacteria as biocatalysts [69]. Prior to the fermentation process, the biomass must be pretreated and the carbohydrate extracted (Figure 2). Yeast, such as _Saccharomyces cerevisiae_, which is a common microorganism used in fermentation, will consume the carbohydrate and convert it into bioethanol via anaerobic fermentation during the fermentation process. Several studies have been conducted to investigate the potential of microalgal carbohydrate as a bioethanol feedstock using various pretreatment methods (Table 4). Clearly, the majority of the research indicated that different pretreatment processes could have a significant impact on the release of microalgal carbohydrates and bioethanol production. Another study on the fermentation of lipid-extracted _Chlorella_ sp. KR-1 biomass found that when the fermentation was performed using _S. cerevisiae_ KCTC 7931 under separate hydrolysis and fermentation (SHF) processes, approximately 0.4 g/g sugar of bioethanol corresponding to 0.16 g/g biomass was produced. Other studies on bioethanol production from microalgal species such as _Tetraselmis suecia_, _Dunaliella tertiolecta_, _Chlamydomonas_ sp., and _Stigecolumum_ sp. have also been published [77; 78; 79; 80]. Biobutanol, on the other hand, is produced through the conversion of microalgal carbohydrate by _Clostridium_ sp. under strictly anaerobic fermentation conditions. Various products such as butyric acid, acetic acid, acetone, ethanol, and butanol are produced during the fermentation process, which involves two major metabolic pathways: acidogenesis and solventogenesis [81]. Acidogenesis is a biological conversion reaction by bacteria of carbohydrate or monomer sugar into volatile fatty acids such as acetic acid and butyric acid. As the second phase of ABE fermentation, the biological reaction is continuing to solventogenesis, which is a biochemical reaction by _Clostridium_ species to produce acetone and butanol. There have been few studies on biobutanol production from microalgal biomass to date. For instance, Hong et al. [82] reported that approximately 3.48 g/L of biobutanol was produced from the fermentation of _Gelidium amansii_ pretreated with 2% sulfuric acid. Another study on ABE fermentation of _Chlamydomonas reinhardtii_ CCAP 11/32C produced 10.31 g butanol/g biomass under optimal conditions. The study also found that pretreatment processes could significantly influence biobutanol production [83]. Investigation on biobutanol from enzymatic hydrolyzed _Nammochloropsis_ sp. also indicated that approximately 2.61% of butanol was produced from the ABE fermentation [84]. In addition, the production of butanol from different microalgae such as _Chlorella_ sp. and _Bordinellopsis texensis_ CCALA [85] has been reported. A study reported that microalgae cultivated using wastewater could accumulate high carbohydrate content, a procedure that can be useful for butanol production from _Bordinellopsis texensis_ CCALA.