Microalgae for Biofuels: Applications, Process Constraints and Future Needs

Microalgae are eukaryotic (e.g. green algae, diatoms) photosynthetic organisms capable of utilizing carbon dioxide and light for the synthesis of carbohydrates as energy compounds. They have been known since many years, but their large-scale cultivation has started a few decades ago. They have the potential to grow in open systems such as raceway ponds, circular ponds and lakes and also in controlled condition like closed photobioreactors. Microalgae are advantageous considering their higher productivity than terrestrial oilseed plants and ease of cultivation in wastewater and saline water. Microalgae do not compete with agricultural land for cultivation. They have dual role such as utilization of CO₂ from atmosphere as well as remediation of wastewater by utilizing nutrients from wastewater to grow into biomass. Microalgae contain different types of major metabolites and high-value products such as proteins, lipids, carbohydrates, vitamins, pigments, antioxidants, minerals, etc. (Gupta et al. 2016; Mata et al. 2010; Rawat et al. 2011; Shriwastav et al. 2014; Francavilla et al. 2015). Their major metabolites are rich in essential amino acids and essential fatty acids, e.g. omega-3 fatty acids. Productivity of these major metabolites can be increased through mode of cultivation and nutrient limitation/stresses. Commonly, the lipids from microalgae are converted into biodiesel by the process of transesterification. After lipid extraction, a huge amount of residual biomass is left that is known as lipid-extracted algae (LEA). LEA still contains the high-value metabolites like proteins and carbohydrates in residual biomass (Ansari et al. 2015; Ju et al. 2012). Lipid-extracted algae can also serve as a good resource for biomethane, bioethanol and syngas production. In addition, protein fraction of LEA has promising potential as food and feed additive for animal and aquaculture. LEA biomass due to rich nitrogen content can also be employed as a fertilizer. Therefore, considering the rich chemical composition of microalgae, it can be considered as a good feedstock for the biorefinery.