Biotechnological Approaches for Biomass and Lipid Production Using Microalgae Chlorella and Its Future Perspectives

Abstract

Heavy reliance on fossil fuels has been associated with increased climate disasters. As an alternative, microalgae have been proposed as an effective agent for biomass production. Several advantages of microalgae include faster growth, usage of non-arable land, recovery of nutrients from wastewater, efficient CO₂ capture, and high amount of biomolecules that are valuable for humans. Microalgae Chlorella spp. are a large group of eukaryotic, photosynthetic, unicellular microorganisms with high adaptability to environmental variations. Over the past decades, Chlorella has been used for the large-scale production of biomass. In addition, Chlorella has been actively used in various food industries for improving human health because of its antioxidant, antidiabetic, and immunomodulatory functions. However, the major restrictions in microalgal biofuel technology are the cost-consuming cultivation, processing, and lipid extraction processes. Therefore, various trials have been performed to enhance the biomass productivity and the lipid contents of Chlorella cells. This study provides a comprehensive review of lipid enhancement strategies mainly published in the last five years and aimed at regulating carbon sources, nutrients, stresses, and expression of exogenous genes to improve biomass production and lipid synthesis.

Keywords: Chlorella; biomass; biotechnology; lipids; microalgae; phycoremediation.

Fig. 1. Overview of the putative metabolic pathways for lipid biosynthesis under heterotrophic cultivation in Chlorella.

Uptake of glucose, acetate, and glycerol to produce lipids. Intermediate products and other metabolic biosynthesis pathways were omitted from this metabolic pathway.

Fig. 2. Overview of the putative metabolic pathways for nitrogen assimilation in Chlorella.

Uptake of nitrate, nitrite, ammonium, and urea to produce amino acids. Intermediate products and other metabolic biosynthesis pathways were omitted from this metabolic pathway. NR, nitrate reductase: NiR, nitrite reductase GS, glutamine synthetase; GOGAT; glutamine oxoglutarate aminotransferase.

Fig. 3. Stress factors that induce lipid production in Chlorella strains.

(A) Diagram of the stress factors that induce lipid production in Chlorella strains. (B) Microscopy images of the Nile Red-stained cells grown in TAP medium with nitrogen (+N) or without nitrogen (-N) for 72 h. Brightfield, Nile Red (lipid droplets), autofluorescence, and merged images with Nile Red and chlorophyll autofluorescence are shown. Scale bars, 10 μm.