Overexpression of S-Adenosylmethionine Synthetase in Recombinant <i>Chlamydomonas</i> for Enhanced Lipid Production

Abstract

consistent with the 2050 Carbon Neutral Strategy recently announced by the Republic of Korea's Ministry of Environment [1,2] Biodiesel is one of the most popular biofuels that can be produced from the lipid accumulated in microalgae.Although microalgae do not accumulate much lipid under normal living conditions, they do accumulate it when exposed to abiotic stress.Two steps are needed for microalgae to produce biofuel: growth under appropriate conditions followed by abiotic stress to accumulate lipids.Since this process is associated with problems due to high cost, a long cultivation period, and low productivity, it is essential to develop microalgae with high lipid content and stress resistance for industrial applications [2][3][4][5][6].Chlamydomonas reinhardtii, a single-celled alga belonging to the Generally Recognized as Safe category, is used as a model organism in various fields such as algal physiology, photosynthesis, and metabolism, as well as biofuel production.Moreover, it has the advantages of easy laboratory-scale experimentation, fast cultivation, and scalability [7][8][9].C. reinhardtii also accumulates large quantities of lipids under abiotic stress, especially in nitrogen-deficient conditions [10].Therefore, many studies are being conducted on the industrial development of the strain.In the studies that have been conducted so far, CRISPR-Cas9 technology has been used to facilitate genetic manipulation; the ferredoxin gene petF was overexpressed to increase heat and H 2 O 2 stress resistance, and putative ACSMD (2-amino-3-carboxymuconate-6semialdehyde decarboxylase) from the macroalga Pyropia yezoensis was transferred to C. reinhardtii to increase stress resistance in nitrogen-deficient conditions and enhance lipid production [11][12][13].Microalgae are attracting much attention as promising, eco-friendly producers of bioenergy due to their fast growth, absorption of carbon dioxide from the atmosphere, and production capacity in wastewater and salt water.However, microalgae can only accumulate large quantities of lipid in abiotic stress, which reduces productivity by decreasing cell growth.In this study, the strategy was investigated to increase cell viability and lipid production by overexpressing S-adenosylmethionine (SAM) synthetase (SAMS) in the microalga Chlamydomonas reinhardtii.SAM is a substance that plays an important role in various intracellular biochemical reactions, such as cell proliferation and stress response, and the overexpression of SAMS could allow cells to withstand the abiotic stress and increase productivity.Compared to wild-type C. reinhardtii, recombinant cells overexpressing SAMS grew 1.56-fold faster and produced 1.51-fold more lipids in a nitrogen-depleted medium.Furthermore, under saline-stress conditions, the survival rate and lipid accumulation were 1.56 and 2.04 times higher in the SAMS-overexpressing strain, respectively.These results suggest that the overexpression of SAMS in recombinant C. reinhardtii has high potential in the industrial-scale production of biofuels and various other high-value-added materials.

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