Genetic Impairment of Cellulose Biosynthesis Increases Cell Wall Fragility and Improves Lipid Extractability from Oleaginous Alga Nannochloropsis salina.

Seok Won Jeong,Jong Min Lim,Yong Keun Chang,Kwon Hwangbo,Won-Joong Jeong,Youn-Il Park,Bong Soo Lee,Seung Won Nam,Byeong-Ryool Jeong

doi:10.3390/microorganisms8081195

Abstract

In microalgae, photosynthesis provides energy and sugar phosphates for the biosynthesis of storage and structural carbohydrates, lipids, and nitrogenous proteins. The oleaginous alga Nannochloropsis salina does not preferentially partition photoassimilates among cellulose, chrysolaminarin, and lipids in response to nitrogenous nutrient deprivation. In the present study, we investigated whether genetic impairment of the cellulose synthase gene (CesA) expression would lead to protein accumulation without the accumulation of storage C polymers in N. salina. Three cesA mutants were generated by the CRISPR/Cas9 approach. Cell wall thickness and cellulose content were reduced in the cesA1 mutant, but not in cesA2 or cesA4 cells. CesA1 mutation resulted in a reduction of chrysolaminarin and neutral lipid contents, by 66.3% and 37.1%, respectively, but increased the soluble protein content by 1.8-fold. Further, N. salina cells with a thinned cell wall were susceptible to mechanical stress, resulting in a 1.7-fold enhancement of lipid extractability. Taken together, the previous and current studies strongly suggest the presence of a controlling mechanism that regulates photoassimilate partitioning toward C and N metabolic pathways as well as the cellulose metabolism as a potential target for cost-effective microalgal cell disruption and as a useful protein production platform.

Highlights

In microalgae, similar to other photoautotrophs, the energy and building blocks used in the C, N, and S metabolism, such as carbohydrates, lipids, nucleic acids, and proteins, are derived from photosynthetic electron transport
Photosynthetic light reactions drive the oxidation of water by PSII and photosynthetic electron flow that leads to the generation of chemical energy (ATP), reducing power (NADPH), and reduced electron carriers such as ferredoxins and thioredoxins [1]
Neutral lipid rather than cytosolic chrysolaminarin is a major storage for C polymers in N. salina grown under high CO2 and N-replete growth conditions [9]

Summary

Introduction

Similar to other photoautotrophs, the energy and building blocks used in the C, N, and S metabolism, such as carbohydrates, lipids, nucleic acids, and proteins, are derived from photosynthetic electron transport. Under nutrient-rich conditions, fixed C is mostly routed into proteins. N limitation favors C allocation that (frequently) leads to increased carbohydrate fraction and a reduced protein fraction. The photosynthetic apparatus is forced to deal with excess photons, since carbohydrate synthesis requires fewer electrons per C than protein synthesis. Understanding this regulatory mechanism is important in terms of the biotechnological and ecological aspects of algae [1]

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Conclusion