Partial decrease of xanthophyll contents by β-carotene hydroxylase knockout enables high-density cultivation of Chlamydomonas

Abstract

Photosynthetic microalgae are sustainable and eco-friendly bioresources enabling CO2 conversion into valuable substances. However, high-density cultivation often decreases biomass productivity due to a decrease in photosynthetic efficiency caused by self-shading. Here, we provide a solution to the self-shading effect by reducing the xanthophyll content, an important component of the photosynthetic antenna complex. We knocked out β-carotene hydroxylase (CHYB) from the model microalga Chlamydomonas reinhardtii using CRISPR-Cas9, a target-specific genome editing method. The decrease in xanthophyll content in CHYB knockout (chyb) mutants decreased the ratio of trimeric to monomeric LHCII, as well as chlorophyll and carotenoid contents. The growth rate of chyb mutants was similar to that of their background strains under mixotrophic and photoautotrophic conditions. The maximal cell density of chyb mutants was higher than that of their background strain under low-light conditions in mixotrophic culture and high-light conditions in photoautotrophic culture. Analysis of oxygen production per chlorophyll confirmed lower photosynthetic activity in chyb mutants under low-light conditions and higher photosynthetic activity under high-light conditions. These results highlight the potential of strategies to decrease xanthophyll content through modulation of the carotenoid biosynthetic pathway to improve high-density cultivation.