Genetic modulation of light-harvesting complex in chlorella to improve photosynthetic efficiency and biomass productivity

Abstract

Microalgae have been proposed as eco-friendly feedstocks for various kinds of products, from biofuels to value-added products. Microalgae can accumulate a large amount of biomass through photosynthesis. However, the photosynthetic efficiency of microalgae is not high enough to achieve economically feasible production system, partly due to oversized antenna that causes shading effects under large scale cultivation. In an effort to overcome this issue, mutants with truncated light-harvesting chlorophyll antenna (tla) were generated by ethyl methanesulfonate (EMS)-mediated mutagenesis of Chlorella vulgaris. The tla mutant, designated E5, exhibited 56.5 and 75.8% decrease in chlorophyll a and b contents, respectively, with partial reductions in the expression levels of light-harvesting complex proteins of the photosystem II. The saturated photosynthetic activity and relative electron transport rate (rETR) of the E5 mutant significantly increased under the high light condition, and non-photochemical quenching (NPQ) was reduced, compared to wild type. Consequently, the E5 mutant showed 44.5% improvement in biomass productivity under high light $(200 μmol photons m^{-2} s^{-1})$. In order to reveal the mutation responsible for the phenotype changes, we carried out the whole genome sequencing, followed by identification of single nucleotide polymorphisms (SNPs) in the coding sequences. A point mutation (T to A) was found in the gene encoding a protein with 468 amino acids, which was homologous to chloroplast-localized signal recognition particle 43 kDa protein (CpSRP43) of Chlamydomonas reinhardtii. The 102nd amino acid in the CpSRP43 homologous protein in E5 was changed from valine to glutamic acid (V102E) in the first chromodomain conserved in plants and green algae. Complementation of E5 with a wild type copy of CpSRP43 partially recovered its phenotypes to wild type, including pigment contents, expression levels of light-harvesting complex proteins, rETR, NPQ and growth. In conclusion, the single amino acid change (V102E) in CpSRP43 resulted in the truncation of light-harvesting complex, and can be the target for genetic engineering to improve photosynthetic efficiency and biomass productivity in industrial microalgae including Chlorella.