Photoprotection capacity of microalgae improved by regulating the antenna size of light-harvesting complexes

The unicellular green microalga Desmodesmus sp. S1 can produce total lipid over 50% of cell dry weight. A mutant D90G-19 induced by C-12(6+) heavy ion irradiation generates more lipid which is 20% higher than its wild type (WT) under high light. The gene expression profiles in the photosynthetic pathway between the Desmodesmus sp. S1 WT and D90G-19 were compared, and several key genes were verified using qRT-PCR and western blotting. The psbA and psbD encoding the D1 and D2 subunits of PSII in D90G-19 were upregulated and higher than WT under high light, while an LHCB gene of Desmodesmus sp. was downregulated dramatically. The expression level of LHCB of D90G-19 was less than that of WT. The improved photoprotection capacity of D90G-19 was attributed to LHCII antennae probably due to the mutation of LHCB gene, which led to less photon into the electron transport chain. Furthermore, LHCBM, a homologous gene of LHCB in model microalgae N. gaditana Cas9+ strain, was knocked out using CRISPR/Cas9. The resulting mutant NgCas9-LHCBM showed an improved capacity to protect the photosynthetic complexes by dissipating extra light energy through non-photochemical quenching pathway. Therefore, the photoprotection capacity of microalgae can be improved by engineering the quantity of PSII subunits to fine-tune PSII reaction centers and the adjacent LHCII antennae.