Double-strand break-free and transgene-free genome editing in the microalga Nannochloropsis oceanica using removable vectors containing the CRISPR base editing system

The accumulation of lipids by algae makes them attractive for carbon-neutral fuel production; however, the industrial-scale production of algal lipids has yet to be achieved. Currently, researchers are trying to improve the lipid productivity of algal strains using genome editing for molecular breeding with CRISPR-Cas9, which allows the efficient alteration of genomic information. However, CRISPR-based gene modification via double-strand breaks sometimes induces unintended large deletions that are toxic to host cells. Here, we applied the cytidine base editor combined with an episomal vector backbone containing a centromere and autonomous replication sequence to the microalga Nannochloropsis oceanica. The cytosine base editor introduces cytidine-to-thymidine base substitutions using deaminase without double-strand breaks, and an episomal vector enables plasmid removal after base substitution. We succeeded in inducing cytidine-to-thymidine substitution at the six target sites of five endogenous genes. The base substitution activity ranged from 29.2% to 47.6% on cytidine bases at the 16th to 19th positions from the protospacer adjacent motifs. The removal of base editor plasmids was also detected, which is essential for constructing transgene-free strains. Our results provide insights into the applicability of further technologies in the genetic modification of microalgae.