Efficient CRISPR–Cas genome editing in brown algae

Brown algae represent the third most complex lineage to evolve multicellularity, independently from plants and animals. However, functional studies of their development, evolution, and biology have been constrained by the lack of efficient and scalable genome editing tools. Here, we report a robust, high-efficiency, and transgene-free CRISPR–Cas12-based genome editing method applicable across four ecologically and biotechnologically important brown algal species. Using Ectocarpus as a model, we optimized a PEG-mediated RNP delivery system employing a temperature-tolerant Cas12 variant, achieving reproducible, high-efficiency editing across multiple loci without the need for cloning or specialized equipment. As proof of concept, we precisely recapitulated the hallmark imm mutant phenotype by editing the IMMEDIATE UPRIGHT (IMM) locus, a phenotype previously described only from a rare spontaneous mutation. APT/2-FA-based selection further improved specificity with minimal false positives. The protocol was readily transferrable to other species, including kelps long considered recalcitrant to transformation. This platform now makes functional genomics accessible in brown algae, enabling mechanistic dissection of developmental processes, life cycle transitions, and the independent origins of complex multicellularity. Our work enables the broader adoption of brown algae as experimental models and provides a valuable platform for marine biotechnology and evolutionary research.