Seaweeds are important and have unique features making them interesting for industrial applications, yet they are underrepresented in functional gene characterization studies. Ulva is currently the only seaweed where transgenes can be efficiently expressed and while gene disruption by CRISPR is possible, it is inefficient. We need to drastically increase CRISPR efficiency in Ulva to allow the generation of large-scale mutant collections for functional studies. In this project we will optimize various CRISPR approaches (knockout, base editing and homology-directed repair) using our modular cloning systems. We will evaluate if environmental conditions (heat, nitrogen starvation) can increase CRISPR efficiency. Using an iterative adaptation of the best-performing factors, our aim is to optimize the mutation rate from currently <1% to >50% of by the end of the project. This will enable large scale investigations of Ulva gene function for both basic and applied research.