Seaweeds have substantial ecological function and economic potential and yet they have been largely ignored in biotechnological research. I have developed a molecular toolkit to help make Sea Lettuce (Ulva mutabilis) a model organism for multicellular green algae. My research has resulted in the efficient expression of transgenes in Ulva and I am now optimizing site-directed mutagenesis using CRISPR/Cas9. I propose to use this molecular toolkit to study of Carbon Concentrating Mechanisms (CCMs) in Ulva. Many algae use CCMs to capture and transport inorganic carbon (CO2 or HCO3-) from the water column to the chloroplasts to sustain photosynthesis and their growth. I will identify and functionally validate the genes responsible for CCM induction in Ulva using a combination of bioinformatics, Chromatin ImmunoPrecipitation (ChIP), RNA-sequencing and mutant analysis using a CRISPR screen. In parallel, I will generate gain-of-function lines to determine the intracellular location of putative CCM regulators and their protein interactors using Affinity Purification coupled to Mass Spectrometry (AP-MS). In summary, I have developed a molecular toolkit for the green seaweed Ulva and propose to use this toolkit to study CCM gene function. This research will enable me to break new ground in seaweed biotechnology and allow for a greater understanding of CCM in Ulva.