A huge variety of organisms adorn sunlit coastal ecosystems, including the colorful and biodiverse group of seaweeds. Seaweeds are primary producers, so they capture carbon (CO2 and HCO3-) from seawater and convert this to the biomass we eat and the oxygen we breathe. In fact, they capture as much carbon from the atmosphere as the Amazon rainforest each year. One group of green seaweeds, Ulva or Sea lettuce, grows all over the world and is particularly known, sometimes notorious, for its fast growth potential. Ulva can produce so much biomass that it overgrows the entire coastal ecosystem. These so-called “green tides” are detrimental to most other marine organisms and several human activities including tourism. SEASYDE wants to understand how Ulva converts carbon to biomass so fast from a molecular biology perspective. To achieve this, we first need to expand our toolset: we are currently able to activate the expression of genes (gain-of-function), but are limited by the efficiency of technology enabling inhibition of gene function (loss-of-function). In SEASYDE, I will optimize the efficiency of the Nobel prize-winning CRISPR technology in Ulva to allow precise gene editing. Using these molecular tools, I propose to study putative regulator genes of carbon capture using both gain- and loss-of-function mutant lines. SEASYDE will therefore establish how the carbon capture pathway is regulated in the ecological and economical important species Ulva.