Rising carbon dioxide levels and the goals of the Paris Agreement have intensified the focus on carbon removal strategies. Blue carbon ecosystems, including mangroves, salt marshes, seagrass meadows and macroalgae are key to carbon sequestration due to their ability to store large amounts of carbon in their biomass. Unlike rooted plants, algae release dissolved organic carbon over their mucilage layer, with up to 50% of brown algae exudates consisting of fucoidan, a complex polysaccharide. This thesis confirms the active release of fucoidan by six brown algae species. Globally, up to 0.1 Gt of fucoidan carbon is exuded annually, equivalent to 0.367 Gt of carbon dioxide. Fucoidan resists microbial degradation, aggregates and is transported to and stored inside coastal ecosystems. This research highlights the collective storage of carbon by coastal vegetated ecosystems and emphasises that algal-derived polysaccharides are promising candidates for carbon sequestration.