In the sunlit surface layer of the ocean, photosynthetic microalgae such as diatoms convert more carbon dioxide into biomass than Earth’s tropical forests. Like land plants, diatoms sequester carbon dioxide into polymeric carbohydrates — in other words: into long-chained sugars called laminarin. However, it has proven difficult to quantify how much carbon dioxide can be stored in the global oceans throughout this process. Photosynthesis in the surface ocean produces on average twelve gigatons of carbon annually in the form of algal laminarin. according to the Global Carbon Budget 2019, humans released 11.5 gigatons of carbon during 2018. However, only a small part of the carbon bound by laminarin is permanently removed from the atmosphere — a large part is subsequently released again through natural processes. In total, the oceans permanently absorbed around 2.6 gigatons of carbon in 2018. laminarin comprises as much as 50 percent of the organic carbon in sinking diatom-containing particles.
— source Max Planck Institute for Marine Microbiology | Mar 18, 2020