Final answer:
Brown algae store food using a polysaccharide called laminarin and rely on mannitol for transport of carbohydrates.
Step-by-step explanation:
Brown algae, known scientifically as Phaeophyta, are multicellular marine organisms that use photosynthesis to produce food. To store the food they produce, brown algae primarily use a carbohydrate storage polysaccharide called laminarin, which consists of glucose units linked by beta 1-3 linkages.
Unlike starch found in plants that has alpha 1-4 linkages, laminarin is unique to brown algae. Additionally, they utilize a compound called mannitol as a transport carbohydrate, which parallels the function of sucrose in most plants.