The aim of this PhD project is to characterize sugar biosynthetic pathways in brown macroalgae at the molecular and physiological levels. Brown macroalgae produce two main types of storage sugars: mannitol (a simple sugar) and laminarin (a polymer made up mainly of glucose units, with the optional addition of a few mannitol units). As these biosynthetic pathways have been poorly described so far, the main aim of this PhD project is to identify and characterize the enzymes responsible for the catalytic activities of these two interconnected biosynthetic pathways in the brown macroalga Ectocarpus. Original molecular and cellular biology approaches, including genome editing, will be used to generate the biological material needed to characterize these biosynthetic pathways and their role in brown macroalgal physiology.