Photosynthesis is the key physicochemical process of the Life through which solar energy is transformed in chemical energy (ATP) and reducing potential (NADPH) to be used in assimilation process of inorganic carbon, nitrogen or sulphur. Photosynthesis can be measured as gas evolution i.e oxygen evolution or carbon assimilation or by in vivo chlorophyll a fluorescence used to estimate productivity of Ulva spp. In addition, light is source of environmental information detected by non-photosynthetic photoreceptor (UVA/Blue, green red/far red) and through a transduction chain, the expression of genes can be modulated controlling growth and the synthesis of bioactive compounds i.e. B light stimulates N metabolism and R light the C metabolism.