The key process for microalgae to obtain energy is photosynthesis. Photosynthesis is the process of using light energy (hν) to fix carbon dioxide into hydrocarbons and discharge oxygen as waste product (Eq.1).
CO2 +H2O ----> Cn(H2O)n + O2 Equation 1
Photosynthesis consists of light and dark reactions (see diagram). In the light reaction, pigments capture light to generate ATP and NADPH2. ATP serves as chemical energy and NADPH2 serves as reducing power. In the dark reaction, these energy-rich components are used in the Calvin cycle to convert carbon dioxide into organic molecules catalyzed by enzymes. Light reactions take place in the thylakoid membranes of chloroplasts. These thylakoid membranes contain the photosynthetic apparatuses consisting of light absorbing pigments and an electron transport chain. Firstly, the photosystem antenna complex, composing out of chlorophyll supported by accessory pigments, absorbs photons with wavelengths between 400 and 700 nm (Photosynthetic Active Radiation).
In the reaction center of the photosystems, chlorophyll absorbs one photon and releases one electron. These electrons are transported via the electron transport chain to photosystem II where reduction takes place and NADPH2 is generated. Via photolysis of water into oxygen and protons, the electron is regenerated at the chlorophyll. As a result, a proton gradient across the thylakoid membrane is created and this gradient is used by ATP synthase to generate ATP.
In the Calvin-Benson cycle, enzymes starting with ribulose-biphosphate carboxylase (Rubisco) use ATP and NADPH to synthesize three-carbon-sugars (C3-sugars) from carbon dioxide. Then, C3-sugars are combined to form molecules of glucose. Glucose can be converted to polysaccharides which serve as building materials or to fatty acids which serve as building blocks for membrane lipids or as a source of energy storage. The enzymes in the dark reaction are temperature dependent and therefore predominantly define the optimal temperature in which the species can grow.
Simplified scheme of photosynthesis.