Light is the most important factor for microalgal growth. The amount of photon energy received by each cell is a combination of several factors: photon flux density, cell density, length of optical path (thick ness of culture layer), and rate of mixing. The light capture by photosynthetic pigments is roughly 10 times higher under full sunlight (2000 p,mol photons m s ) than that required to saturate growth. In other words, up to 90% of the photons captured in full sunlight by chlorophyll and other pigments are not being used for photosynthesis and instead must be dissipated as heat and fluorescence. Consequently, the efficiency of light utilization usually drops from a theoretical value of 20% (based on photosynthetically active irradiance) to lower than 4%, roughly corresponding to an annual biomass yield of about 40tha 1.

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