Optimization of light for growth, photosynthesis, and hydrocarbon production by the colonial microalga Botryococcus braunii BOT-22

► Hydrocarbon production by B. braunii saturated at 200μmolphotonsm−2s−1. ► Hydrocarbons stored in the colony were not used as substrate for respiration. ► Neutral lipid body in the cell was degraded by consumption in the dark. Optimization of the light conditions for biofuel production by the microalga Botryococcus braunii BOT-22 (race B) was performed using monochromatic red light. The lipid and sugar contents were approximately 40% and 20–30% of the cell dry weight, respectively, and about half of the lipids were liquid hydrocarbons. The half-saturation intensities for the production rate of lipids, hydrocarbons, and sugars were 63, 49, and 44μmolm−2s−1, respectively. Fluorescence microscopic images of Nile Red-stained cells showed an increased number of intracellular neutral lipid granules due to increased light intensity. After 16days of incubation in the dark, lipid and sugar, but not hydrocarbon content decreased. Growth, metabolite production, and photosynthesis were saturated at 100, 200 and 1000μmolm−2s−1, respectively. These results indicate that photosynthetically captured energy is not used efficiently for metabolite production; thus, improvements in metabolic regulation may increase hydrocarbon production.