Timing is everything: Diel metabolic and physiological changes in the diatom Cyclotella cryptica grown in simulated outdoor conditions

Microalgal cultures grown on a light-dark cycle experience diel patterns in metabolic and physiological processes, including cell cycle synchronization, but the implications for productivity in terms of biomass and commercially-appealing molecules are not commonly appreciated. Despite a long history of diel response studies, only recently have photobioreactor technology advances enabled the use of sinusoidal light and temperature to more accurately mimic outdoor conditions. The present study investigates cell cycle progression and dynamic changes on a 24-hour scale in triacylglycerol (TAG), photosynthetic pigments, and optical density at 750 nm (OD750) as a proxy for biomass in the production candidate diatom Cyclotella cryptica grown using a sinusoidal light and temperature regime. Cell division synchronized to occur predominantly in the middle of the light period while OD750 started to increase several hours earlier, as the cells prepared to divide. TAG levels increased during the day and decreased at night, with a mid-day dip corresponding to the time when lipid needs for cell division-associated membrane biosynthesis would be high. Photosynthetic pigment dynamics data was overlaid with cell cycle progression, indicating that while some photosynthetic pigments respond primarily to light, others are influenced by the cell cycle. Additionally, the results indicate that in a synchronized culture, potential product yields change substantially throughout the day. This may inform harvest timing to significantly increase yield.