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 commerciallyappealing 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 in biomass, triacylglycerol (TAG), and photosynthetic pigments on a 24-hour scale in the diatom Cyclotella cryptica grown using a sinusoidal light and temperature regime. Cell division synchronized to occur predominantly in the middle of the light period and biomass 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 divisionassociated membrane biosynthesis would be high. For the first time, photosynthetic pigment dynamics data, obtained with higher temporal resolution than previously reported for microalgae, was overlaid with cell cycle progression, indicating that while some photosynthetic pigments respond primarily to light, others are influenced by the cell cycle. Another novel observation was that the abundance of most photosynthetic pigments also fluctuated substantially on a rapid time scale during dawn, dusk, and the beginning of the dark period. 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.