The Fluctuating Cell-Specific Light Environment and Its Effects on Cyanobacterial Physiology

Individual cells of cyanobacteria or algae are supplied with light in a highly irregular fashion when grown in industrial-scale photobioreactors (PBRs). These conditions coincide with significant reductions in growth rate compared to the static light environments commonly used in laboratory experiments. We grew a dense culture of the model cyanobacterium Synechocystis sp. PCC 6803 under a sinusoidal light regime in a bench-top PBR (the Phenometrics environmental PBR [ePBR]). We developed a computational fluid dynamics model of the ePBR, which predicted that individual cells experienced rapid fluctuations (∼6 s) between 2,000 and <1 µmol photons m^-2 s^-1, caused by vertical mixing and self-shading. The daily average light exposure of a single cell was 180 µmol photons m^-2 s^-1 Physiological measurements across the day showed no in situ occurrence of nonphotochemical quenching, and there was no significant photoinhibition. An ex situ experiment showed that up to 50% of electrons derived from PSII were diverted to alternative electron transport in a rapidly changing light environment modeled after the ePBR. Collectively, our results suggest that modification of nonphotochemical quenching may not increase cyanobacterial productivity in PBRs with rapidly changing light. Instead, tuning the rate of alternative electron transport and increasing the processing rates of electrons downstream of PSI are potential avenues to enhance productivity. The approach presented here could be used as a template to investigate the photophysiology of any aquatic photoautotroph in a natural or industrially relevant mixing regime.