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

Abstract

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 mmol photons m 22 s 21 , caused by vertical mixing and self-shading. The daily average light exposure of a single cell was 180 mmol photons m 22 s 21. 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.

Fluctuating light experiments and semi-automated plant phenotyping enabled by self-built growth racks and simple upgrades to the IMAGING-PAM

“…Over the last decade, plant science and photosynthesis research has made a big push towards gaining insights into complex physiological, biochemical, and genetic processes under more realistic growth conditions than the traditional lab regimes in which growth environments are kept as constant as possible (Vialet-Chabrand et al, 2017;Andersson et al, 2019). In this respect, light regimes represent a good example because light intensities in nature change frequently (Slattery et al, 2018).…”

A Chlorophyte alga utilizes alternative electron transport for primary photoprotection

“…We have previously modelled the light conditions experienced by individual cells in 255 an ePBR system (see Andersson et al, 2019 for modelling details). We translated the output 256 of this single-cell model at solar noon to a shortened script for employment on a Dual-PAM 257 100 coupled with MIMS ( Fig.…”

“…Testing of NPQ capacity ex situ was required to rule it out as a major photoprotective process 391 in this organism, but it might have suggested an exaggerated role for AET in situ. Thus, we 392 utilized the light environment previously used by Andersson et al (2019) to assay the 393 utilization of AET vs NPQ in industrially relevant, in situ light conditions (ZT 6 h; Fig S5). 394…”

“…Chlorophyll PAM fluorescence and membrane-inlet mass spectrometry analysis 495A Dual-PAM 100 fluorometer (Walz, Germany), coupled with a quad mass 496 spectrometer (Pfeiffer PrismaPlus QMS200; PTM28612), was employed for simultaneous 497 chlorophyll a fluorescence and oxygen evolution and consumption measurements in a 498 custom-made 2 ml glass cuvette with plastic stopper (seeAndersson et al, 2019 for system 499 details). Cells were harvested during the exponential growth phase, centrifuged (1000 x g, 10 500 min) and resuspended in fresh culture medium to reach a final chl a concentration of 1.4 μg 501 chl a mL −1 .…”

“…Rates of oxygen evolution 513 and consumption were calculated with the following equations(Beckmann K et al, ] and [18O2] reflect the ratios of 16 O2 and 18 O2 at the start of each two-minute 517 actinic light illumination step. Oxygen concentrations were normalized to argon 518 concentrations, chosen for similar solubility as O2, to account for abiotic variance due to the 519 MIMS gas consumption(Andersson et al, 2019). Ion current slopes were calibrated based on 520 oxygen values for saturated and anoxic media values.…”