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In photobioreactors and natural systems, microalgae are subjected to rapidly changing light intensities (LI) due to light attenuation and mixing. A controlled way to study the effect of rapidly changing LI is to subject cultures to flashing light. In this study, series of flashing‐light experiments were conducted using Synechocystis sp. PCC6803 with constant overall average LI of approximately 84 μmol·m−2·s−1 and relative times in the light and dark varied. The results were also compared with simulated results using a mathematical model including an absorbed pool of light energy, photoacclimation, and photoinhibition. With equal time in light and dark, the specific growth rate (μ) systematically decreased with increasing light duration, and µ decreased further when the ratio of light to dark was decreased. The model captured both trends with the mechanistic explanation that when the light duration was very short the changes in the pool of absorbed LI were smoothed out across the light and dark periods, whereas longer durations caused the biomass to experience discrete light and dark conditions that lead to reduced light absorption, more energy loss to nonphotochemical quenching, and more photodamage. These growth effects were accentuated as the ratio of light to dark decreased. A series of flashing‐light experiments with equal total photon flux was conducted with Synechocystis sp. PCC 6803. Longer periods of equal light and dark led to a decrease in growth rate, and a larger ratio of time in the light versus dark further decreased the growth rate. A dynamic model was used to explain the trends: Light and dark periods were "blended" with rapid flashes, but decreases in growth rate during longer flashes caused less light absorption and more photodamage. [ABSTRACT FROM AUTHOR]

The viability of large-scale microalgae cultivation depends on providing optimal growth conditions, for which a key operational parameter is culture density. Using Synechocystis sp. PCC 6803, we conducted a series of fixed-density, steady-state experiments and one batch-growth experiment to investigate the role of culture density on biomass production and light utilization efficiency. In all cases, the fixed-density, steady-state experiments and batch-growth experiment showed good agreement. The highest biomass production rates (260 mg L−1 d−1) and efficiency for converting light energy to biomass (0.80 μg (μmol photons)−1) occurred together at a culture density near 760 mg L−1, which approximately corresponded to the lowest culture density where almost all incident light was absorbed. The ratio of OD680/OD735 increased with culture density up to the point of maximum productivity, where it plateaued (at a value of 2.4) for higher culture densities. This change in OD680/OD735 indicates a photoacclimation effect that depended on culture density. Very high culture densities led to a sharp decline in efficiency of biomass production per photons absorbed, likely due to a combination of increased decay relative to growth, metabolic changes due to cell-cell interactions, and photodamage due to mixing between regions with high light intensity and zero light intensity. [ABSTRACT FROM AUTHOR]

Abstract The success of algal and cyanobacterial cultivation hinges on the robustness of the processes, and a key to this success is managing the microbial community co-existing with the algae or cyanobacteria. In this work, we grew the cyanobacterium Synechocystis sp. PCC6803 in a flat-plate photobioreactor operated as a sequencing batch reactor using two light conditions: continuous incident photosynthetically active radiation of 112 or 364 μmol m−2 s−1 (32 or 90 μmol m−2 s−1 average internal light intensity, respectively). As expected, the higher light condition supported more Synechocystis biomass, but the proportion of photosynthetically produced electrons released as soluble microbial products (SMP; cellular components released during growth and decay) also increased – ~42% compared to 20% at the lower light condition – which corresponded to an increase in heterotrophic biomass that grew by oxidizing the SMP. After the increase in incident light intensity, the co-existing community of heterotrophic bacteria showed a large taxonomical transition, with Sphingobacteriales declining and Comamonadaceae increasing. Over the course of 7 days, however, the microbial community structure stabilized, gradually moving towards the initial composition. We suggest that changes in the microbial community resulted from changes in SMP composition from Synechocystis at the different light intensities. Graphical abstract Unlabelled Image Highlights • Synechocystis sp. PCC6803 was grown under 112 or 364 μmol m−2 s−1 of PAR light. • Heterotrophic bacteria fed on soluble microbial products (SMP) from Synechocystis. • Higher light produced more biomass, but with a higher proportion of heterotrophs. • The heterotrophic community structure shifted with a change in light intensity. • The changes in the heterotrophic community resulted from changes in SMP composition. [ABSTRACT FROM AUTHOR]