Abstract
Light attenuation in photobioreactors is a major bottleneck in microalgal production. A possible strategy for artificial light-based microalgal production to deliver light deep inside the culture is through the periodical emission of high intensity light flashes (so-called flashing light). However, our results did not show improved photosynthetic rates compared to continuous light for dilute and concentrated Tetraselmis chui cultures exposed to flashing light with various repetition rates (frequencies 0.01 Hz–1 MHz), light-dark ratios (duty cycles: 0.001–0.7) or time-averaged light intensity (50–1000 μmol s−1 m−2). Likewise, flashing light applied to Chlorella stigmatophora and T. chui batch cultures could not enhance growth. However, we observed flashing light effects at different duty cycles and frequencies, depending on cell acclimation, culture concentration, and light intensity. In conclusion, artificial flashing light does not improve microalgal biomass productivities in photobioreactors, but low frequencies (f < 50 Hz) may be still used to improve light harvesting-associated biomolecules production.
Highlights
Research and development on microalgal biotechnology dates back to the 19th century, and the first commercial cultivation was reported in the 1960s (Milledge, 2011)
A photosynthetic oxygen evolution rate vs irradiance curve (P-I curve) of T. chui cultures obtained under continuous irradiance shows the photo-acclimation stages of cultures exposed to the average light intensity used in the flashing light experiments (Ia = 50, 500 and 1000 μmol s−1 m−2; stock 2.1–2.3, 3; Fig. 1)
The maintenance re spiration of the 50, 500 and 1000 μmol s−1 m−2 adapted cultures
Summary
Research and development on microalgal biotechnology dates back to the 19th century, and the first commercial cultivation was reported in the 1960s (Milledge, 2011). The limitation of any PBR is the inefficiency in delivering photons at optimum wave lengths and quantities to drive photosynthesis in all microalgal cells within a culture (Schulze et al, 2017; Schulze et al, 2014). The cells at the periphery prevent penetration of light into the PBR, limiting the photosynthetic efficiency and productivity of the whole culture (AbuGhosh et al, 2016). To improve the delivery of photons to cells in a culture, light in tensities as well as culture mixing velocities should be increased. While high-intense light penetrates deeper into a PBR, appropriate culture mixing rates allow the algal cells to move faster from the light-limited (or dark) to light-saturated (or inhibiting-) zones (Abu-Ghosh et al., 2016; Brindley et al, 2016). The fast transition from light- to dark zones helps to avoid photoinhibition of cells at the periphery but ensures the sufficient absorption of light energy by most cells to carry out photo synthesis and convert this energy into energy-bound molecules (e.g., sugars in the Calvin cycle, NADPH, ATP; Sivakaminathan et al, 2018)
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