Effect of dark period and reactor thickness on Arthrospira maxima continuous cultivation under high frequency pulsed light

High-frequency pulsed-light is interesting to improve the efficiency of artificially-illuminated reactors. Even though it is demonstrated that certain pulsed conditions can increase productivity, the flashing-light effect strongly depends on the operative conditions. In this study, Arthrospira maxima was cultivated under pulsed-light, focusing in particular on the effect of dark time duration and reactor thickness, to find the best parameters for a more efficient process. Being the nature of pulsed light complex in terms of number of interdependent parameters, a novel respirometric protocol was developed to obtain a quick response of the effect of pulsed light on photosynthetic efficiency. The protocol was then validated by continuous cultivation experiments at steady state. Results confirmed that a light period between 50 μs and 100 μs ensure the highest Oxygen Production Rate, while a minimum dark time of about 5 ms is necessary to maximize photosynthetic efficiency. Increasing the dark periods does not improve biomass productivity. The light pulse intensity plays also a role, with values above 17,850 μmol m⁻² s⁻¹ causing photoinhibition. Interestingly, the effect of pulsed light is also strongly linked to the thickness of the photobioreactor: at increased light paths, pulsed-light allows better photosynthetic efficiency and biomass concentration compared to continuous light, thanks to its higher penetration. The light regime parameters, thought, need to be set properly to avoid photoinhibition. In summary, this study allows a deeper understanding of the best light parameters for continuous cultivations exploiting pulsed light technology and provides a foundation for the comprehension of the flashing light effect, particularly focusing on the effect of dark time.