Green microalgae in intermittent light: a meta-analysis assisted by machine learning

Microalgal biotechnology still needs to alleviate the productivity bottleneck before achieving the full extent of its promises. With this goal in mind, many studies have investigated the impact of light/dark cycles on microalgae growth. In particular, two timescales of intermittent light have been investigated for operating a culture in a PBR: medium (0.1 to 10 Hz) and high (> 10 Hz) frequencies. However, regardless of the light regime, many conflicting results have been reported. This can be attributed to the belonging of the studied strain to different phyla and the variety of experimental designs. In this review, we propose a comparison of the results published for both frequencies. To compare likes to likes, only studies on green microalgae involving low-density cultures in an optically thin photobioreactor have been selected. Collected data were analyzed using machine learning and inferential statistics. First, the choice of the method for monitoring culture photosynthetic activity (growth rate or oxygen concentration) has an effect on the direction of the results. Second, two trends emerge. For medium frequencies, with respect to continuous light, lower duty cycle values (e < 0.1) hinder photosynthetic activity (p < 0.001), while higher values (0.5 < e) produce similar results (p = 0.557). In high frequency regime, lowering the cycle time furthers culture performances (+ 20 to + 73% depending on the monitoring procedure, p< 0.001). In addition, based on the reviewed evidence, recommendations are drawn to avoid a waste of effort in future works: flat panel airlift combined with intermittent lighting should be the reference experimental device for this type of investigation. Furthermore, growth rate monitoring should be preferred to its dissolved gases counterpart.