The impact of light supply to moving photosynthetic biofilms

The production of microalgae using biofilm-based processes is becoming popular because of their higher productivity compared to traditional culture systems. Another advantage of microalgal biofilms is the straightforward harvesting procedure achieved by scraping off the biofilm, significantly reducing the energy demand required when concentrating liquid culture. Here, a promising way to grow microalgae is explored, examining a biofilm developed on a moving conveyor belt. Algae are then successively exposed to light and dark periods as the conveyer belt rotates. A lab-scale biofilm-based reactor mimicking the light pattern of the moving system was first used to study the effect of light/dark cycles on a Chlorella autotrophica biofilm. The succession of light and dark phases (in the order of minutes) effectively dilutes the light over a given time period and mitigates overexposure of light, which can lead to photoinhibition. When the illumination time represents one-third of the cycle period (light dilution factor of 3), the biofilm seems to deal with photoinhibition better than when the biofilm is exposed to permanent illumination. Extrapolations for a rotating conveyer belt in such conditions points out twofold productivity compared to a static biofilm exposed to continuous light. However, when the periods in the dark extend too long, respiration decreases the carbon pool, hindering the benefit of photosynthesis, and a trade-off must be achieved.