The effect of light intensity on microalgae biofilm structures and physiology under continuous illumination

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Biofilm-based microalgae technology improves productivity, reduces energy consumption and facilitates harvesting. However, the effect of light received less attention than for planktonic cultures. This work assessed the effect of Photon Flux Density (PFD) on Chlorella vulgaris biofilm dynamics (structure, physiology, activity). Microalgae biofilms were cultivated in a flow-cell system with PFD from 100 to 500 μmol·m-2·s-1. In the first stage of biofilm development, uniform cell distribution was observed on the substratum exposed to 100 μmol·m-2·s-1 while cell clusters were formed under 500 μmol·m-2·s-1. Though similar specific growth rate in exponential phase (ca. 0.3 d-1) was obtained under all light intensities, biofilm cells at 500 μmol·m-2 ·s-1 seem to be ultimately photoinhibited (lower final cell density). Chlorella vulgaris showed a remarkable capability to cope with high light. This was marked for sessile cells at 300 μmol·m-2·s-1, which reduce very rapidly (in two days) their chlorophyll-a content, most probably to reduce photodamage, while maintaining a high final cell density. Besides cellular physiological adjustments, our data demonstrate that cellular spatial organization is light-dependent.