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

Abstract The interest by biofilm-based microalgae technologies has increased lately due to productivity improvement, energy consumption reduction and easy harvesting. However, the effect of light, one key factor for system’s operation, received less attention than for planktonic cultures. This work assessed the impact 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 $${\upmu {\textrm{mol}} \, \textrm{m}^{-2} \, \textrm{s}^{-1}}$$ μ mol m - 2 s - 1 . In the first stage of biofilm development, uniform cell distribution was observed on the substratum exposed to 100 $${\upmu \textrm{mol} \, \textrm{m}^{-2} \, \textrm{s}^{-1}}$$ μ mol m - 2 s - 1 while cell clusters were formed under 500 $${\upmu \textrm{mol} \, \textrm{m}^{-2} \, \textrm{s}^{-1}}$$ μ mol m - 2 s - 1 . Though similar specific growth rate in exponential phase (ca. 0.3 $${\textrm{d}^{-1}}$$ d - 1 ) was obtained under all light intensities, biofilm cells at 500 $${\upmu \textrm{mol} \, \textrm{m}^{-2} \, \textrm{s}^{-1}}$$ μ mol m - 2 s - 1 seem to be ultimately photoinhibited (lower final cell density). Data confirm that Chlorella vulgaris showed a remarkable capability to cope with high light. This was marked for sessile cells at 300 $${\upmu \textrm{mol} \, \textrm{m}^{-2} \, \textrm{s}^{-1}}$$ μ mol m - 2 s - 1 , which reduce very rapidly (in 2 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.