Photoacclimation strategies in Phaeodactylum tricornutum biofilms

Microalgal biofilms are a promising alternative to conventional suspended cultures, offering increased biomass densities with reduced water and energy demands for cultivation and harvesting. However, to fully demonstrate their interest for large-scale production, a better understanding of the impact of operational factors on biofilm structure is required. This study explores the effect of four different photon flux densities (75, 150, 300 and 600 μmol m-2 s-1) on Phaeodactylum tricornutum biofilms cultivated in a millifluidic system. For the first time, biofilm structure and physiology were characterised in situ, non-destructively by complementary imaging tools (Confocal Laser Scanning Microscopy, CLSM, and Optical Coherence Tomography, OCT) and dissolved oxygen measurements. Biofilms cultivated at 150 and 300 μmol m-2 s-1 presented the highest growth rates (0.31 and 0.38 d-1) while higher light intensities (600 μmol m-2 s-1) induced photoinhibition. On the other hand, biofilms at 75 μmol m-2 s-1 exhibited the lowest growth rate (0.23 d-1) but they were extremely efficient in converting absorbed light into biomass (2.5 times more efficient than the biofilms grown at 300 μmol m-2 s-1). Interestingly, adjustments in the optical properties of biofilms exposed to different light conditions were observed through changes in the light extinction coefficient with biofilms becoming more transparent when their thickness was higher than 200–300 μm. These findings reveal important photoacclimation strategies in P. tricornutum biofilms and highlight the potential of exploiting different light intensities and harvesting strategies to optimise the operation of biofilm-based processes.