Biomass production and physiology of Chlorella vulgaris during the early stages of immobilized state are affected by light intensity and inoculum cell density

The interest for biofilm-based systems for microalgae and related compounds production has been increasing lately. Although extensive literature has been reported on productivity, the physiological characterization (photosynthetic activity and composition) of attached cells at early stages of biofilm development has seldom been investigated. In this work, the effect of light intensity and inoculum cell density on 3-days Chlorella vulgaris biofilms developed on membranes was studied. Biomass production was clearly impacted by mechanism of photo-limitation occurring in biofilms acclimated to low light intensity (50 μmol m−2 s−1). A higher electron transport capacity and lower chlorophyll content in biofilms at high light intensity (500 μmol m−2 s−1) were also measured which are in line with patterns observed for suspended microalgae cultures. In addition, optimal conditions in terms of light (250 μmol m−2 s−1) combined with low (4.8 × 106 cells cm−2) or high inoculum density (28.8 × 106 cells cm−2) were identified to optimize biomass and lipids production, respectively. On the whole, measuring physiological profiles of immobilized cells at the initial stages of biofilm development provides information to efficiently operate and optimize biofilm-based systems.