Physiological transition of Chlorella vulgaris from planktonic to immobilized conditions

Biofilm-based technologies present many advantages (e.g., higher productivity, lower water demand and harvesting costs) compared with the conventional planktonic approaches for microalgae cultivation. A better understanding of photosynthetic biofilm formation is though still needed in order to develop and run biofilm-based systems at large-scale. In this study, for the first time the physiological transition of C. vulgaris cells from planktonic to immobilized state was tracked (cell number, morphology, photosynthetic performance, and cellular composition) during the first 24 h of immobilization. The results clearly confirmed that microalgae rapidly respond to immobilization via physiological adjustments. Over very short time-scales (3 h), cells used photosynthesis to grow (increase in size, 140 %), at expense of cell division, and adjusted their carbon allocation patterns (increase in the relative carbohydrates pool, 135 %). The experiments confirmed that this behavior is specific to cells in the immobilized state. Triggering factors such as water and/or nutrient availability may be responsible for this fast acclimation process. Additionally, lipid content doubled by the end of cultivation, possibly due to imbalanced carbon and nitrogen metabolisms. These results offer new insights for understanding the mechanisms involved in microalgae biofilm formation and development, further helping the operators to optimize the biofilm-based systems.