Microalgae, sunlight, and starch: low cell concentration is optimal for outdoor production under nutrient stress

Microalgae, particularly Chlorella vulgaris, are promising sustainable sources of starch for bioplastics and biofuels. Economically viable production requires optimizing starch accumulation, typically triggered by nutrient starvation, a process often facilitated by culture dilution to enhance light homogeneity. This study proposes an empirical model to predict the optimal cell concentration for starch accumulation under nitrogen starvation based on available light. The model, developed using 1-L outdoor cultures, was successfully applied to a 180-L photobioreactor. The theoretical optimal concentration under 330 µmol_photons m^-2 s^-1 (average absorbed photon flux density during daylight) was found at 0.8 g_DWL^-1. The corresponding carbohydrate productivity (43 mg_carbohydrate L^-1 h^-1 during daylight), carbohydrate content (57 %) and photosynthesis efficiency (6.8 % expressed as kJ_carbohydrate kJ_photonsPAR^-1) aligned with experimental results. This study indicates that optimal carbohydrate accumulation occurs at low cell concentration, which is a critical guideline for the future industrialization of microalgae production.