Effect of Light Intensity on the Growth and Nutrient Uptake of the Microalga Chlorella sorokiniana Cultivated in Biogas Plant Digestate

This study investigated the effect of light intensity on the growth and nutrient uptake of Chlorella sorokiniana cultivated in nitrogen-rich anaerobic digestion wastewater. Three light intensities (20, 68, and 162 µmol m⁻2 s⁻1) were applied over a 30-day period with a 16:8 h light–dark photoperiod. The goal was to understand how light affects biomass productivity, nutrient assimilation, and biochemical composition under varying nitrogen concentrations originating from biogas plant digestate, up to 5 g L⁻1. The results showed that higher light intensities significantly boosted biomass production, achieving a five-fold increase at 162 µmol m⁻2 s⁻1 compared to 20 µmol m⁻2 s⁻1. Nutrient uptake followed a similar pattern, with 94% of ammonium nitrogen removed in 7 days under high light, compared to 55% after 30 days under low light. Phosphorus content was also completely removed after 7 days under light intensities of 68 and 162 µmol m⁻2 s⁻1. Additionally, elevated light intensity led to increased lipid accumulation (from 29.7% to 34%) and reduced protein content (from 30.9% to 26.1%), with carbohydrate content not being affected by light intensity. These findings highlight light intensity as a critical factor for optimizing microalgae cultivation in nitrogen-rich biogas digestate, promoting both effective nutrient removal and biomass production for potential bioenergy applications.