Insight into the mechanism of nitrogen sufficiency conversion strategy for microalgae-based ammonium-rich wastewater...

The strategy of nitrogen sufficiency conversion can improve ammonium nitrogen (NH₄⁺-N) removal with microalgal cells from ammonium-rich wastewater. We selected and identified one promising isolated algal strain, NCU-7, Chlorella sorokiniana, which showed a high algal yield and tolerance to ammonium in wastewater, as well as strong adaptability to N deprivation. The transition from N deprivation through mixotrophy (DN, M) to N sufficiency through autotrophy (SN, P) achieved the highest algal yields (optical density = 1.18 and 1.59) and NH₄⁺-N removal rates (2.5 and 4.2 mg L^-1 d^-1) from synthetic wastewaters at two NH₄⁺-N concentrations (160 and 320 mg L^-1, respectively). Algal cells in DN, M culture obtained the lowest protein content (20.6%) but the highest lipid content (34.0%) among all cultures at the end of the stage 2. After transferring to stage 3, the lowest protein content gradually recovered to almost the same level as SN, P culture on the final day. Transmission electron microscopy and proteomics analysis demonstrated that algal cells had reduced intracellular protein content but accumulated lipids under N deprivation by regulating the reduction in synthesis of protein, carbohydrate, and chloroplast, while enhancing lipid synthesis. After transferring to N sufficiency, algal cells accelerated their growth by recovering protein synthesis, leading to excessive uptake of NH₄⁺-N from wastewater. This study provides specific insights into a nitrogen sufficiency conversion strategy to enhance algal growth and NH₄⁺-N removal/uptake during microalgae-based ammonium-rich wastewater treatment.