Impact of the NH4+/NO3− ratio on growth of oil-rich filamentous microalgae Tribonema minus in simulated nitrogen-rich wastewater

This study delves into the impact of varying NH₄⁺-N: NO₃⁻-N ratios in simulated nitrogen-rich wastewater on the physiological and biochemical responses of the Xanthophyceae Tribonema minus. Our findings revealed intriguing patterns: the maximum biomass observed was 5 g/L, which occurred in treatments containing NH₄⁺-N at a concentration of 60 mg/L and NO₃⁻-N at a concentration of 180 mg/L (mass concentration ratio of 1:3). Due to the excessive concentration of NH₄⁺-N, the photosynthetic apparatus is damaged. When NH₄⁺-N exceeds 120 mg/L (ratio 1:1 of NH₄⁺-N: NO₃⁻-N), growth is seriously hindered. However, increased NH₄⁺-N levels foster lipid accumulation. Notably, NH₄⁺-N removal was high across all NH₄⁺-N treatments, ranging from 78 %–90 %. Nitrate, ammonium, and total phosphorus removal declined with increasing NH₄⁺-N proportions due to limited growth of Tribonema minus. When NH₄⁺-N was 120 mg/L (ratio 1:1 of NH₄⁺-N: NO₃⁻-N), the lipid content accounted for 50 % of the biochemical components, surpassing the control group without ammonium addition by 15 %. The lipid composition of T. minus primarily comprises palmitoleic acid, constituting around 50 % of total fatty acids, with its concentration rising at higher NH₄⁺-N supply. Increasing NH₄⁺-N also improved fatty acid profiles and biodiesel properties according to international standards. Overall, this work adds knowledge to stimulate microalgal lipid production. Not only should absolute amounts of nutrients be considered, but also the NH₄⁺-N: NO₃⁻-N ratio to optimize biomass yields with high lipid content.