Impact of the NH 4 + /NO 3 − ratio on growth of oil-rich filamentous microalgae Tribonema minus in simulated nitrogen-rich wastewater

This study delves into the impact of varying NH 4 + -N: NO 3 − -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 NH 4 + -N at a concentration of 60 mg/L and NO 3 − -N at a concentration of 180 mg/L (mass concentration ratio of 1:3). Due to the excessive concentration of NH 4 + -N, the photosynthetic apparatus is damaged. When NH 4 + -N exceeds 120 mg/L (ratio 1:1 of NH 4 + -N: NO 3 − -N), growth is seriously hindered. However, increased NH 4 + -N levels foster lipid accumulation. Notably, NH 4 + -N removal was high across all NH 4 + -N treatments, ranging from 78 %–90 %. Nitrate, ammonium, and total phosphorus removal declined with increasing NH 4 + -N proportions due to limited growth of Tribonema minus. When NH 4 + -N was 120 mg/L (ratio 1:1 of NH 4 + -N: NO 3 − -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 NH 4 + -N supply. Increasing NH 4 + -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 NH 4 + -N: NO 3 − -N ratio to optimize biomass yields with high lipid content.