Effect of N/P ratio on attached microalgae growth and the differentiated metabolism along the depth of biofilm

Attached microalgae cultivation coupled with wastewater treatment could convert pollutants into bioresource with high efficiency and low cost. Nitrogen to phosphorus ratio (N/P ratio) is considered as an important factor on microalgae growth. Due to spatially heterogeneous distribution of nutrient, how N/P ratio affected attached microalgae growth in both macro- and micro-scopes was explored in this study. The findings revealed that an optimal N/P ratio of 10:1 promoted attached microalgae growth, while unsuitable ratios hampered algal growth by inhibiting photosynthesis, lowering oxidative resistance and decreasing metabolism activity. Long-term cultivation with improper N/P ratios resulted in a gradual decrease in actual photosynthetic rates, implying 50 days as the upper culture time limit for high-efficiency growth. Moreover, the study highlighted the uneven distribution of light and nutrients in algal biofilms, causing cells in different biofilm layers with variability of metabolism and composition. However, the 15N isotopic distribution demonstrated that even bottom cells were equally capable of nitrogen assimilation. [Display omitted] •The optimal N/P ratio in wastewater for attached microalgae growth was around 10:1.•Excessive or insufficient P supplement both gradually destroyed algal photosynthesis.•Long-term transport of phosphorus, other than N, in microalgae biofilm was blocked.•Uneven light and N/P distribution changed algal compositions along the biofilm.•Pollutant utilization occurred throughout the biofilm, including the dark layers.