Synergistic microalgal-bacterial interactions enhance nitrogen removal in membrane-aerated biofilm photoreactors treating aquaculture wastewater under salt stress: Insights from metagenomic analysis.

This study investigates the membrane-aerated biofilm photoreactor (MABPR) for treating aquaculture effluents with low C/N ratio and elevated salinity (0.5%-3.2%). The MABPR integrated biofilm reactors with microalgal-bacterial consortia, achieving superior total inorganic nitrogen (TIN) removal by leveraging counter-diffusional biofilm properties, bubbleless aeration, and enhanced microalgal productivity. The system consistently outperformed conventional reactors, achieving 84.7 ± 1.9% TIN removal at 3.2% salinity with TIN removal flux increasing from 0.82 ± 0.04 to 1.22 ± 0.07 g/m² d. The MABPR promoted microalgal proliferation (Chl-a/VSS: 8.08-15.04 mg/g) and higher biomass productivity (1.83 g/m² d) compared to SBBPR and MABR. Elevated salinity stimulated extracellular polymeric substance (EPS) production, reinforcing biofilm stability and microbial resilience. The MABPR demonstrated 22%-65% higher nitrogen removal efficiency than controls at the highest salinity. Canonical nitrification-denitrification remained the primary nitrogen removal pathway, with short-cut nitrification-denitrification contributing under salt stress. Metagenomic analysis revealed bidirectional adaptation between microalgae and bacteria, with enriched nitrogen assimilation (GS/GOGAT pathway) compensating for bacterial deficits. Microalgae facilitated pollutant removal through ammonia uptake and dissolved organic matter release, supporting denitrification. At 3.2% salinity, Nitrosomonas and Nitrobacter abundance increased by 42.6% and 35.8%, while denitrifiers Denitromonas and Hoeflea dominated, comprising 59.4% and 35.9% of the population. The MABPR further promoted the synthesis of growth cofactors (vitamins, phytohormones), enhancing microalgal productivity and stress resilience. These synergistic microalgal-bacterial interactions supported pollutant removal, showcasing the MABPR as a robust, sustainable solution for aquaculture wastewater treatment and resource recovery under salt stress.