Layer-specific photo-metabolic specialization of encapsulated microalgae: A strategy for synchronous multi-nitrogen elimination from wastewater

Conventional wastewater treatment (WWT) systems face persistent challenges in simultaneous ammonium (NH₄⁺-N) and nitrate (NO₃^--N) removal due to substrate competition and energy-intensive multi-stage processes. This study presents an innovative strategy leveraging encapsulated microalgal systems to achieve synchronous 94.45 % NH₄⁺-N and 98.47 % NO₃^--N removal within a single reactor through spatial reprogramming of photosynthetic energy allocation. By exploiting the structural heterogeneity within alginate-encapsulated beads, depth-stratified metabolic zones were created that challenge the long-held dogma of microalgae's inherent NH₄⁺-N preference. Multidimensional analyses, including spatial distribution mapping, molecular dynamics simulations, metagenomic profiling and photosynthetic regulation, further revealed that light-modulated oxygen gradients, polymer-mediated solute transport, and stratified photo-metabolic specialization synergistically reprogramed microalgal nitrogen metabolism, enabling co-utilization of NH₄⁺-N and NO₃^--N. The system demonstrated robust dual-nitrogen assimilation efficiencies under varying environmental conditions, transcending conventional substrate utilization hierarchies. This transformative approach not only resolves the dilemma of mixed nitrogen pollution but also advances sustainable WWT by integrating pollutant removal with biomass valorization. The findings provide mechanistic insights into microalgal metabolic plasticity and offer a scalable, energy-efficient solution to upgrade traditional denitrification technologies, aligning with urgent demands for circular economy in water resource management.