Red Light Enhanced Nitrogen Removal Efficiency by Bacterial–Algae Biofilm Reactor in Recirculating Aquaculture Systems

This study aimed to evaluate the effects of different light wavelengths on nitrogen removal efficiency and microbial community dynamics in a bacterial–algal biofilm reactor (BABR) within recirculating aquaculture systems (RASs). Four RAS units were operated under red, blue, red–blue (1:1), and white light, and their performance in nitrogen transformation, microbial community composition, extracellular polymeric substances (EPSs), and gene abundance was systematically assessed. The results showed that red light markedly improved ammonia removal and overall nitrogen transformation stability, particularly under high nitrogen loading, by enabling faster recovery and suppressing nitrite accumulation. Microbial analyses revealed that red light enriched key algae (e.g., Scenedesmus) and functional bacteria (e.g., Bosea and Nitrospirota), supporting efficient nitrification and denitrification. Furthermore, gene annotation demonstrated that red light enhanced the abundance of photosynthetic proteins and nitrogen metabolism pathways, including biofilm formation, quorum sensing, and amino acid biosynthesis. Collectively, these findings highlight red light as a promising regulatory factor for enhancing biofilm-based nitrogen removal in RASs, providing a theoretical basis for light-assisted aquaculture wastewater treatment.