Low-light and tannin functionalization promote high-density filamentous cyanobacterial biofilms for low-C/N aquaculture tailwater treatment: Stratified structures and metabolic networks.

Algal-bacterial biofilms show great promise for treating aquaculture tailwater with low carbon-to-nitrogen (C/N), yet their practical application is hindered by limited light penetration, which restricts biomass accumulation and reduces algal-to-bacterial ratios. Here, we developed a tannin-functionalized optical fiber (OTL) system that integrates internal light delivery with a bio-flocculant tannin interface to promote the formation of high-density cyanobacterial biofilms. The OTL system achieved removal efficiencies of 95.73%, 96.93%, 84%, and 89% for NH4+-N, NO3--N, COD, and PO43⁻-P, respectively, and increased biofilm biomass by 2.37-fold compared with illuminated bare fibers (OL). Multiscale imaging and spectral analyses showed that the middle biofilm layer harbored the highest cyanobacterial-to-bacterial ratio. Filamentous cyanobacteria enriched in this layer secreted abundant exopolysaccharides, forming a robust biological scaffold that recruited coccoid cyanobacteria and heterotrophic bacteria and generated a stratified architecture favorable for mass transfer and algal-bacterial interactions. Metagenomics analysis identified Desertifilum as the dominant filamentous cyanobacterium in OTL. The relative abundances of exopolysaccharide biosynthesis and export genes, including wzx, wzy, and wza, were 1.11- to 1.22-fold higher in OTL, indicating enhanced potential for exopolysaccharide polymerization and export. Metagenome-assembled genomes further revealed that complex exopolysaccharides produced by Desertifilum were degraded by epiphytic bacteria such as Flavobacterium and Sediminibacterium, supplying bioavailable carbon to Thauera and establishing a cooperative metabolic network associated with enhanced nitrogen uptake, nitrate reduction, and coupled GS/GOGAT-mediated nitrogen assimilation. This study demonstrates that optical fiber-guided photochemical regulation can engineer stratified, high-density algal-bacterial biofilms, offering a nature-inspired and low-carbon strategy for aquaculture tailwater treatment and ecological water restoration.