Cross-feeding rewires carbon and nitrogen fluxes in anammox consortia treating long-chain fatty acids.

Long-chain fatty acids (LCFAs) are common in lipid-rich wastewater, yet the dose-dependent effects on anaerobic ammonium oxidation (anammox) remain unclear. In this work, the effect of oleate on anammox was evaluated in short- and long-term experiments at different C/N ratios using multi-omics approaches. Oleate showed a dose-dependent effect. Low oleate dosages, particularly at a C/N ratio of 0.1, improved nitrogen removal performance, whereas high dosages shifted nitrite consumption from anammox toward heterotrophic reduction. Multi-omics analysis showed that appropriate oleate dosages also stimulated central carbon metabolism, while the Wood-Ljungdahl pathway remained comparatively stable. 13C-isotope tracing results demonstrated that oleate-derived carbon entered acetate, central-carbon intermediates, amino acids, peptides, and vitamin-related metabolites, supporting a cross-feeding framework involving LCFA-degrading heterotrophs and anammox bacteria. In contrast, excessive oleate caused incomplete oleate degradation, membrane damage, and community restructuring. These findings provide a basis for controlling LCFA dosage to improve anammox-based treatment of organic-rich side streams.