Microbial niches and metabolism drive spatial heterogeneity of hydroxyapatite precipitation in aerobic granular...

Biologically induced phosphate precipitation (BIPP) in aerobic granular sludge (AGS) provides a promising approach to address phosphorus removal instability and granule structural fragility in practical applications. However, the roles of microbial communities, ecological niches, and metabolic activities in driving phosphate precipitation and shaping its spatial distribution within AGS remain underexplored. This study systematically investigates AGS physicochemical properties, reactor performance, phosphorus speciation, precipitation composition and distribution, microbial community structure, and metabolic activity using sodium propionate (R_P) and sodium acetate (R_A) as sole carbon sources. The findings reveal for the first time the mechanisms by which microbial communities, ecological niches, and metabolic functions regulate phosphate precipitation and determine its spatial heterogeneity. BIPP contributes 22.6% and 60.1% of total phosphorus removal in R_P and R_A, respectively, thereby enhancing phosphorus removal efficiency and granule structural stability. Multi-scale analyses-including Standards, Measurements and Testing, Raman spectroscopy, X-ray diffraction, scanning electron microscopy-energy dispersive X-ray spectroscopy, and micro-computed tomography-reveal that hydroxyapatite (HAP) predominantly accumulates in the outer region of R_P granules but in the inner region of R_A granules. Periodic water quality variations, fluorescence in situ hybridization, granule-stratified sequencing, and metagenomic analyses indicate that the spatial heterogeneity of HAP is driven by the ecological niche separation and metabolic activities of polyphosphate-accumulating organisms (PAOs) and glycogen-accumulating organisms (GAOs). In R_P granules, PAO‑driven anaerobic phosphate release creates a high‑phosphate microenvironment, which promotes HAP formation in the granule outer region. In R_A, GAO‑mediated endogenous denitrification increases local pH, thereby inducing HAP precipitation in the granule interior. Overall, this study elucidates the mechanisms underlying the spatial heterogeneity of phosphate precipitation in AGS from the perspectives of microbial community structure, ecological niches, and metabolic pathways. These findings provide guidance for optimizing AGS systems to achieve efficient phosphorus removal and stable operation.