Microbial factors behind struvite precipitation: enzymes and extracellular polymeric substances (EPS) as key players

Struvite is a phosphate-based mineral commonly formed in ammonia-rich settings such as soils, aquatic environments, wastewater systems, and the urinary tract. Its distinct morphology and mineral properties have drawn interest across disciplines, including microbiology, geology, medicine, and materials science, especially for its ammonia capturing ability. Microbial metabolic activities have been recently recorded to play a critical role in its formation. However, the influence of microbial characteristics, such as enzymatic activity, biofilm, and extracellular polymeric substances (EPS), on struvite morphology and precipitation mechanisms remains underexplored. This study investigates microbial struvite precipitation mediated by three distinct microbes, Sporosarcina pasteurii, Bacillus subtilis, and Pseudomonas fluorescens to investigate the role of different enzymes and EPS on struvite mineralisation. The bacterial cultures selected for the study have varying ureolytic and alkaline phosphatase activity, as well as EPS and biofilm production potential. Struvite precipitation kinetics under different microbial conditions were studied along with its morphology and biofilm-mineral interactions employing microstructural and mineralogical analyses (via field emission scanning electron microscopy, confocal laser scanning microscopy, X-ray diffraction, and Fourier transform infrared spectroscopy). This study reports that urease and phosphatase activities (between 7 and 7.5 U/mL) along with moderate EPS production (~6.5 g/L), lead to the generation of the highest quantities of struvite crystals with moderate size (~103 ± 5 µm), contrary to the larger crystals (~180 ± 5 µm) produced with a higher urease activity (8.4 U/mL) and lower EPS (2 g/L). The experimental findings suggest a complex interplay between enzymatic activity and EPS in shaping crystal structure and its features, which have a significant impact on mineral characteristics. Additionally, moderate biofilm formation (OD₅₇₀ ~1.2) resulted in maximal struvite precipitation, indicating that structured biofilms support efficient mineral nucleation and accumulation. Overall, this study has unravelled key insights into microbial struvite precipitation mechanisms and can pave the way for tailored engineering and biotechnological applications utilising this phosphate mineral. KEY POINTS: Biogenic struvite precipitation is structurally different from abiogenic struvite Type of microbial species, surface features, enzymes and EPS impact crystallinity of struvite Composition of EPS effect phase and pattern of biogenic struvite crystal.