Examining Phosphate Precipitation: Impact of Biological Nucleation Sites on the Precipitation of Fluorapatite Crystals as Liesegang Bands in a Double-Diffusion Setup

The deposition and burial of phosphate minerals is generally rare as this limiting nutrient is more likely to be recycled by organisms than precipitated into benthic environments. Phosphorites within the geologic record thus present themselves as an area of study. Common methods to explain the presence of phosphorites in the rock record rely upon microbial metabolic pathways to increase pore concentrations of phosphate, allowing for spontaneous precipitation of the phosphorus-rich mineral apatite may occur. A less studied area of microbial influence upon benthic phosphate deposition is the use of microbial substrates as scaffolding for crystal growth. Using a double- diffusion setup supersaturated with respect to fluorapatite, the effects of microbial scaffolding in the form of extracellular polymeric substance (EPS) on the precipitation of phosphate minerals were studied. Precipitation occurred in Liesegang bands, a phenomenon associated with discrete rather than continuous spatial crystallization within a substrate. The microbial presence provided preferential nucleation sites for precipitation, shown by the morphological development of the bands with microbial substrate versus the abiotic controls. These results show that biology impacts precipitation of phosphates through the growth mechanisms of the mineral by lowering the activation energy for precipitation