The Effect of Polymeric Substances on Apatite Reactivity in the Presence of a Freshwater Cyanobacterium

Abstract

Marine cyanobacteria have evolved phosphate uptake systems to live where P is limiting. Ultimately, P derives from apatite in rocks but little is known about apatite solubilization by cyanobacteria. Fluorapatite (FAP) was added to a P-free culture medium with and without Anabaena, as well as in cell-free supernatant to test reports that the cyanobacterium Anabaena PCC 7120 enhances the dissolution of fluorapatite (FAP). Anabaena, a fresh-water organism, is an analogue to harder-to-grow marine cyanobacteria. The initial abiotic release rate ratios (Ca/P) are significantly higher than the similar stoichiometric ratio of apatite. The majority of the “missing” P in abiotic experiments is sorbed to precipitates formed from Fe added as a nutrient to the culture medium. In contrast to literature reports, Ca release from FAP in the presence of Anabaena or culture supernatant was lower than abiotic release. However, Anabaena cultures aggregate and cells attach to the FAP when P-limited. Adhesion is explained by surface charge rather than P content because significant adhesion on boehmite occurred in similar experiments. X-ray photoelectron spectroscopic (XPS) analysis of the FAP surface after incubation with Anabaena is consistent with sorbed proteins that promote adhesion. These released polymers are also inferred to complex with Ca during dissolution, contributing to nucleation of Ca-P precipitates in experiments with cells or supernatant. These experiments emphasize that for Anabaena growing under P-limited conditions, polymeric material is important for cellular adhesion to mineral particles and for the concentration of phosphate as Ca-or Fe-complexes or nanoparticles.