Strategies of organic phosphorus recycling by soil bacteria: acquisition, metabolism, and regulation.

Critical to meeting cellular phosphorus (P) demand, soil bacteria deploy a number of strategies to overcome limitation in inorganic P (Pi ) in soils. As a significant contributor to P recycling, soil bacteria secrete extracellular enzymes to degrade organic P (Po ) in soils into the readily bioavailable Pi . In addition, several Po compounds can be transported directly via specific transporters and subsequently enter intracellular metabolic pathways. In this review, we highlight the strategies that soil bacteria employ to recycle Po from the soil environment. We discuss the diversity of extracellular phosphatases in soils, the selectivity of these enzymes towards various Po biomolecules and the influence of the soil environmental conditions on the enzyme's activities. Moreover, we outline the intracellular metabolic pathways for Po biosynthesis and transporter-assisted Po and Pi uptake at different Pi availabilities. We further highlight the regulatory mechanisms that govern the production of phosphatases, the expression of Po transporters and the key metabolic changes in P metabolism in response to environmental Pi availability. Due to the depletion of natural resources for Pi , we propose future studies needed to leverage bacteria-mediated P recycling from the large pools of Po in soils or organic wastes to benefit agricultural productivity.

Molecular Coordination, Structure, and Stability of Metal-Polyphosphate Complexes Resolved by Molecular Modeling and X-ray Scattering: Structural Insights on the Biological Fate of Polyphosphate.

Polyphosphate-accumulating organisms (PAOs), which can store high levels of phosphate (Pi) in the form of polyphosphate (polyP), are employed to engineer enhanced biological P removal (EBPR) from wastewaters. Co-localization of Mg and K in polyP granules of PAOs has been reported, and higher abundance of Mg-polyP granules relative to other metal complexes was correlated positively with EBPR performance stability. However, the underlying mechanism remains unknown. Here, we obtained molecular structural information of hydrated polyP complexes with four physiologically relevant metal cations (Na+, K+, Ca2+, and Mg2+) using computational and experimental techniques. Molecular dynamics simulations revealed that Mg-polyP and K-polyP complexes were the most and least stable of the complexes, respectively, suggesting that the co-occurrence of these complexes facilitates variable polyP bioavailability. The relative thermodynamic stability reflected the strength of metal chelation whereby the coordination distance between the polyP ligand O and the metal was 1.71-2.01 Å for Mg2+ but this distance was 2.64-2.70 Å for K+. Pair distribution function analysis of X-ray scattering data obtained with a Mg-polyP solution corroborated the theoretical Mg-polyP coordination geometry. These findings implied a possible mechanistic role of metal complexation in the P cycling traits of PAOs in engineered and natural systems.