Birnessite mediated abiotic protein fragmentation may supply oligopeptides to soil biota

Abstract:

The ability of plants and microorganisms to take up organic nitrogen in the form of free amino acids and oligopeptides has received increasing attention over the last two decades, yet the mechanisms for the formation of such compounds in soil environments remain poorly understood. We used Nuclear Magnetic Resonance (NMR) and Electron Paramagnetic Resonance (EPR) spectroscopies to distinguish the reaction of a model protein with a redox active mineral surface (Birnessite, MnO₂) from its response to a common phyllosilicate (Kaolinite). Our data demonstrate that birnessite fragments the model protein while kaolinite does not. The reaction of birnessite with protein produced soluble peptides that were released from the birnessite surface where they would be available to soil biota. Soluble Mn(II) production during fragmentation of the protein by birnessite was measured by EPR spectrscopy. NMR and EPR spectroscopies are shown to be valuable tools to observe these reactions and capture the extent of protein transformation together with the extent of mineral response. Our data reveal an abiotic pathway for the formation of organic nitrogen compounds for direct uptake by plants and microorganisms as well as lay the foundation to identify soil environments with a particular propensity to act in this way.