Persistence of Protein-Mineral Associations Controlled By Protein Type, Mineral Surface Reactivity and Energy Input.

The propensity of protein to become altered or even broken apart when exposed to a gradient of energy input while in (dry) contact with mineral surfaces was investigated. Recent evidence shows that soil can oxidize organic matter (OM) even when microbiota are killed off. Combustion treatments on soils were found to enhance the oxidizing capability compare to the natural soils with intact microbiota. Several theories have been offered, but little information is available on how energy inputs affect the molecular integrity and the functionality of proteins in contact with mineral surfaces. In aqueous suspension, two proteins were brought in contact with two minerals representing different types of surface reactivity. Kaolinite was hypothesized to act as a plain sorbent while Birnessite was chosen for its known ability to oxidize OM. Proteins and minerals were allowed to interact at neutral and at acidic pH to allow for variation in the strength of resulting protein-mineral associations. After dry down, the res ponse of the respective systems to a gradient of energy input was observed using Laser Desorption Post Ionization Mass Spectrometry (LDPI-MS) at the Advanced Light Source (ALS) in Lawrence Berkeley National Laboratory. Variations in total ion counts as well as variations in fragmentation patterns were quantified and used to draw inference on the role of mineral surfaces for the fate of the proteins. At all energies, Birnessite, was more effective at fragmenting proteins than the phyllosilicate by several orders of magnitude. While fragmentation patterns did not significantly vary as a function of energy applied, total ion counts showed a nonlinear increase with energy input for all systems. Our data suggest that, in line with the observations offered by Blankinship et al. (2014), the propensity of mineral surfaces to fragment organic macromolecules such as proteins increases with energy input. In both minerals studied but depending on the type of protein tested, an initial protective effect can be observed until a certain energy threshold is reached. This threshold is much lower for Birnessite than it is for kaolinite. The implications of this work are threefold. (1) Proteins in soil have a higher likelihood to remain functional following modest energy inputs when in contact with phyllosilicates, while contact with MnO2 will lead to fragmentation even at very low energy levels. (2) Natural soils and sediments with significant Birnessite concentrations (such as most moist forest topsoils) will lose some of their ability to protect organic matter after exposure to high energy inputs as they may occur following natural or accidental wildfires. (3) When exposed to periodic burns, Forest soils with an active Mn cycle (Keiluweit et al 2015) may serve as an effective transformation agent for organic matter, thereby accelerating organic matter turnover within the mineral soil and beyond the C-losses associated with the incineration of surface vegetation.