Towards the Quantification of Organic Matter Degradation in the Deep Biosphere

The oxidative degradation of organic matter is a key process in the biogeochemical functioning of the earth system. Quantitative models of organic matter degradation are therefore essential for understanding the chemical state and evolution of the Earth's near-surface environment. In order for these models to capture large-scale patterns of change, the underlying formulations in them must account for organic matter transformations throughout a range of environments characterized by differing temperatures, pressures and compositions over short and long time scales. However, the complex nature of biologically produced organic matter, and it's evolution in the deep biosphere, represent a major obstacle to the development of such models. Here, a new energetic description of organic matter is combined with bioenergetic theory to rationalize observed patterns in the decomposition of natural organic matter by comparing the energetics of the oxidative degradation of a large number of naturally occurring organic compounds. By relating the Gibbs energies of half reactions describing the complete mineralization of the compounds to their average nominal carbon oxidation state, it becomes possible to estimate the energetic potential of the compounds based on major element (C, H, N, O, P, S) ratios. Furthermore, a new thermodynamic limiting function is introduced that can be used to model microbial metabolism throughout the range of conditions in which they are known to be active. This formulation is based upon a comparison of the amount of energy available from any redox reaction to the energy required to a maintain membrane potential, a proxy for the minimum amount of energy a microbe requires in order to be considered active. This function does not require species- or metabolism-specific parameters, and can be used to model metabolisms that capture any amount of energy. The proposed approaches open the way for the inclusion of the thermodynamic properties of organic compounds in kinetic models of organic matter degradation.