A stochastic model for the synthesis and degradation of natural organic matter. Part I. Data structures and reaction kinetics

Here we present a stochastic biogeochemical model for the formation, transformation and mineralization of natural organic matter (NOM). The model is agent-based, with each software agent representing a single molecule of defined composition. Molecular properties and reactivities are estimated from composition and environmental parameters. Environmental parameters including temperature, pH, light intensity, dissolved O2, moisture and enzyme activities are user controlled. Time is treated in discrete steps, and during each step potential reaction probabilities are evaluated for each molecule based on its structure and the environmental parameters. When reactions occur, the molecular composition is modified accordingly. The model uses small natural products and biopolymers for inputs, and the composition of the molecules produced is constrained only by the inputs and reaction stoichiometries, not by pre-defined structures. Example simulations using the program AlphaStep are presented, in which the breakdown of biopolymers and the condensation of small molecules both lead to molecular assemblages with elemental composition and average properties similar to those of aquatic NOM. This batch-reactor model can be expanded to include spatial information and environmental feedback. Abbreviations: A – Arrhenius constant; CE – capillary electrophoresis; Ea – activation energy, J mol ; HPLC – high-pressure liquid chromatography; IHSS – International Humic Substances Society; k – rate constant; Mn – number-average molecular weight, amu; Mw – weight-average molecular weight, amu; NOM – natural organic matter; NMR – nuclear magnetic resonance; P – probability of a reaction occurring; PRN – pseudo-random number; R – universal gas constant in J mol 1 K ; RSD – relative standard deviation; SRFA – Suwannee River fulvic acid; T – temperature in Kelvins; t – time in hours (Dt = change in time); UV-Vis – ultraviolet-visible