PNNL

Abstract

The increasing availability of high-resolution characterization of natural organic matter (OM) data has shifted the paradigm of lumped descriptions of microbial activities and OM components. It presents an exciting opportunity for understanding the fundamental mechanisms governing the oxidation of various OM molecules. Our recent development of a substrate-explicit thermodynamic model uniquely enables incorporating complex OM pools to formulate biogeochemical reaction models based on their elemental compositions. While this previous work facilitates prediction of aerobic respiration of complex OM, it is equally imperative to consider the role of non-oxygenic electron acceptors in regulating OM turnover and the fate of carbon. Here we significantly expand the previous model by flexibly incorporating both detailed OM chemistry and electron acceptors other than oxygen. Our modeling analysis has revealed substantial variations in the energy status of OM molecules across different soils, which drive the co-occurrence of different electron-accepting processes. We demonstrated the effectiveness of the proposed model using a consistency check with experimental data. Through systematic evaluation of the impact of diverse chemical inputs (both electron donors and acceptors) on OM decomposition, the new model also revealed how key microbial growth parameters such as carbon use efficiency (CUE) and reaction rates vary across different electron-accepting processes. Our model provides a unified framework integrating thermodynamic and kinetic constraints on microbial metabolic activities. It complements traditional kinetic models that are often solely designed to capture mass fluxes. We conclude that thermodynamic modeling is a powerful tool for describing the dynamic interplay between microbial growth and the redox dynamics at molecular-level and enhancing our ability to project complex ecosystem behaviors in dynamic environments.