PNNL

The Science

The study demonstrates a new model that integrates complex organic matter (OM) chemistry and multiple electron acceptors to predict kinetic rates of OM oxidation. In this model, shown to be consistent with experimental data, the dynamic distribution of energy associated with multiple redox pathways enables their simultaneous occurrence, challenging the traditional paradigm of strict redox hierarchies. The size of OM molecules emerges as a key factor, affecting energetic and kinetic efficiencies. The research demonstrates the application of thermodynamic laws to quantify mass and energy balances, predicting microbial growth kinetics parameters and providing valuable insights for biogeochemical modeling.

The Impact

This research offers a groundbreaking thermodynamic insight into microbial redox dynamics by considering a continuum of OM molecules and their reactions with different electron acceptors. Using bioenergetics as a unified theory, the study integrates thermodynamic and kinetic controls on OM decomposition, addressing a central problem in biogeochemical modeling: the thermodynamics of microbial growth, which is being increasingly recognized as central for predicting soil carbon cycling. Moreover, this work explicitly links thermodynamics to kinetics, paving the way for describing and capturing the relationship between microbial growth rates and yields—two critical parameters for robust biogeochemical modeling.

Summary

In soils, redox reactions serve as the cornerstone driving all biogeochemical cycles. This research employs a novel thermodynamic approach to unravel the dynamics of microbial redox processes in complex soil systems. Leveraging high-resolution metabolite data and thermodynamic principles, the study presents a quantitative modeling approach that explicitly considers the energy status of both the electron donors and acceptors in driving the progression of different redox pathways. This approach enables the prediction of both aerobic and anaerobic respiration pathways for complex OM substrates, challenging conventional assumptions of a static thermodynamic hierarchy of redox reactions. Complementing traditional biogeochemical models, this thermodynamic perspective integrates energy parameters alongside mass fluxes that capture the interplay among thermodynamic efficiency, kinetic rate, and substrate availability. These insights hold profound implications for understanding and modeling soil redox processes.

PNNL Contact

Tim Scheibe, Pacific Northwest National Laboratory, tim.scheibe@pnnl.gov

Funding

This research was supported by the Department of Energy (DOE), Office of Science, Biological and Environmental Research program, Environmental System Science program through the River Corridor Science Focus Area project. A portion of this research was performed using capabilities at the Environmental Molecular Sciences Laboratory, a DOE Office of Science user facility.