The biogeochemical redox paradox: how can we make a foundational concept more predictive of biogeochemical state changes?

Oxidation–reduction (redox) ladders are a fundamental biogeochemical concept that explain basic and applied phenomena including microbial metabolic evolution and chemical pollution remediation. However, it is difficult to measure and predict which redox couples dominate an environment. This discrepancy manifests a redox paradox: while redox ladders are a fundamental biogeochemical concept, it is difficult to use existing measurements to predict which redox couples dominate in an environment or use redox to predict biogeochemical state changes. Additionally, there is often confusion around the concept due to the differential emphasis in biological vs. geochemical sub-disciplines in explanations of the: (1) roles of actual vs. standard free energy calculations, (2) thermodynamics vs. enzymatic kinetics in reactions, (3) ubiquity of alternative electron donors as opposed to assuming processes are solely driven by organic carbon, and (4) difference between redox intensity and redox buffering capacity. Redox buffering capacity is a particularly underdeveloped concept within biogeochemical studies, which have largely used redox potential, a measurement with myriad analytical concerns. We review the small number of studies that have quantified redox buffering capacity. We highlight four challenges to overcome before redox buffering capacity can be a useful concept. Measuring redox buffering capacity may be key to understanding the ability of the ecosystems to resist change in redox conditions. Biogeochemists are often most interested in predicting these state changes to understand hot spot/hot moment phenomena such as greenhouse gas fluxes; thus, incorporating redox buffering capacity into our predictive frameworks and methodological toolbox may be key to unlocking the redox paradox. We postulate that the well-known ecological resilience and resistance framework constitutes the needed theoretical foundation on which measurements of redox buffering capacity can be incorporated.