The biogeochemical redox ladder of alternative electron acceptors constitutes a first principle of understanding energy and matter transformations in ecosystems. The theoretical underpinning is that electron acceptors should be reduced in a predictable sequence, provided that microbes compete for electron donors in a homogeneous environment. However, we see many lines of evidence that theory does not work as cleanly as predicted in heterogeneous, natural settings. Furthermore, the main method of measuring the dominant redox process, redox potential (also known as Eh; defined as a measure of electron availability) is often not reproducible or predictive of the dominant redox couple in an environment. This contradiction creates a paradox: while it is a fundamental concept, it is difficult to measure and predict which redox couples should dominate an environment. What factors contribute to failure of the predictive ability of the redox ladder? Do better measurement options exist?
Results/Conclusions
We find that the redox ladder works in highly simplified or homogeneous environments (e.g., oligotrophic lake water columns), but fails in complex soil environments or environments with hydrologic variation. Issues such as coupled elemental cycling, prevalence of dissolved vs. solid phase reactants and shifting environmental controls (e.g., pH) can alter the favorability of redox reactions, resulting in percieved "failure" of the redox ladder to apply to a given system. Furthermore, these environmental shifts can feed back to affect the capacity for microbes to harness the energy released from redox reactions. Finally, we argue that simply measuring redox potential isn’t a powerful enough predictor of the dominant redox couple, but instead posit that a combination of redox potential and redox buffering capacity will give superior predictive power. Incorporating these concepts into our understanding of redox ladders in naturally heterogeneous environments will improve the predictive capacity of biogeochemical studies.