This website features an interactive redox tower, a static version of which is explained in Fig. 1 of Scarampi et al. (Citation to follow). In brief, the redox tower lists biologically relevant, theoretical redox half reactions on a reduction potential scale going from negative values at the top to positive values at the bottom. Each half reaction is listed in the direction of “reduction”, i.e. acceptance of electrons.

To explore possible microbial growth-supporting reactions, one should consider pairs of two redox half reactions: on the redox tower each half reaction can be reversed and coupled with another half-reaction that is below it on the tower. This way, the resulting overall redox reaction would have a positive reduction potential, which corresponds to a negative Gibbs free energy, fulfilling the requirement of the 2nd Law of Thermodynamics.

On the static redox tower, it is customary to list all half-reaction reduction potentials under biological standard conditions, i.e. 1 M concentrations for all metabolites, except for hydrogen, listed at pH = 7. The interactive version allows varying metabolite concentrations, such as hydrogen, thereby exploring shifts in thermodynamic feasibility of different microbial growth-supporting metabolisms.

The Calculation Details page explains how the Nernst equation is applied to compute condition-dependent reduction potentials, and includes a worked example for aerobic respiration.