Extracellular Electron Uptake by Acetogenic Bacteria: Does H Consumption Favor the H Evolution Reaction on a Cathode or Metallic Iron?

Some acetogenic bacteria are capable of using solid electron donors, such as a cathode or metallic iron [Fe(0)]. Acetogens using a cathode as electron donor are of interest for novel applications such as microbial electrosynthesis, while microorganisms using Fe(0) as electron donor cause detrimental microbial induced corrosion. The capacity to use solid electron donors strongly differs between acetogenic strains, which likely relates to their extracellular electron transfer (EET) mechanism. Different EET mechanisms have been proposed for acetogenic bacteria, including a direct mechanism and a H dependent indirect mechanism combined with extracellular hydrogenases catalyzing the H evolution reaction on the cathode or Fe(0) surface. Interestingly, low H partial pressures often prevail during acetogenesis with solid electron donors. Hence, an additional mechanism is here proposed: the maintenance of low H partial pressures by microbial H consumption, which thermodynamically favors the H evolution reaction on the cathode or Fe(0) surface. This work elaborates how the H partial pressure affects the H evolution onset potential and the H evolution rate on a cathode, as well as the free energy change of the anoxic corrosion reaction. In addition, the H consumption characteristics, i.e., H threshold (thermodynamic limit for H consumption) and H consumption kinetic parameters, of acetogenic bacteria are reviewed and evidence is discussed for strongly different H consumption characteristics. Different acetogenic strains are thus expected to maintain different H partial pressures on a cathode or Fe(0) surface, while those that maintain lower H partial pressures (lower H threshold, higher H affinity) more strongly increase the H evolution reaction. Consequently, I hypothesize that the different capacities of acetogenic bacteria to use solid electron donors are related to differences in their H consumption characteristics. The focus of this work is on acetogenic bacteria, but similar considerations are likely also relevant for other hydrogenotrophic microorganisms.