Extracellular Electron Uptake by Acetogenic Bacteria: Does H2 Consumption Favor the H2 Evolution Reaction on a Cathode or Metallic Iron?

Abstract

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 2 dependent indirect mechanism combined with extracellular hydrogenases catalyzing the H 2 evolution reaction on the cathode or Fe(0) surface. Interestingly, low H 2 partial pressures often prevail during acetogenesis with solid electron donors. Hence, an additional mechanism is here proposed: the maintenance of low H 2 partial pressures by microbial H 2 consumption, which thermodynamically favors the H 2 evolution reaction on the cathode or Fe(0) surface. This work elaborates how the H 2 partial pressure affects the H 2 evolution onset potential and the H 2 evolution rate on a cathode, as well as the free energy change of the anoxic corrosion reaction. In addition, the H 2 consumption characteristics, i.e., H 2 threshold (thermodynamic limit for H 2 consumption) and H 2 consumption kinetic parameters, of acetogenic bacteria are reviewed and evidence is discussed for strongly different H 2 consumption characteristics. Different acetogenic strains are thus expected to maintain different H 2 partial pressures on a cathode or Fe(0) surface, while those that maintain lower H 2 partial pressures (lower H 2 threshold, higher H 2 affinity) more strongly increase the H 2 evolution reaction. Consequently, I hypothesize that the different capacities of acetogenic bacteria to use solid electron donors are related to differences in their H 2 consumption characteristics. The focus of this work is on acetogenic bacteria, but similar considerations are likely also relevant for other hydrogenotrophic microorganisms.

Electrophysiology of the Facultative Autotrophic Bacterium Desulfosporosinus orientis

“…This possible EEU mechanism in D. orientis should be further investigated via BES experiments with cell-free spent medium as catholyte (Rowe et al, 2019). Moreover, very recently, the maintenance of low H 2 partial pressures via microbial H 2 consumption was proposed as additional mechanism by which hydrogenotrophic microorganisms could increase the H 2 evolution rate on a cathode or Fe 0 (Philips, 2020).…”

Parameters influencing the development of highly conductive and efficient biofilm during microbial electrosynthesis: the importance of applied potential and inorganic carbon source

“…The difference between the abundance of Clostridium and Acetobacterium enriched from mixed inocula could be justified by low H 2 partial pressure in BES. In a recent study 85 , it was discussed that Acetobacterium has higher cells growth at low H 2 pressures (e.g. at the cathode), while Clostridium growth requires higher H 2 partial pressures (e.g.…”

“…at the cathode), while Clostridium growth requires higher H 2 partial pressures (e.g. gas fermentation) 85 . The low levels of Clostridium spp, could explain the low concentration of longer chain products such as butyrate and butanol that were detected in BES 1 (−1.0 V/ CO 2 ).…”

Electrochemical removal of sulfur pollution

“…One cannot exclude a very slow H 2 generation even at this relatively high cathode potential, because it is thermodynamically feasible at low H 2 partial pressure at the electrode interface, e.g. if the bacteria readily scavenge the electrogenerated H 2 (Philips, 2020).…”