Revealing extracellular electron transfer mediated parasitism: energetic considerations
Résumé
Extracellular electron transfer (EET) is a mechanism that allows energetic coupling between two
microorganisms or between a microorganism and an electrode surface. EET is either supported by
direct physical contacts or mediated by electron shuttles. So far, studies dealing with interspecies
EET (so-called IET) have mainly focused on possible syntrophic interactions between microorganisms
favoured by this mechanism. In this article, the case of fermentative bacteria receiving extracellular
electrons while fermenting a substrate is considered. A thermodynamical analysis based on metabolic
energy balances was applied to re-investigate experimental data from the literature. Results suggest
that the observations of a decrease of cell biomass yields of fermentative electron-accepting species,
as mostly reported, can be unravelled by EET energetics and correspond to parasitism in case of IET.
As an illustration, the growth yield decrease of Propionibacterium freudenreichii (−14%) observed in
electro-fermentation experiments was fully explained by EET energetics when electrons were used by
this species at a potential of −0.12 ± 0.01 V vs SHE. Analysis of other cases showed that, in addition to
EET energetics in Clostridium pasteurianum, biological regulations can also be involved in such biomass
yield decrease (−33% to −38%). Interestingly, the diminution of bacterial biomass production is always
concomitant with an increased production of reduced compounds making IET-mediated parasitism and
electro-fermentation attractive ways to optimize carbon fluxes in fermentation processes.
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