Abstract
Interspecies electron transfer is a common way to couple metabolic energy balances between different species in mixed culture consortia. Direct interspecies electron transfer (DIET) mechanism has been recently characterised with Geobacter species which couple the electron balance with other species through physical contacts. Using this mechanism could be an efficient and cost-effective way to directly control redox balances in co-culture fermentation. The present study deals with a co-culture of Geobacter sulfurreducens and Clostridium pasteurianum during glycerol fermentation. As a result, it was shown that Geobacter sulfurreducens was able to grow using Clostridium pasteurianum as sole electron acceptor. C. pasteurianum metabolic pattern was significantly altered towards improved 1,3-propanediol and butyrate production (+37% and +38% resp.) at the expense of butanol and ethanol production (−16% and −20% resp.). This metabolic shift was clearly induced by a small electron uptake that represented less than 0.6% of the electrons consumed by C. pasteurianum. A non-linear relationship was found between G. sulfurreducens growth (i.e the electrons transferred between the two species) and the changes in C. pasteurianum metabolite distribution. This study opens up new possibilities for controlling and increasing specificity in mixed culture fermentation.
Highlights
C. pasteurianum Initial Final able to consume electrons from a cathode in microbial electrolysis cells to convert CO2 into methane[16], or reduce nitrates, respectively[17,18]
To study the possible interactions that could exist between G. sulfurreducens and C. pasteurianum, the two strains were inoculated in a medium containing glycerol and acetate
The range of known electron acceptors that can be used by G. sulfurreducens is limited, and consists in some metal ions, elemental sulfur, malate and fumarate[27,28]
Summary
C. pasteurianum Initial Final able to consume electrons from a cathode in microbial electrolysis cells to convert CO2 into methane[16], or reduce nitrates, respectively[17,18]. Clostridium pasteurianum was reported to be able to consume cathodic electrons during fermentation, and produced more butanol and 1,3-propanediol (PDO) from glucose and glycerol respectively than during classic fermentation[22]. As this fermenter was able to uptake extracellular electrons from a cathode, it is not excluded that electrons provided by DIET with an exoelectrogen organism could be consumed and lead to a similar metabolic shift. The aim of this study is to provide a proof-of-concept experiment showing that electron coupling between fermenters and exoelectrogens is possible the other way around: in this case, the fermentative species would be the electron acceptor while the exoelectrogens would provide electrons This could serve to trigger a metabolic shift towards the production of more reduced products such as PDO. This experiment was conducted for glycerol fermentation using a defined co-culture of G. sulfurreducens and C. pasteurianum as model partners for DIET
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