Electron conductive compounds alter fermentative pathways and cooperation in <i>Clostridium carboxidivorans</i> and <i>Clostridium acetobutylicum</i> in co-culture

The addition of conductive materials promotes interactions between bacteria as they facilitate the exchange of reducing equivalents among cells. In this work, the impact of electron conductive compounds (magnetite, activated carbon, or iron salts) was investigated on a Clostridium acetobutylicum/Clostridium carboxidivorans co-culture. Co-culturing both species with soluble iron salts or magnetite significantly improved carbon recovery in liquid end-products (75%-85% of added carbon) compared to control and activated carbon supplementation (50%-55% of added carbon). The addition of magnetite enhanced the production of longer-chain acids and alcohols (C4 and C6) when compared to all other treatments and reached the highest production after 44 h of fermentation. This effect was not observed in C. carboxidivorans nor in C. acetobutylicum pure cultures, advocating for a cooperation between the two species. Among comparisons to the behaviour observed in pure cultures, we suggest magnetite was first used as a sink of reduced equivalents produced by C. carboxidivorans and later as a source of energy for C. acetobutylicum for the production of elongated short-chain fatty acids and alcohols. We propose that adding magnetite (iron) could be an effective strategy to enhance alcohol production in synthetic clostridia consortia.