Flavin secretion of Clostridium acetobutylicum in a bioelectrochemical system - Is an iron limitation involved?

A flavin-based extracellular electron transfer mechanism (EET) has recently been described for the gram-positive Listeria monocytogenes. The gram-positive, solvent producing Clostridium acetobutylicum is a known flavin producer. Since flavin secretion in C. acetobutylicum can be triggered by a low-iron environment, the interaction of iron with an electrochemical system as well as the consequences for flavin production are investigated. It is shown that iron adsorbs onto the electrode's surface in the form of iron phosphorus compounds but that this iron is still bioavailable. Moreover, a shift in the flavin spectrum of the supernatant from high flavin mononucleotide percentages of 59% to high riboflavin (43-45%) and flavin adenine dinucleotide (FAD, 40-48%) content can be seen by limiting or omitting the iron source from the culture medium. When additionally an electric potential of -600 mV vs. Ag/AgCl (saturated KCl) is applied, the same overall trend is obtained but an increase in flavin concentration and especially in the FAD share between 6 and 27% is observed. This study is a first hint that a flavin-based EET might also take place in solventogenic Clostridia and highlights the importance of further investigation of flavin production and their involvement in EET mechanisms in different species.