Geobacter grbiciae—A New Electron Donor in the Formation of Co-Cultures via Direct Interspecies Electron Transfer

Geobacter grbiciae can grow via coupling oxidation of ethanol to the reduction of various forms of soluble Fe(III) and poorly crystalline Fe(III) oxide, suggesting that G. grbiciae can act as an electron-donor microbe for forming co-cultures through direct interspecies electron transfer (DIET). In this report, potential co-cultures through DIET of G. grbiciae and Methanosarcina barkeri 800, G. sulfurreducens Δhyb, or Methanospirillum hungatei, as electron-acceptor microbes, were examined. Co-cultures of G. grbiciae and G. sulfurreducens Δhyb were performed with ethanol as the sole electron-donor substance and fumarate as the electron-acceptor substance in the presence of granular activated carbon (GAC), magnetite, or polyester felt. The conditions for co-culturing G. grbiciae and M. barkeri 800 (or M. hungatei) were the same as those for G. grbiciae and G. sulfurreducens Δhyb, except fumarate was absent and different cultivation temperatures were used. All co-cultures were anaerobically cultivated. Samples were regularly withdrawn from the co-cultures to monitor methane, fumarate, and succinate via gas or high-performance liquid chromatography. G. grbiciae formed functional co-cultures with M. barkeri 800 in the presence of GAC or magnetite. No co-culture of G. grbiciae with the H2/formate-utilizing methanogen M. hungatei was observed. Additionally, G. grbiciae formed functional co-cultures with H2/formate-un-utilizing G. sulfurreducens Δhyb without the GAC or magnetite supplement. These findings indicate electron transfer between G. grbiciae and M. barkeri 800/G. sulfurreducens Δhyb is via DIET rather than H2/formate, confirming that G. grbiciae acts as an electron-donor microbe. Although the co-cultures of G. grbiciae and M. barkeri 800 syntrophically converted ethanol to methane through DIET, the conversion of propionate or butyrate to methane was not observed. These findings expand the range of microbes that can act as electron donors for interaction with other microbes through DIET. However, propionate and butyrate metabolism through DIET in mixed microbial communities with methane as a product requires further analysis. This study provides a framework for finding new electron-donor microbes.