Evaluation of electron-shuttling compounds in microbial ferric iron reduction

Iron-reducing bacteria can transfer electrons to ferric iron oxides which are barely soluble at neutral pH, and electron-shuttling compounds or chelators are discussed to be involved in this process. Experiments using semipermeable membranes for separation of ferric iron-reducing bacteria from ferric iron oxides do not provide conclusive results in this respect. Here, we used ferrihydrite embedded in 1% agar to check for electron-shuttling compounds in pure and in enrichment cultures. Geobacter sulfurreducens reduced spatially distant ferrihydrite only in the presence of anthraquinone-2,6-disulfonate, a small molecule known to shuttle electrons between the bacterial cell and ferrihydrite. However, indications for the production and excretion of electron-shuttling compounds or chelators were found in ferrihydrite-containing agar dilution cultures that were inoculated with ferric iron-reducing enrichment cultures.