| Summary: | Despite the worldwide growth of Anaerobic Digestion (AD) plants able to produce biogas, further investigations on the cooperation of syntrophic organisms within mixed cultures are necessary. Microbiomes in AD bioreactors depend on process conditions, heterogeneities and on the intrinsic characteristics of microorganisms. However, chemotropic organisms obtain energy by extracting an electron flow from different organic compounds that are successively passed through different Red-Ox cycles, supported by enzymes under different red-ox potentials (E°). Oxidation-reduction cycles are present not only at intracellular level, but also as interspecies (exocellular) electron exchange, mediated through chemical compounds (Shuttled Interspecies Electron Transfer, SIET) or based on electrical interactions between bacteria (Direct InterspeciesElectron Transfer, DIET). Metal particles, namely Fe, via anaerobic corrosion (AC) generates Fe2+ and e- thatinteract with the biological system suggesting that a higher energy recovery is achievable, due to modification of syntrophic relationships and to positive bio-stimulatory effects. This review addresses the state-of-the-art of exocellular syntrophy among microorganisms SIET and DIET, introduces the AC of Fe° under the operative conditions, summarizes the different laboratory-scale studies that have been performed with metals particles and hypothesizes energy improvement mechanisms in the light of enhancement of AD performances.
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