Elemental metals as electron sources for biological methane formation from CO2

Several elemental metals were examined as potential electron donors for methanogenic bacteria, using both a single tube system where the metal was in direct contact with the cells, and a two-flask system, where metal and cells were not in direct contact, but had contact via the gas phase. With all organisms examined in the direct contact system, Fe^o, Al^o and Zn^o served as electron donors for methanogenesis; some organisms used Ni^o or Sn^o as low-level electron donors. Of the metals tested, methanogenesis from H₂+CO₂ was inhibited by direct contact with Zn^o or Cu^o, but not by Fe^o or Al^o. Ni^o and Co^o were inhibitory to some methanogens, with Ni^o being particularly inhibitory to the thermophilic strains tested. With all organisms examined in the two-flask system, Fe^o and Zn^o served as good electron sources for both methanogenesis and growth; Co^o generated a very low level of methane and Cu^o did not work at all. In either system V^o, Ti^o or Cd^o did not serve as electron donors. The results suggest that some elemental metals (notably Fe^o, Al^o and Zn^o) produce gaseous H₂ by cathodic depolarization which is then consumed by the methanogen, thus accelerating oxidation of the metal by its metabolic activity. All of these reactions are thermodynamically favorable; however, some other metals that are clearly favorable for such a reaction on thermodynamic grounds (Ti^o and V^o) are very stable and do not serve as electron donors.Preliminary results on this topic were presented at the Pan American Biodeterioration Society Meeting in Washington, D.C. July 28–31, 1988.