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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, Feo, Alo and Zno served as electron donors for methanogenesis; some organisms used Nio or Sno as low-level electron donors. Of the metals tested, methanogenesis from H2+CO2 was inhibited by direct contact with Zno or Cuo, but not by Feo or Alo. Nio and Coo were inhibitory to some methanogens, with Nio being particularly inhibitory to the thermophilic strains tested. With all organisms examined in the two-flask system, Feo and Zno served as good electron sources for both methanogenesis and growth; Coo generated a very low level of methane and Cuo did not work at all. In either system Vo, Tio or Cdo did not serve as electron donors. The results suggest that some elemental metals (notably Feo, Alo and Zno) 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 (Tio and Vo) are very stable and do not serve as electron donors.
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