ANAEROBIC OXIDATION OF ELEMENTAL METALS COUPLED TO METHANOGENESIS BY METHANOBACTERIUM-THERMOAUTOTROPHICUM

Methanobacterium thermoautotrophicum Marburg, a thermophilic, obligately hydrogenotrophic, methanogenic bacterium, was used as a model organism to investigate anaerobic metal oxidation coupled to H-2 utilization. Methane was produced with elemental aluminum, cobalt, copper, indium, iron, magnesium, manganese, nickel, tin, titanium, or zinc as the sole electron source. The extent of corrosion, and concomitant methanogenesis, was greatest in medium reduced with sulfide and buffered with carbonate under 100% CO2 headspace. Comparisons between the concentrations of reducing equivalents generated in inoculated and uninoculated media revealed that M. thermoautotrophicum increased corrosion of copper, nickel, and zinc. Iron and magnesium (0.1 g/culture) were completely oxidized abiotically under 100% CO2; M. thermoautotrophicum converted all reducing equivalents to methane. Under 20% CO2, H-2 utilization by M. thermoautotrophicum doubled the rate and increased the extent of iron oxidation. Incomplete oxidation of the other metals with M. thermoautotrophicum implied inhibition of methanogenesis by metal ions. Ni2+ (2.5 mM) completely inhibited methanogenesis, but Mg2+ (200 mM) only slowed the rate. Cell numbers increased in cultures of M. thermoautotrophicum with aluminum, cobalt, iron, magnesium, manganese, or tin as the sole energy source, demonstrating that oxidation of these metals can support autotrophic growth of a methanogen.