methanogenic bacteria , which normally use molecular hydrogen ( h ( 2 ) ) and carbon dioxide ( co ( 2 ) ) to produce methane ( ch ( 4 ) ) and which are major inhabitants of most anaerobic ecosystems , use either pure elemental iron ( fe ( 0 ) ) or iron in mild steel as a source of electrons in the reduction of co ( 2 ) to ch ( 4 ) .
C0020275
hydrogen
C0011744
h ( 2