Ferrous iron is a soluble form of iron that is stable at extremely low pHs or under anaerobic conditions.  Under aerobic, moderate pH conditions ferrous iron is oxidized spontaneously to the ferric () form and is hydrolyzed abiotically to insoluble ferric hydroxide ().  There are three distinct types of ferrous iron-oxidizing microbes.  The first are acidophiles, such as the bacteria Acidithiobacillus ferrooxidans and Leptospirillum ferrooxidans, as well as the archaeon Ferroplasma.  These microbes oxidize iron in environments that have a very low pH and are important in acid mine drainage.  The second type of microbes oxidize ferrous iron at near-neutral pH. These micro-organisms (for example Gallionella ferruginea, Leptothrix ochracea, or Mariprofundus ferrooxydans) live at the oxic-anoxic interfaces and are microaerophiles.  The third type of iron-oxidizing microbes are anaerobic photosynthetic bacteria such as Rhodopseudomonas, which use ferrous iron to produce NADH for autotrophic carbon dioxide fixation.  Biochemically, aerobic iron oxidation is a very energetically poor process which therefore requires large amounts of iron to be oxidized by the enzyme rusticyanin to facilitate the formation of proton motive force.  Like sulfur oxidation, reverse electron flow must be used to form the NADH used for carbon dioxide fixation via the Calvin cycle.