[7] In soil and sediment environments, where the pH is often neutral, metals like iron are found in their solid oxidized forms, and exhibit variable reduction potential, which can affect their use by microorganisms.
[5][9] Cytochromes c, which are transmembrane proteins, play an important role in transporting electrons from the cytosol to enzymes attached to the outside of the cell.
[11] Other organic compounds frequently found in soils and sediments, such as humic acids, may also act as electron shuttles.
[10] A wide range of Fe(III)-bearing minerals have been observed to function as terminal electron acceptors, including magnetite, hematite, goethite, lepidocrocite, ferrihydrite, hydrous ferric oxide, smectite, illite, jarosite, among others.
[16] Commonly observed secondary minerals produced during experimental bio-reduction by dissimilatory metal reducers include magnetite, siderite, green rust, vivianite, and hydrous Fe(II)-carbonate.