Figure 1.

Possible mechanisms for energetic benefit from iron oxidation coupled to nitrate reduction. (A) A true Fe(II):menaquinone oxidoreductase accepts electrons from iron and reduces the quinone pool, (B) Nar accepts electrons from Fe(II) and reduces nitrate cytoplasmically, consuming protons to generate a PMF, (C) the cytochrome bc₁ complex accepts electrons from Fe(II) and reduces the quinone pool, (D) more protons are pumped per electron at Nar than other nitrogen oxide reductases. If abiotic reactions dominate for the reduction of ${\text{NO}}_2^{-},$ an energetic benefit is conferred through an electron sparing mechanism. Nar, nitrate reductase; Nir, nitrite reductase; Nor, nitric oxide reductase; Nos, nitrous oxide reductase; QH₂, reduced quinone; Q, oxidized quinone; bc₁, cytochrome bc₁; cyt c, cytochrome c.