Ferrous iron oxidation by anoxygenic phototrophic bacteria

NATURAL oxidation of ferrous to ferric iron by bacteria such as Thiobacillus ferrooxidans or Gallionella ferruginea¹, or by chemical oxidation^2,3 has previously been thought always to involve molecular oxygen as the electron acceptor. Anoxic photochemical reactions^4-6 or a photobiological process involving two photosystems^7-9 have also been discussed as mechanisms of ferrous iron oxidation. The knowledge of such processes has implications that bear on our understanding of the origin of Precambrian banded iron formations^10-14. The reducing power of ferrous iron increases dramatically at pH values higher than 2-3 owing to the formation of ferric hydroxy and oxyhydroxy compounds^1,2,15 (Fig. 1). The standard redox potential of Fe³⁺/Fe²⁺ (E₀ = +0.77 V) is relevant only under acidic conditions. At pH 7.0, the couples Fe(OH)₃/Fe²⁺ (E'₀ = -0.236V) or Fe(OH)₃ + HCO^-₃FeCO₃ (E'₀ = +0.200 V) prevail, matching redox potentials measured in natural sediments^9,16,17. It should thus be possible for Fe(n) around pH 7.0 to function as an electron donor for anoxygenic photosynthesis. The midpoint potential of the reaction centre in purple bacteria is around +0.45 V (ref. 18). Here we describe purple, non-sulphur bacteria that can indeed oxidize colourless Fe(u) to brown Fe(in) and reduce CO₂ to cell material, implying that oxygen-independent biological iron oxidation was possible before the evolution of oxygenic photosynthesis.