ANAEROBIC OXIDATION OF FERROUS IRON BY PURPLE BACTERIA, A NEW-TYPE OFPHOTOTROPHIC METABOLISM

Anoxic iron-rich sediment samples that had been stored in the light sh owed development of brown, rusty patches. Subcultures in defined miner al media with ferrous iron (10 mmol/liter, mostly precipitated as FeCO 3) yielded enrichments of anoxygenic phototrophic bacteria which used ferrous iron as the sole electron donor for photosynthesis. Two differ ent types of purple bacteria, represented by strains L7 and SW2, were isolated which oxidized colorless ferrous iron under anoxic conditions in the light to brown ferric iron. Strain L7 had rod-shaped, nonmotil e cells (1.3 by 2 to 3 mu m) which frequently formed gas vesicles. In addition to ferrous iron, strain L7 used H-2+CO2, acetate, pyruvate, a nd glucose as substrates for phototrophic growth. Strain SW2 had small rod-shaped, nonmotile cells (0.5 by 1 to 1.5 mu m). Besides ferrous i ron, strain SW2 utilized H-2+CO2, monocarboxylic acids, glucose, and f ructose; Neither strain utilized free sulfide; however, both strains g rew on black ferrous sulfide (FeS) which was converted to ferric iron and sulfate. Strains L7 and SW2 grown photoheterotrophically without f errous iron were purple to brownish red and yellowish brown, respectiv ely; absorption spectra revealed peaks characteristic of bacteriochlor ophyll a. The closest phototrophic relatives of strains L7 and SW2 so far examined on the basis of 16S rRNA sequences were species of the ge nera Chromatium (gamma subclass of proteobacteria) and Rhodobacter (al pha subclass), respectively. In mineral medium, the new isolates forme d 7.6 g of cell dry mass per mol of Fe(II) oxidized, which is in good agreement with a photoautotrophic utilization of ferrous iron as elect ron donor for CO2 fixation. Dependence of ferrous iron oxidation on li ght and CO2 was also demonstrated in dense cell suspensions. In media containing both ferrous iron and an organic substrate (e.g., acetate, glucose), strain L7 utilized ferrous iron and the organic compound sim ultaneously; in contrast, strain SW2 started to oxidize ferrous iron o nly after consumption of the organic electron donor. Ferrous iron oxid ation by anoxygenic phototrophs is understandable in terms of energeti cs. In contrast to the Fe3+/Fe2+ pair (E(0) = +0.77 V) existing in aci dic solutions, the relevant redox pair at pH 7 in bicarbonate-containi ng environments, Fe(OH)(3)+HCO3-/FeCO3, has an E(0) of +0.2 V. Ferrous iron at pH 7 can therefore donate electrons to the photosystem of ano xygenic phototrophs, which in purple bacteria has a midpoint potential around +0.45 V. The existence of ferrous iron-oxidizing anoxygenic ph ototrophs may offer an explanation for the deposition of early banded- iron formations in an assumed anoxic biosphere in Archean times.