Deposition of banded iron formations by anoxygenic phototrophic Fe(II)-oxidizing bacteria

Figure 1. Wavelength-dependent light intensity at different water depths (indicated by numbers assigned to contour lines). Light intensity is given in percent of surface light intensity, where 100% equals ~30,000 lux (~600 mmol quanta m22 s21), estimated to be 24 h average expected for light intensity on horizontal surface on clear summer day (Thimijan and Heins, 1983). Contour lines were derived using attenuation coefficients given for clear ocean water by Smith and Baker (1981). Dashed lines represent light intensities at 25 and 100 m depth, respectively, including absorption by 25 and 100 m water column and 17.6-m-thick layer of anoxygenic phototrophic Fe(II) oxidizers containing 106 cells/mL. Shaded area indicates wavelength region where carotenoids absorb light. ABSTRACT The mechanism of banded iron formation (BIF) deposition is controversial, but classically has been interpreted to reflect ferrous iron [Fe(II)] oxidation by molecular oxygen after cyanobacteria evolved on Earth. Anoxygenic photoautotrophic bacteria can also catalyze Fe(II) oxidation under anoxic conditions. Calculations based on experimentally determined Fe(II) oxidation rates by these organisms under light regimes representative of ocean water at depths of a few hundred meters suggest that, even in the presence of cyanobacteria, anoxygenic phototrophs living beneath a windmixed surface layer provide the most likely explanation for BIF deposition in a stratified ancient ocean and the absence of Fe in Precambrian surface waters.