lower concentrations of Fe(II), and light attenuation by cells and/or (in)organic particles, it appears that anoxygenic phototrophic bacteria alone could have precipitated enough Fe(III) to account for the im- mense amounts of Fe minerals present in BIFs. Given that the cells closely associate with the Fe(III) precipitates (Kappler and Newman, 2004), it seems likely that deposition of these cell-mineral aggregates would have fostered diagenetic re-reduction of Fe(III) coupled to the mineralization of cell material. As hypothesized by Walker, such pro- cesses not only could explain the low content of organic carbon in most BIFs, but also the presence of magnetite and siderite (Walker, 1984).
In conclusion, our experimental and theoretical results suggest that, in a stratified Precambrian ocean, a layer of anoxygenic photo- trophic bacteria living beneath the wind-mixed surface layer was the most likely catalyst for BIF deposition, even in the presence of cya- nobacteria, and could explain the absence of Fe that has been inferred for shallow-water environments (Tice and Lowe, 2004), as well as ac- count for the deposition of BIFs on the continental shelf. Even in the event of pycnocline deepening due to strong winds or intense surface cooling, upwelling of Fe(II) into the surface layer would have been limited by the metabolism of the anoxygenic phototrophs, serving ef- fectively as a buffer to separate the deep iron reservoir from O2 at the surface. If Fe(II) was consumed prior to reacting with cyanobacterial oxygen, the dominant sink for oxygen on the early Earth becomes even more enigmatic.
The Rhodobacter ferrooxidans strain SW2 was obtained from F. Widdel, MPI Bremen, Germany. Chlorobium ferrooxidans strain KoFox and Thiodictyon sp. strain F4 were provided by B. Schink, University of Konstanz, Germany. This work was supported by the German Research Foundation, DFG (Kappler), the Natural Sciences and Engineering Research Council of Canada (Konhauser), and the Luce and Packard Foundations (Newman). We thank A. Tobias for help with carotenoid analysis, and P. Falkowski, J. Gibson, B. Kopp, B. Schink, K. Straub, and the members of the Newman lab for comments on the manuscript.
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Manuscript received 25 February 2005 Revised manuscript received 21 June 2005 Manuscript accepted 28 June 2005
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GEOLOGY, November 2005