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Banded iron formations (BIF) are iron-rich (~20-40% Fe), siliceous (~40-50%), and organic-lean (<0.5 wt.%) sedimentary deposits that precipitated widely during the late Archean (2.7-2.5 Ga) and Paleoproterozoic (2.5-1.8 Ga). As cyanobacteria evolved, the reaction between dissolved Fe(II) and  photosynthetically produced oxygen would have became a viable mechanism for Fe(II) oxidation near the surface of the Precambrian oceans.

The association or encrustation of cells with minerals not only influences the amount of organic carbon in Fe deposits, but the rate and extent of microbial cell sedimentation. To date, studies directed at cell-ferrihydrite aggregate formation have exclusively focused on photoferrotrophs. Here, we examine (1) the growth of the marine cyanobacterium strain Synechococcus sp. PCC 7002 in the presence of both dissolved Fe(II) and silica under various conditions, and (2) the resultant cell-ferrihydrite aggregates. This allows for the characterization of the size and morphology of Fe(III) minerals produced by cyanobacteria, as well as the rates by which the cell-mineral aggregates sediment. This work has implications for understanding the potential role of cyanobacteria in BIF precipitation and deposition.

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