Fig. S1. DGGE analysis of 16S rRNA gene amplicons retrieved from different generations of Fe(III)-Si coprecipitate reducing enrichment cultures.

Fig. S2. Mixing lines used to estimate δ56Fe values for the Fe(III) component of the 0.5 m HCl extracts calculated assuming that δ56FeFe(II)0.5 M HCl = δ56FeFe(II)0.1 M HCl.

Fig. S3. Mixing lines used to estimate δ56Fe values for the Fe(II) component of the 0.5 m HCl extracts.

Fig. S4. Plot of estimated δ56FeFe(II)0.5 M HCl calculated values versus measured δ56FeFe(II)0.1 M HCl values.

Fig. S5. Mixing lines employed to resolve δ56FeFe(III)0.5 M HCl Fe(III)0.5 M HCl given the constraints of Fe(II)0.5 M HCl expressed in equation  7 from the text.

Table S1. Artificial Archaean seawater composition.

Table S2. Fe:Si ratio of Fe–Si coprecipitate.

Table S3. Fe isotope composition of partial dissolutions of the Fe–Si coprecipitate.

Table S4. Phylogenetic association of the most common 16S rRNA gene sequences from 454 pyrosequencing.

Table S5. Aqueous silica and Fe concentrations in DIR experiments.

Table S6. Determination of isotopic composition of 0.5 m HCl-extractable Fe(II) and Fe(III).

Table S7. Raw Fe isotope data.

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