Fig. 2.
Concentration and speciation of iron in surface seawater through time. Putative Fe concentrations were taken from the literature (SI Appendix has details). The redox state of the oceans in the Early Archean is not precisely constrained but was certainly many orders of magnitude more reducing than the present day (uncertainty of prevailing oxygen fugacity, fO2, represented by light blue bands). Assuming the Archean oceans were broadly in equilibrium with an atmosphere derived primarily from gases emanating from the mantle implies that the oceanic near-surface waters were significantly more reducing and lower in pH than present (76) and importantly, Fe(II) enriched (77). Local oxidation of Fe by either biological or abiogenic actions would have led to significant temporal iron limitation. Additionally, significantly higher Fe concentration also implies a possible Fe(III) component in the reduced oceans of similar magnitude to present. The evolution of Fe(III) selecting siderophores may then be a response to both temporal Fe limitations and also, a mechanism to both select and control cellular Fe acquisition.