Iron is essential to all life forms. Assess the solubility of Fe in +2 and +3 oxidation states; combine this with likely stable oxidation state under normal conditions to assess the bioavailability of iron. Consider how the atmosphere changed as oxygenic photosynthesis took over (Box 16.1) and comment on how this might have affected the bioavailability of iron.

Data from Box 16.1.

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BOX 16.1 What are the consequences of an oxygen atmosphere on the environment? In the course of the evolution of the Earth's atmosphere, the proliferation of oxygen-evolving photosynthesis eventually resulted in the presence of O, in the atmosphere at the present level of 21 per cent by volume. Oxygen was a toxic constituent of the atmosphere of the early Earth and led to the extinction of many species. Some species retreated to habitats deeper in the soil or waters, where anaerobic conditions persisted and where their descendants remain today. Other organisms adapted differently and evolved to exploit this now abundant and powerful oxidant. These organisms are the aerobes, among which were our ancestors. The shift from an anaerobic atmosphere to an oxygenic atmosphere had a profound effect on the composition of the waters. Sulfur, which was present largely in the form of sulfide in anaerobic waters, was oxidized to sulfate. Metal ion concentrations also changed dramatically. Two of the metals most profoundly affected were molybdenum and iron. In Earth's modern oceans, molybdenum is the most abundant d metal (at 0.01 ppm). However, before the oxygenation of the oceans and atmosphere, molybdenum was present as insoluble solids, mainly MoO, and MoS₂. Oxidation of these solids produced the soluble molybdate ion: 2 MoS₂(s)+70₂(g) + 2H₂O(l)→ 2[MOO 1²(aq) +450₂(g) + 4H+ (aq) The molybdate ion became available to aquatic organisms and is now transported into cells by methods that differ dramatically from those used to acquire the more widespread cationic d-metal species in the marine environment. Iron suffered the opposite fate to molybdenum. In the ancient oceans, the element was present as Fe(II). Iron(II) hydroxide and sulfide are essentially soluble and so iron would have been readily available to aquatic organisms. However, on oxygenation of the atmosphere, the oxidation of Fe²+ to Fe³+ led to the precipitation of iron(III) hydroxides and oxides, and uptake of Fe by living organisms depended on special ligands known as siderophores (Section 26.6). Massive banded formations containing magnetite (Fe,O,) and hematite (Fe₂O₂) in Canada and Australia are testimony to the precipitation of iron from the oceans between 2 and 3 Ga (2-3x10 years) ago.