Iron is a major component of the Earth's crust, but like other reactive trace elements, its dissolved con- centration in open-oceanic waters remains very low (<1.0 nM). It is an essential micronutrient for organ- isms and in certain high-nutrient, low-chlorophyll areas of the world's oceans, iron appears to limit phytoplankton growth [1,2], which may have impor- tant implications for global carbon cycles [3]. Such hypotheses have recently been tested in the under- productive waters of the equatorial Paciฎc, where seeding an expanse of surface water with low concentrations of iron sulphate triggered a massive phytoplankton bloom [4,5], and resulted in a transi- ent increase in the atmosphereฑocean CO2 flux Fe(III) is the thermodynamically stable form inoxygenated seawater, existing predominantly as insoluble oxyhydroxides or colloidal matter . Fe(II)is a transient species in surface oxic waters, existing via chemical or photochemical Fe(III) reduction, or via atmospheric deposition . At seawater pH, Fe(II) is oxidised rapidly by O2 and H2O2. Recently, organic complexation has been thought to occur to a signiฎcant extent in marine systems [17ฑ19]. Laboratory studies have shown that phytoplankton are only able to utilise dissolved Fe2 or Fe3
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