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 con-centrations of iron sulphate triggered a massive phytoplankton bloom [4,5], and resulted in a transi- ent increase in the atmosphereฑocean CO2 flux [6].Fe(III) is the thermodynamically stable form inoxygenated seawater, existing predominantly as inso-luble oxy-hydroxides or colloidal matter [7ฑ9]. Fe(II)is a transient species in surface oxic waters, existingvia chemical or photochemical Fe(III) reduction 12], or via atmospheric deposition [13ฑ15]. At sea-water pH, Fe(II) is oxidised rapidly by O2 and H2O2[16]. Recently, organic complexation has been thoughtto occur to a signicant extent in marine systems ton are only able to utilise dissolved Fe2 or Fe3
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