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In many parts of the global ocean, iron bioavailability is a critical factor controlling primary productivity. Because colloidal Fe (cFe) makes up a substantial proportion of the surface water Fe inventory, it is critical to evaluate the biogeochemical behaviour and availability of this quantitatively important Fe pool. Here we present and review a compilation of over 1300 soluble and corresponding dissolved Fe concentration measurements from which operationally-defined colloidal Fe data have been determined. Partitioning between cFe and the smaller soluble Fe (sFe) size fraction shows a high degree of variability in the surface ocean caused by the range of factors affecting size fractionation (e.g., Fe sources, ligand controls, thermodynamic controls, biological utilisation). Evaluation of the seasonality of sFe and cFe partitioning indicates that a strong decoupling between their respective biogeochemical cycling occurs in the spring time. This suggests that a seasonal component (e.g., seasonal biological growth, seasonal changes in Fe supply) is responsible for a proportion of the observed surface ocean variability. Below depths of 200 m, the partitioning becomes more constant (49 ± 17%). To better understand the bioavailability of the cFe pool, future work should complement traditional size-fractionated data with molecular level investigation into the functional group chemistry of organic Fe colloids and the mineralogy and surface chemistry of inorganic Fe colloids. [ABSTRACT FROM AUTHOR]

Abstract: Experiments were conducted using a semi-continuous batch protocol to leach iron from aerosol samples. In this procedure, aerosol samples were leached in an all-Teflon flow-through reaction chamber by multiple (3–4) aliquots (~40 mL) of leaching solution per predetermined leaching period. Several types of aerosols and leaching solutions were tested. The percent iron solubility obtained from aerosols collected in two urban regions was lower (~1%) than the solubility from samples collected in the open ocean (6–13%). This regional variability was greater than the variability obtained when leaching the same aerosols using different leaching solutions. A review of field and laboratory studies from the literature also suggests that different types of aerosols produce more variability in aerosol iron solubility estimates than do different leaching protocols. Colloidal iron was the dominant fraction of aerosol derived dissolved iron in all the experiments carried out during this study, suggesting that the dissolution of aerosol iron in seawater is dominated by the colloidal iron fraction. [Copyright &y& Elsevier]