Evidence for geochemical control of iron by humic substances in seawater

Abstract

A new method, based on the direct detection of iron-humic substance (HS) species by cathodic stripping voltammetry (CSV), is used to determine the iron binding capacity and complex stability of fulvic acid (FA), humic acid (HA), and the natural HS in the seawater. The FA binds 16.7 6 2.0 nmol iron (Fe) (mg FA) 21 , whereas the HA and the marine HS bind 32 6 2.2 nmol Fe (mg FA) 21 . The complex stabilities are (log K9 Fe9HS values) 10.6 for FA and 11.1 for HA and coastal HS. Measurements of coastal waters (Irish Sea) show that the HS occur in a widespread fashion, the HS concentration decreasing with increasing salinity and occurring at levels of 400 (at salinity 30) to 70 (at salinity 34) mg L 21 . A sample from the deep Pacific was found to contain 36 mg HS L 21 , amounting to 4% of the dissolved organic matter. Comparative measurement of the total iron complexing capacity by CSV with competitive ligand exchange showed that the natural HS can account for the entire ligand concentration in the shallow coastal and deep ocean waters tested. Measurements of the iron solubility showed that FA added to seawater and the HS in coastal waters maintain iron in solution at a level just below the iron binding capacity. The preliminary data for the open ocean indicate that the same may be true for HS in deep ocean waters. The data are consistent with a mechanism by which iron is transported from land to sea, associated with land-derived HS.Humic substances (HS) are ubiquitous hydrophobic components of the natural organic matter present in soil and aquatic environments that consist predominantly of polyphenols and benzoic/carboxylic acids (Buffle 1990). HS are operationally divided into humic acids (HAs) and fulvic acids ( Previous work has shown that HS in seawater can be determined at good sensitivity by cathodic stripping voltammetry (CSV), which takes advantage of adsorption of the complex on the mercury drop electrode as a concentration step prior to the CSV scan; bromate is added to enhance the sensitivity through a catalytic effect (Laglera et al. 2007). A very similar procedure (Obata and Van Den Berg 2001) is used to determine iron in seawater by means of CSV in the presence of a specific adsorptive ligand (2,3-dihydroxynaphthalene; DHN). The iron (Fe)-HS peak occurs at ,20.6 V, at nearly the same potential as the peak for Fe-DHN. The CSV response for FA (Suwannee River FA) and HA (SRHA) standards in ultraviolet (UV)-digested seawater in the presence of bromate was found to be the same as that for HS in natural waters (fresh, estuarine, and coastal waters), indicating that FA and HA are good model compounds for the natural HS.HS and iron are known to co-precipitate at the lowsalinity end of estuaries (Sholkovitz and Copland 1981), removing more than 99% of the dissolved iron and lowering its concentration from 0.5-10 mmol L 21 , which is typical for freshwaters (Nagai et al. 2007), to the 1-20 nmol L 21 range found in end-estuarine and coastal waters (Sanudo-Wilhelmy et al. 1996;Buck and Bruland 2007). The...

Saccharides enhance iron bioavailability to Southern Ocean phytoplankton

“…Not surprisingly, the concentrations of SRFA-equivalent substances in the euphotic zone of the Southern Ocean (this study) were much lower than for the Irish Sea and at depth in the Pacific Ocean (36-370 ppb of SRFA equivalents; refs. 16,40). In our study, the SRFA-equivalent concentrations detected, would correspond to 4 nM of EPS in the PF and 20 nM of EPS in the SAZ.…”

“…However, a photoactive siderophore that belongs to the L2 class has now been reported (3). Moreover, uncertainties remain about the nature of organic ligands that bind iron (12,16). Ligands too weak to fall within the analytical window of the competitive ligand exchange technique are also not detected (17); thus, their importance for iron biogeochemistry is likely to be underestimated.…”

The organic complexation of iron and copper: an intercomparison of competitive ligand exchange‐adsorptive cathodic stripping voltammetry (CLE‐ACSV) techniques

“…The larger standard deviations reported in the deeper samples between both Fe and Cu speciation results may also reflect aspects of the quality of the ligand pools present for these elements-such as fresher, biologically produced ligands in the chlorophyll maximum and a "ligand soup" of various organic breakdown materials at depth (Hirose 2007;Hunter and Boyd 2007). This ligand soup at depth in the North Pacific may also include humic-like substances (Laglera and van den Berg 2009;Yamashita and Tanoue 2009). Labs E and F employed the same experimental approach and instrumentation for Cu speciation measurements.…”

Fractionation of iron species and iron isotopes in the Baltic Sea euphotic zone

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