Collection and detection of natural iron-binding ligands from seawater

Iron (Fe) is an essential element for the biochemical and physiological fun ctioning of terrestrial and oceanic organisms, including phytoplankton, whi ch are responsible for the primary productivity in the world's oceans. Howe ver, due to the low solubility of Fe in seawater, phytoplankton are often l imited by their inability to incorporate enough Fe to allow for optimal gro wth rates in regions with dissolved Fe concentrations below 1 nM. It has be en postulated that certain phytoplankton may produce compounds to facilitat e the uptake of Fe from seawater to overcome this limitation. Dissolved Fe in the oceans is overwhelmingly complexed (> 99%) by strong organic ligands that may control the uptake of Fe by microbiota; however, the identity, or igin, and chemical characteristics of these organic chelates are largely un known. Although it has been implied that some components of natural Fe-bind ing ligands are siderophores, no direct analyses of such compounds from nat ural seawater have been conducted. Here, we present a simple solid-phase ex traction technique employing Biobeads. SM-2 and Amberlite XAD-16 resins for concentrating naturally occurring dissolved iron-binding compounds from la rge volumes (> 200 1) of seawater. Additionally, we report on the first suc cessful determination of molecular weight size classes and preliminary iron -binding functional group characterization within those size classes for is olates collected from the surface and below the photic zone (150 m) in the central California coastal upwelling system. Electrochemical analyses using competitive ligand equilibration/adsorptive cathodic stripping voltammetry (CLE-ACSV) showed that isolated compounds had conditional Fe-binding affin ities (with respect to inorganic iron-Fe') of K-FeL,Fe'(cond) = 10(11.5)-10 (11.9) M-1, similar to purified marine siderophores produced in laboratory cultures and to the ambient Fe-binding ligands observed in seawater. In add ition, 63% of the extracted compounds from surface-collected samples fall w ithin the defined size range of siderophores (300-1000 Da). Hydroxamate or catecholate Fe-binding functional groups were present in each compound for which Fe binding was detected. These results illustrate that the functional groups previously shown to be present in marine and terrestrial siderophor es extracted and purified from laboratory cultures are also present in the natural marine environment. These data provide evidence that a significant fraction of the organic Fe-binding compounds we collected contain Fe-bindin g functional groups consistent with biologically produced siderophores. The se results provide further insight into characteristics of the Fe-binding l igands that are thought to be important in controlling the biological avail ability of Fe in the oceans. (C) 2001 Elsevier Science B.V. All rights rese rved.