Iron is an essential element in many key biochemical processes, including respiration, nitrogen fixation, nitrate reduction, and chlorophyll biosynthesis. Iron limits phytoplankton production and regulates plankton community structure over large areas of the ocean, and for some marine bacteria, iron stress stimulates expression of high affinity iron acquisition systems, including extracellular iron specific chelators, or siderophores. Iron speciation measurements show that >99% of dissolved iron is complexed by organic ligands of unknown composition, and that these complexes have conditional stability constants characteristic of siderophores, low molecular mass molecules that have a high specificity for chelating or binding iron. Siderophores in marine heterotrophic bacteria have been identified, and siderophore-like compounds have been detected in exudates of marine phytoplankton. It has been postulated that siderophores or their breakdown products make up a large fraction of the strong iron-binding ligands in surface seawater, and that they play an important role in microbial iron uptake. Nevertheless, only one marine cyanobacterium siderophore has been definitively characterized, and no siderophores from any photoautotrophs have ever been isolated from seawater. Are siderophores present in the mixture of iron binding ligands in seawater, and do marine photoautotrophs use siderophores to acquire iron? Or are natural iron binding ligands a by-product of generic organic matter cycling with little affect on iron uptake? The evidence that a large fraction of dissolved iron is complexed by organic ligands is compelling. However, we do not yet know the nature of the ligands, or if indeed (as many have suggested), they mediate iron acquisition by marine microbes. Here I propose to collect iron-binding ligands from very large volume surface seawater samples for detailed spectroscopic characterization (mass spectrometry and nuclear magnetic resonance spectroscopy).
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