Dissolved-iron availability plays a critical role in controlling phytoplankton growth in the oceans 1,2 . The dissolved iron is overwhelmingly (ϳ99%) bound to organic ligands with a very high affinity for iron 3-5 , but the origin, chemical identity and biological availability of this organically complexed Fe is largely unknown 6 . The release into sea water of complexes that strongly chelate iron could result from the inducible iron-uptake systems of prokaryotes (siderophore complexes) 7-9 or by processes such as zooplankton-mediated degradation and release of intracellular material (porphyrin complexes). Here we compare the uptake of siderophore-and porphyrin-complexed 55 Fe by phytoplankton, using both cultured organisms and natural assemblages. Eukaryotic phytoplankton efficiently assimilate porphyrin-complexed iron, but this iron source is relatively unavailable to prokaryotic picoplankton (cyanobacteria). In contrast, iron bound to a variety of siderophores is relatively more available to cyanobacteria than to eukaryotes, suggesting that the two plankton groups exhibit fundamentally different iron-uptake strategies. Prokaryotes utilize iron complexed to either endogenous 7-9 or exogenous siderophores 9 , whereas eukaryotes may rely on a ferrireductase system 10,11 that preferentially accesses iron chelated by tetradentate porphyrins, rather than by hexadentate siderophores. Competition between prokaryotes and eukaryotes for organicallybound iron may therefore depend on the chemical nature of available iron complexes, with consequences for ecological niche separation, plankton community size-structure and carbon export in low-iron waters.Extracellular chelators are produced by many marine cyanobacteria and heterotrophic bacteria as part of high-affinity Fe uptake systems 7,9,12 . Several investigators have noted the similarity between Fe-binding conditional stability constants (K cond ) of these siderophores and the uncharacterized ligands in sea water (for both, log K cond ¼ 19-23) 4,7,8 . The siderophores produced by various marine prokaryotes have diverse molecular structures, including hydroxamates, catecholates 7,9 , and mixed functional group (-hydroxy aspartate/catecholate) ligands 12 .Another likely source of strong chelators in the water column is through release of intracellular ligands by zooplankton grazing 13 or viral lysis 14 . Cells contain many organic molecules that can strongly complex iron. We chose tetrapyrroles to represent this type of cell breakdown ligand, since they are among the most abundant strong chelators of Fe in cells. Tetrapyrroles include porphyrins such as those in chlorophylls, cytochromes and haem proteins 6 , and linear tetrapyrroles such as the phycobilin pigments of cyanobacteria. Like siderophores, tetrapyrroles have very high conditional stability constants for Fe(III) binding that resemble those of the seawater ligands (ref. 4, and A.E.W., unpublished results). Faecal pellets of metazoan and protozoan grazers are highly enriched with phaeopigment porphyrins 15,16 ,...
