Availability of iron from iron-storage proteins to marine phytoplankton

We examined the bioavailability of iron-storage proteins—including representatives of maxiand miniferritins—to various species of marine phytoplankton. Both eukaryotic and prokaryotic species were able to grow rapidly with horse spleen ferritin (HoSF) or deoxyribonucleic acid (DNA)-binding proteins from starved cells (Dps) from Trichodesmium erythraeum as the sole Fe source in the medium. In the presence of ethylenediaminetetra-acetic acid (EDTA), cells grown with HoSF or Dps maintained exponential growth rates similar to those obtained at the same concentration of FeEDTA. Growth was also observed in the absence of EDTA, showing that a complexing agent is not necessary for Fe availability. The bioavailability of Fe in these storage proteins apparently results from a spontaneous release of Fe(III) to solution with an effective first-order rate constant ,0.15 d21. Genes coding for iron-storage proteins are common in DNA samples from seawater. In iron-deprived marine ecosystems, iron-storage proteins may be important constituents of the recycled iron pool and modulate its availability to phytoplankton. Iron, an essential element for virtually all living organisms, is present at extremely low concentrations in the oxygenated waters of most oceanic regions because of the insolubility of Fe(III) (Bruland et al. 1994; Martin et al. 1994). Numerous field studies have demonstrated that Fe limits phytoplankton productivity in high-nitrate, lowchlorophyll (HNLC) regions of the ocean (Martin et al. 1994; Coale et al. 1996, 2004). The interaction of oxygen, light, and Fe leads to the production of reactive oxygen species that cause oxidative damage to cells. Iron-storage proteins are used by organisms to sequester and store intracellular iron in a safe manner and provide a source of iron that can be drawn on when external iron supplies are limited (Harrison and Arosio 1996; Andrews 1998). Ferritin, bacterioferritin (heme-containing ferritin), and deoxyribonucleic acid (DNA) binding protein from starved cells (Dps) compose a superfamily of iron-storage proteins that are found in both prokaryotes and eukaryotes. Although ferritin, bacterioferritin, and Dps proteins are distantly related they retain similar structural and functional properties. Ferritin and bacterioferritin are composed of 24 subunits, whereas Dps proteins are composed of 12 subunits (Andrews 1998; Stefanini et al. 1999). The subunits are assembled to form a spherical protein with a central cavity capable of storing a maximum of 4,500 Fe(III) atoms in ferritins, 2,000 Fe(III) atoms in bacterioferritins, and 500 Fe(III) atoms in Dps proteins. Iron is incorporated into the central cavity by the oxidation of Fe(II) followed by the formation of a microcrystalline ferrihydrite–phosphate core usually referred as the ‘‘iron core.’’ The oxidation of Fe(II) is carried out by the ferroxidase center that is located within the subunits of ferritins and bacterioferritins and between subunits in Dps proteins (Andrews 1998; Ilari et al. 2000). The physiological processes that lead to the incorporation of Fe into the core and its release from iron-storage proteins to other molecules in vivo remain not fully understood. The Fe core can vary in composition and crystallinity. Ferritin isolated from animals tends to contain ordered cores that resemble the ferrihydrite mineral and are low in phosphate (Pi : Fe 1 : 8), whereas bacterial and plant ferritin are high in phosphate (Pi : Fe 1 : 1 to 1 : 3) and amorphous (Treffry et al. 1987). The Fe core of bacterioferritins tends to be disordered and normally contains more phosphate (Pi : Fe 1 : 1 to 1 : 2) (Andrews 1998; Treffry et al. 1987). The core composition of Dps proteins is less known, but phosphate incorporation into the iron core of Trichodesmium erythraeum Dps protein has been reported (Pi : Fe 1 : 4) (Castruita et al. 2006). The three types of iron-storage proteins can exist in the same bacterium and multiple copies of ferritin or bacter1 Corrsponding author ([email protected]). 2 Deceased September 2006.