Although the role of iron in marine productivity has received a great deal of attention, no iron storage protein has been isolated from a marine microorganism previously. We describe an Fe-binding protein belonging to the Dps family (DNA binding protein from starved cells) in the N 2 -fixing marine cyanobacterium Trichodesmium erythraeum. A dps gene encoding a protein with significant levels of identity to members of the Dps family was identified in the genome of T. erythraeum. This gene codes for a putative Dps T. erythraeurm protein (Dps tery ) with 69% primary amino acid sequence similarity to Synechococcus DpsA. We expressed and purified Dps tery , and we found that Dps tery , like other Dps proteins, is able to bind Fe and DNA and protect DNA from degradation by DNase. We also found that Dps tery binds phosphate, like other ferritin family proteins. Fe K near-edge X-ray absorption of Dps tery indicated that it has an iron core that resembles that of horse spleen ferritin.

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We examined the bioavailability of iron-storage proteins-including representatives of maxi-and 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 d 21 . 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.