Availability of iron from iron-storage proteins to marine phytoplankton

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^-1. 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.