Probing the bioavailability of organically bound iron: a case study in the Synechococcus-rich waters of the Gulf of Aqaba

The bioavailability of organically chelated and ambient iron to phytoplankton in the Gulf of Aqaba was examined in 2 sets of grow-out incubations amended with major nutrients and increasing concentrations of the model organic ligand desferrioxamine B (DFB). Short-term uptake of pre-complexed (FeDFB)-Fe-55 was then conducted with the DFB-incubated natural populations. Since incubation communities were dominated by Synechococcus spp., short-term FeDFB uptake experiments with Synechococcus WH8102 cultures complemented the field study. The role of extracellular reduction in FeDFB acquisition was studied in accordance with the Fe(II)s model (Shaked et al. 2005) by applying large excesses of DFB. Ambient iron was found adequate in meeting natural phytoplankton iron demands in incubations containing major nutrients. A gradually increasing iron-stress response was observed as DFB concentration increased and titrated out ambient iron. Nevertheless, both natural phytoplankton and Synechococcus WH8102 cultures were able to acquire DFB-bound iron. FeDFB uptake rates were inhibited by large excesses of DFB. This may indicate the operation of an extracellular reduction step in the acquisition of FeDFB by natural populations in the Gulf of Aqaba and Synechococcus WH8102.