Luxury uptake, iron storage and ferritin abundance in Prochlorococcus marinus (Synechococcales) strain MED4

We measured the capacity for luxury iron (Fe) uptake and ferritin abundance in the marine cyanobacterium Prochlorococcus marinus (strain MED4) acclimated to various Fe concentrations. We also followed changes in ferritin abundance and physiological measurements after abruptly changing Fe availability by adding Fe-EDTA or desferrioxamine B (DFB) to cultures. Fe quotas (normalized to carbon) increased from 46 to 112 Fe:C (μmol:mol) in cultures grown at Fe′ (the summed concentrations of all Fe species not complexed with EDTA) between 21 and 180 pM. No further increase in Fe:C was observed at 684 pM Fe′. Growth rates and variable fluorescence did not vary significantly in cultures acclimated to Fe0 values ranging from 15 to 679 pM, demonstrating that Prochlorococcus can store ∼ 2.4-fold more Fe than it requires for maximum growth. Ferritin abundance increased from below detection at 15 pM Fe′ to maximum values at 175 to 679 pM Fe′. Ferritin abundance, growth, and variable fluorescence decreased after the addition of DFB, and ferritin abundance increased upon addition of Fe-EDTA. These results further support the protein's role in Fe storage in Prochlorococcus. This capacity for luxury Fe uptake may help Prochlorococcus survive periods of low Fe availability in the open ocean, and the recycling of ferritin-stored Fe may be an important component of the marine iron cycle.