Iron (Fe) is an essential micronutrient to sustain primary production in the contemporary ocean. Here, the Fe homeostasis of the only available Fe bioreporter (based on the cyanobacterium Synechococcus sp. PCC7002) acclimated to increasing Fe limitation and Fe replete conditions was characterized on basis of its transcriptome, growth, stoichiometric composition and photophysiology. Then, the bioreporter assay was optimized to sense Fe bioavailability in marine systems, showing that the application of the bioreporter was limited in regions of the ocean with low Fe concentrations. Fe bioavailability was alternatively inferred from Fe uptake rates determined for two phytoplankton species using 55Fe bioaccumulation experiments and compared to the concentration of labile Fe species in chemically characterized artificial seawater and natural seawater samples. Finally, the effect of different organic ligands on the growth and structure of natural phytoplankton communities naturally Fe limited phytoplankton were tested using on board bottle incubations during the PS97 expedition.