Bioavailability of Iron Sensed by a Phytoplanktonic Fe-Bioreporter

This study describes a short-term (12 h) evaluation of iron (Fe) bioavailability to an Fe-dependent cyanobacterial bioreporter derived from Synechococcus PCC 7942. Several synthetic ligands with variable conditional stability constants for Fe(III) (K* of 1019.8 to 1030.9), in addition to several defined natural Fe-binding ligands and a fulvic acid of aquatic origin (Suwannee River), were used to elucidate the forms of Fe that are discerned by this phytoplanktonic microbe: Fe−HEBD (log conditional stability constant, K*, = 28.1, HEBD = N,N‘-di(2-hydroxybenzyl)ethylenediamine-N,N‘-diacetic acid monohydrochloride hydrate), Fe−HDFB (K* = 30.9, DFB = desferroxamine B), Fe−ferrichrome (K* = 23.2), Fe−DTPA (K* = 21.1, DTPA = diethylenetrinitrilopentaacetic acid), Fe−(8HQS)2 (K* = 20.4, 8HQS = 8-hydroxyquinoline-5-sulfonic acid), Fe−CDTA (K* = 19.8, CDTA = trans-1,2-cyclohexylenedinitrilotetraacetic acid), and Fe−EDTA (K* = 19.2). Iron bioavailability sensed by the bioreporter was related to diffusion limitation and activity of high-affinity transporters rather than by siderophore secretion. Iron complexed with a K* < 23.2 contributes to the bioavailable pool; bioavailability could be explained by disjunctive ligand exchange considerations and fully, partially, and nonbioavailable complexes could be distinguished according to their conditional stability constant. The use of Fe-bioreporters provides a relevant measurement of bioavailability to an important group of primary producers in freshwaters (cyanobacteria) and is thus a promising technique for understanding Fe cycling in aquatic systems.