Utilization of iron/organic ligand complexes by marine bacterioplankton

Nearly all of the dissolved iron in the ocean is bound in very strong organic complexes, but how heterotrophic bacteria obtain Fe from these uncharacterized natural ligands is not understood. We examined Fe uptake from model ligand/(55) Fe complexes by cultures of gamma and alpha marine proteobacteria grown under Fe-replete and Fe-limited conditions, and by natural bacterio-plankton communities from the Sargasso Sea and California upwelling region. Model chelators included siderophores and porphyrins chosen to represent the possible structures of the unknown natural ligands. Complexed-Fe uptake was compared to uptake of Fe added in inorganic form. In all cases, the Fe-stressed cultures had higher uptake rates than the Fe-replete bacteria. The gamma proteobacterium Vibrio natriegens was best able to use siderophore-bound Fe, especially from the catecholate siderophore enterobactin and the dihydroxamate rhodotorulic acid, but had very little success at utilizing porphyrin-bound Fe. In contrast, the alpha proteobacterium IRI-16 could use very little of the Fe bound to any of the model ligands in comparison to iron added in inorganic form. We observed variable degrees of Fe availability relative to ligand structure in natural communities. In some cases, Fe uptake by the prokaryotic size-class was most efficient from siderophores; in other cases, porphyrin-bound iron was more available. To better understand organically-bound Fe utilization by natural bacterioplankton communities, further studies combining ligand bioavailability measurements with detailed analyses of bacterioplankton community composition are needed.