Bioavailability of iron to Trichodesmium colonies in the western subtropical Atlantic Ocean

Trichodesmium provides new nitrogen (N) to marine surface waters via N2 fixation, a process that requires a substantial amount of iron (Fe). Organic ligands in seawater that bind Fe could either increase or reduce the bioavailability of Fe. Electrochemical techniques indicate that these naturally occurring ligands have Fe-binding constants similar to those of siderophores and porphyrins, suggesting that these chelators play an important role in determining the bioavailability of Fe to cyanobacteria. We conducted Fe uptake experiments using model ligands labeled with 55Fe to compare the bioavailability of inorganic Fe(III), porphyrin-bound Fe(III), and siderophorebound Fe(III) to field-collected Trichodesmium colonies. Inorganic Fe(III) and siderophore-bound Fe(III) were more bioavailable to Trichodesmium colonies than was porphyrin-bound Fe(III). Furthermore, the bioavailability of the siderophore-bound Fe(III) can be characterized by the functional groups of the siderophore. The dihydroxamate siderophore and an uncharacterized ligand from a cultured Synechococcus sp. increased the bioavailability of Fe compared to the trihydroxamate siderophores. Except for experiments with desferrioxamine B, dark incubations resulted in lower Fe uptake rates for all treatments, relative to parallel lighted incubations. This suggests that light enhances the photochemical dissociation of most of the ligand complexes or that light energy is required for the active transport of Fe complexed to the model ligands. The Fe uptake rate of Trichodesmium colonies also differed slightly on the basis of colony morphology, with higher uptake rates with ‘‘puffs’’ than ‘‘tufts.’’ These experiments show that Trichodesmium colonies are capable of discriminating between Fe bound to different organic complexes. Cyanobacteria are ubiquitous in marine environments and play a significant role in the global carbon (C) and N cycles (Capone et al. 1997). The bioavailability of Fe is an important factor affecting the productivity of cyanobacteria, as Fe is necessary for essential metabolic processes such as photosynthesis and N2 fixation (Wilhelm 1995). Although cyanobacteria require relatively low amounts of Fe compared to C, N, and phosphorus (P), their biological Fe requirements are not always met because of the low inputs of Fe that are common to the open ocean and the low solubility of Fe in 1 Corresponding author ([email protected]).