Different iron availability to two marine diatoms revealed by different iron-uptake strategies

In the open ocean, extremely low iron (Fe) concentrations limit the growth of marine phytoplankton. To cope with Fe scarcity and its diverse chemical forms in seawater, phytoplankton have evolved distinct uptake strategies. Here, we compared two low-Fe-adapted diatoms, Thalassiosira oceanica and Phaeodactylum tricornutum, to assess the bioavailability of unchelated Fe and siderophore-bound forms, as well as their underlying Fe-uptake strategies. Growth inhibition assays with bathophenanthroline disulfonic acid (BPDS) indicated that both species acquired unchelated Fe at environmentally relevant concentrations by reductive pathways in which Fe(III) was reduced to Fe(II) prior to internalization. In contrast, their responses to hydroxamate siderophores, desferrioxamine B (DFB) and ferrichrome (FCH), diverged: P. tricornutum maintained substantial growth and Fe-uptake rates when reductive pathways were inhibited, consistent with a receptor-mediated non-reductive Fe-uptake pathway, whereas T. oceanica relied predominantly on reductive processes and transported FeDFB and FeFCH at much lower rates. These differences suggest that P. tricornutum may gain a competitive advantage in hydroxamate siderophore-rich waters. Overall, the diverse Fe-acquisition strategies among marine phytoplankton may determine their ecological success and spatial distribution in the ocean.