Low iron bioavailability limits phytoplankton growth across vast oceanic regions. Although iron acquisition mechanisms have been relatively well characterized in model diatoms, they remain poorly understood in other marine eukaryotic phytoplankton. Coccolithophores are the most abundant calcifying organisms in modern oceans, and important primary producers in many marine ecosystems. We show that the model coccolithophore Emiliania huxleyi exhibits distinct iron uptake strategies depending on the iron source. For Fe(III)' acquisition, it utilizes a phytotransferrin-mediated pathway similar to that in diatoms, displaying low reductase activity; the uptake rate is co-limited by carbonate availability. In contrast, for natural ligand-complexed iron, the presence of ferrozine significantly reduces its short-term uptake rate. These distinct iron uptake strategies likely enable E. huxleyi to adapt to iron-limited open-ocean conditions.

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