Carbonate-sensitive phytotransferrin controls high-affinity iron uptake in diatoms.

Area of Science:

• Marine biology
• Biochemistry
• Oceanography

Background:

• Iron is a limiting nutrient for phytoplankton in vast ocean areas.
• Diatoms, crucial eukaryotic phytoplankton, require labile iron for growth.
• The mechanism of picomolar labile iron acquisition by diatoms was previously unknown.

Purpose of the Study:

• To elucidate the mechanism of labile iron acquisition by diatoms.
• To characterize the function of the ISIP2A protein in iron uptake.
• To investigate the role of carbonate ions in diatom iron acquisition.

Main Methods:

• Genome-enabled transcriptomics to identify iron-responsive genes.
• Gene deletion and complementation experiments in the diatom Phaeodactylum tricornutum.
• Manipulation of the seawater carbonic acid system to assess ion dependencies.

Main Results:

• ISIP2A was identified as a phytotransferrin with carbonate-coordinated ferric iron binding.
• Deletion of ISIP2A impaired high-affinity iron uptake, which was restored by human transferrin.
• Iron uptake showed a second-order dependence on labile iron and carbonate ion concentrations, indicating co-limitation.

Conclusions:

• ISIP2A is essential for high-affinity iron uptake in diatoms, functioning as a phytotransferrin.
• Carbonate ion availability co-limits iron uptake, highlighting a synergistic interaction with labile iron.
• Ocean acidification, by reducing carbonate ion concentrations, may negatively impact this critical iron acquisition pathway for marine ecosystems.