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

McQuaid JB, Kustka AB, Oborník M, Horák A, McCrow JP, Karas BJ, Zheng H, Kindeberg T, Andersson AJ, Barbeau KA, Allen AE

Nature. 2018 Mar 22;555(7697):534-537. PDF

Commentary by Dr Paolo Arosio

Of importance is the role of iron not only in mammals, but also in the oceans, particularly after the various fertilization experiments that showed that iron is the limiting factor for the growth of phytoplankton. Iron is known to come from the dust of the continents and from hydrothermal vents at the bottom of the ocean, and it is present in a particulate form and in a dissolved form. Iron in the ocean is in the oxidised Fe(III) form, that is highly insoluble, so that it easily sinks and it is not clear how the organism living in the ocean can take it up. A partial answer is from a recent paper in nature by McQuaid and collaborators, in which they shown that the diatom Phaeodactylum tricornutum expresses on its membrane a transferrin-like molecule with high affinity for iron, ISIP2A, the deletion of which reduced iron incorporation and cell proliferation. The defects were restored after complementation with human transferrin. This phytotransferrin is a trans-membrane protein like the melanotransferrin, that also may transfer iron. This phytotransferrin is able to bind the small amount of soluble Fe(III) present in the ocean and to deliver it to the cell after internalization by endocytosis. Transferrin was previously thought to have originated in multicellular metazoa, but these data show that transferrin-like iron acquisition occurs also in monocellular organisms, the phytotransferrin probably evolved from phosphonate-binding periplasmic proteins before 671 million years ago. An important implication is the observation that transferrin needs carbonate to bind iron, and the affinity strong pH dependent, since it diminishes with carbonate protonization. This is exploited for the mammalian recycling of transferrin, but it may be a problem with the acidification of the ocean due to the CO2 recent excess. In fact the papers shows  that an addition of CO2 in the medium reduces diatom iron uptake and cell replication. For example, under constant iron constant iron a doubling of CO2 concentration reduced iron uptake rates by 44% in the P. tricornutum. This may be relevant in an environment in which productivity and CO2 consumption is limited by iron.