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

TitleCarbonate-sensitive phytotransferrin controls high-affinity iron uptake in diatoms
Publication TypeJournal Article
Year of Publication2018
AuthorsMeQuaid J.B, Kustka A.B, Obornik M., Horak A., McCrow J.R, Karas B.J, Zheng H., Kindeberg T., Andersson AJ, Barbeau K.A, Allen A.E
Date Published2018/03
Type of ArticleArticle
ISBN Number0028-0836
Accession NumberWOS:000427937900056
Keywordsalga dunaliella; evolution; marine-phytoplankton; ocean acidification; phaeodactylum-tricornutum; plasma-membrane; reporter genes; Science & Technology - Other Topics; sequence alignment; signal peptides; transferrin

In vast areas of the ocean, the scarcity of iron controls the growth and productivity of phytoplankton(1,2). Although most dissolved iron in the marine environment is complexed with organic molecules(3), picomolar amounts of labile inorganic iron species (labile iron) are maintained within the euphotic zone(4) and serve as an important source of iron for eukaryotic phytoplankton and particularly for diatoms(5). Genome-enabled studies of labile iron utilization by diatoms have previously revealed novel iron responsive transcripts(6,7), including the ferric iron-concentrating protein ISIP2A(8), but the mechanism behind the acquisition of picomolar labile iron remains unknown. Here we show that ISIP2A is a phytotransferrin that independently and convergently evolved carbonate ion-coordinated ferric iron binding. Deletion of ISIP2A disrupts high-affinity iron uptake in the diatom Phaeodactylum tricornutum, and uptake is restored by complementation with human transferrin. ISIP2A is internalized by endocytosis, and manipulation of the seawater carbonic acid system reveals a second order dependence on the concentrations of labile iron and carbonate ions. In P. tricornutum, the synergistic interaction of labile iron and carbonate ions occurs at environmentally relevant concentrations, revealing that carbonate availability co-limits iron uptake. Phytotransferrin sequences have a broad taxonomic distribution(8) and are abundant in marine environmental genomic datasets(9,10), suggesting that acidification-driven declines in the concentration of seawater carbonate ions will have a negative effect on this globally important eukaryotic iron acquisition mechanism.

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