Ferritin is used for iron storage in bloom-forming marine pennate diatoms

Ferritin's role in the oceans The non-haem protein ferritin is used by many plants, animals and microorganisms to store iron in a non-toxic soluble form that can be readily mobilized when required. Ferritin has now been found in the two diatoms, Pseudo-nitzschia and Fragilariopsis , that dominate phytoplankton blooms induced by both natural and artificial oceanic iron supplementation. This is the first report of ferritin in any member of the Stramenopila, the eukaryote lineage that includes many plankton components including unicellular algae, diatoms and macroalgae. Phylogenetic analysis suggests that ferritin arose in this small subset of diatoms via lateral gene transfer, and it may be key to their success in the 30–40% of ocean waters in which iron availability is the factor limiting primary productivity. Natural or artificial oceanic iron supplementation induces blooms that are dominated by pennate diatoms. It is shown that these diatoms contain the iron storage protein ferritin, which may explain their success in iron-limited waters. Primary productivity in 30–40% of the world’s oceans is limited by availability of the micronutrient iron 1 , 2 . Regions with chronically low iron concentrations are sporadically pulsed with new iron inputs by way of dust 3 or lateral advection from continental margins 4 . Addition of iron to surface waters in these areas induces massive phytoplankton blooms dominated primarily by pennate diatoms 5 , 6 . Here we provide evidence that the bloom-forming pennate diatoms Pseudo-nitzschia and Fragilariopsis use the iron-concentrating protein, ferritin, to safely store iron. Ferritin has not been reported previously in any member of the Stramenopiles, a diverse eukaryotic lineage that includes unicellular algae, macroalgae and plant parasites. Phylogenetic analyses suggest that ferritin may have arisen in this small subset of diatoms through a lateral gene transfer. The crystal structure and functional assays of recombinant ferritin derived from Pseudo-nitzschia multiseries reveal a maxi-ferritin that exhibits ferroxidase activity and binds iron. The protein is predicted to be targeted to the chloroplast to control the distribution and storage of iron for proper functioning of the photosynthetic machinery. Abundance of Pseudo-nitzschia ferritin transcripts is regulated by iron nutritional status, and is closely tied to the loss and recovery of photosynthetic competence. Enhanced iron storage with ferritin allows the oceanic d