Marine Diatoms Survive Iron Droughts in the Ocean by Storing Iron in Ferritin
summary written by Raven Hanna
Diatoms, unicellular algae that exist almost anywhere there is water, have recently attracted attention as potential thwarters of climate change. Diatoms go through cycles of blooms, where they grow and multiply rapidly near the ocean's surface. Scarcity of a nutrient will trigger the end of a bloom and the algae sink, taking with them large amounts of sequestered carbon from the air to the bottom of the ocean. Because iron is a limiting nutrient in about 30-40% of the world's oceans, some researchers propose that artificially enriching iron in oceans would promote diatom growth and carbon dioxide capture similar to the hypothesized scenario that occurs during glacial periods when iron input into the ocean is higher. At least two types of diatoms that grow well in iron enrichment experiments use a protein called ferritin to grab and store iron. By stockpiling iron when it is plentiful, they can release it when the cell needs it. One of these species is the pennate diatom, Pseudo-nitzschia multiseries, a thin eukaryotic cell about a tenth of a millimeter long that most likely acquired ferritin through lateral gene transfer.
A team of researchers from the University of Washington and the University of British Columbia used SSRL Beam Lines 9-2 and 7-1, as well as the Canadian Light Source, to determine the macromolecular crystal structure of the pennate diatom's ferritin. They found that the ferritin forms a hollow ball made from 24 monomer subunits. Minimizing the potential for cell damage from storing such a reactive metal, the ferritin first oxidizes the iron atoms into a less reactive form then stores thousands of them in the hollow center of the complex. The ferritin storage subunits are localized to the chloroplasts, which require iron for capturing solar energy through photosynthesis.
The researchers found that diatoms containing iron-storing ferritin performed more cell divisions when grown in iron-free seawater than their counterparts that lack ferritin, suggesting that ferritin gives the cells an advantage in iron-limited ocean environments that receive intermittent iron fertilization. This work was published in the January 22 issue of the journal Nature.
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Marchetti A., Parker M.S., Moccia L.P., Lin E.O., Arrieta A.L., Ribalet F., Murphy M.E.P., Maldonado M.T., Armbrust E.V. (2009) Ferritin is used for iron storage in bloom-forming marine pennate diatoms. Nature, 457, 467-470.
