Iron uptake by diatoms is a biochemical process with global biogeochemical implications. In large regions of the surface ocean diatoms are both responsible for the majority of primary production and frequently experiencing iron limitation of growth. The strategies used by these phytoplankton to extract iron from seawater constrain carbon flux into higher trophic levels and sequestration into sediments. In this study we use reverse genetic techniques to target putative iron-acquisition genes in the model pennate diatom Phaeodactylum tricornutum We describe components of a reduction-dependent siderophore acquisition pathway that relies on a bacterial-derived receptor protein and provides a viable alternative to inorganic iron uptake under certain conditions. This form of iron uptake entails a close association between diatoms and siderophore-producing organisms during low-iron conditions. Homologs of these proteins are found distributed across diatom lineages, suggesting the significance of siderophore utilization by diatoms in the marine environment. Evaluation of specific proteins enables us to confirm independent iron-acquisition pathways in diatoms and characterize their preferred substrates. These findings refine our mechanistic understanding of the multiple iron-uptake systems used by diatoms and help us better predict the influence of iron speciation on taxa-specific iron bioavailability.

Biology Centre Institute of Parasitology Academy of Sciences of the Czech Republic 370 05 České Budějovice Czech Republic Faculty of Science University of South Bohemia 370 05 České Budějovice Czech Republic

Microbial and Environmental Genomics J Craig Venter Institute La Jolla CA 92037

Scripps Institution of Oceanography University of California San Diego La Jolla CA 92093

Scripps Institution of Oceanography University of California San Diego La Jolla CA 92093 Microbial and Environmental Genomics J Craig Venter Institute La Jolla CA 92037

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