SEQUENCE ANALYSIS AND TRANSCRIPTIONAL REGULATION OF IRON ACQUISITION GENES IN TWO MARINE DIATOMS1

Authors

  • Adam B. Kustka,

    1. Department of Geosciences, Guyot Hall, Princeton University, Princeton, New Jersey 08544, USA
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    • 2

      Author for correspondence: e-mail kustka@marine.rutgers.edu.

    • 3

      Present address: Institute of Marine and Coastal Sciences, Rutgers University, New Brunswick, New Jersey 08901, USA.

  • Andrew E. Allen,

    1. École Normale Supérieure, Organismes Photosynthétiques et Environment, Paris, France Department of Geosciences, Guyot Hall, Princeton University, Princeton, New Jersey 08544, USA
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  • François M. M. Morel

    1. Department of Geosciences, Guyot Hall, Princeton University, Princeton, New Jersey 08544, USA
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  • 1

    Received 25 May 2006. Accepted 8 March 2007.

Abstract

The centric diatom Thalassiosira pseudonana Hasle et Heimdal and the pennate diatom Phaeodactylum tricornutum Bohlin possess genes with translated sequences homologous to high-affinity ferric reductases present in model organisms. Thalassiosira pseudonana also possesses putative genes for membrane-bound ferroxidase (TpFET3) and two highly similar iron (Fe) permeases (TpFTR1 and TpFTR2), as well as a divalent metal (M2+) transporter belonging to the NRAMP superfamily (TpNRAMP). In baker’s yeast, the ferroxidase–permease complex transports Fe(II) produced by reductases. We investigated transcript abundances of these genes as a function of Fe quota (QFe). Ferric reductase transcripts are abundant in both species (15%–60% of actin) under low QFe and are down-regulated by 5- to 35-fold at high QFe, suggesting Fe(III) reduction is a common, inducible strategy for Fe acquisition in marine diatoms. Permease transcript abundance was regulated by Fe status in T. pseudonana, but we did not detect significant differences in expression of the copper (Cu)-containing ferroxidase. TpNRAMP showed the most dramatic regulation by QFe, suggesting a role in cellular Fe transport in either cell-surface uptake or vacuolar mobilization. We could not identify ferroxidase or permease homologues in the P. tricornutum genome. The up-regulation of genes in T. pseudonana that appear to be missing altogether from P. tricornutum as well as the finding that P. tricornutum seems to have an efficient system to acquire Fe′, suggest that diverse (and uncharacterized) Fe-uptake systems may be at play within diatom assemblages. Different uptake systems among diatoms may provide a mechanistic basis for niche differentiation with respect to Fe availability in the ocean.

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