Copper-dependent iron transport in coastal and oceanic diatoms

Maldonado, Maria T., Andrew E. Allen, Joanne S. Chong, Kevin Lin, Dan Leus, Nataliya Karpenko, Shannon L. Harris

Limnol. Oceanogr., 51(4), 2006, 1729-1743 | DOI: 10.4319/lo.2006.51.4.1729

ABSTRACT: We investigated the presence of plasmalemma-bound copper-containing oxidases associated with the inducible iron (Fe) transport system in two diatoms of the genus Thalassiosira. Under Fe-limiting conditions, Thalassiosira oceanica, an oceanic isolate, was able to enzymatically oxidize inorganic Fe(II) extracellularly. This oxidase activity was dependent on copper (Cu) availability and diminished by exposure to a multi-Cu oxidase (MCO) inhibitor. The rates of Fe uptake from ferrioxamine B by Fe-limited T. oceanica were also dependent on Cu availability in the growth media. The effects of Cu limitation on Fe(II) oxidation and Fe uptake from ferrioxamine B were partially reversed after a short exposure to a Cu addition, indicating that the putative oxidases contain Cu. Limited physiological experiments were also performed with the coastal diatom Thalassiosira pseudonana and provided some evidence for putative Cu-containing oxidases in the high-affinity Fe transport system of this isolate. To support these preliminary physiological data, we searched the newly available T. pseudonana genome for a multi-Cu-containing oxidase gene and, using real-time polymerase chain reaction (PCR), quantified its expression under various Fe and Cu levels. We identified a putative MCO gene with predicted transmembrane domains and found that transcription levels of this gene were significantly elevated in Fe-limited cells relative to Fe-replete cells. These data collectively suggest that putative MCOs are part of the inducible Fe transport system of Fe-limited diatoms, which act to oxidize Fe(II) following reductive dissociation of Fe(III) from strong organic complexes.

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