Spring 2008 Seminar Series

Dr. Adam Kustka

Abstract

Most data concerning the availability of iron in eukaryotic phytoplankton suggest that unchelated Fe, Fe prime (Fe'), is the available form, yet most of the iron in seawater at equilibrium is bound to strong organic ligands.  Biological reduction of Fe(III) chelates is one way which eukaryotes may access Fe from the FeL pool.  We explore whether reduction at cell surfaces and/or diffusible reductants may be responsible for Fe uptake, and therefore reconcile discrepancies between the Fe' model and the concept of ferric reduction.  We measured the production of a diffusible reductant, superoxide, from diatoms, as well as the superoxide-mediated production of Fe(II) from Fe(III) and the effect of superoxide  on the availability of Fe as inorganic Fe complexes, ferrihydrite, Fe bound to model porphyrin compounds and Fe regenerated from grazers.  We also developed a cell surface model of Fe transport in two diatoms, Thalassiosira pseudonana and T. weissflogii.  Empirical data supporting this model will be presented. We investigated the relationship between cellular Fe quota and the relative transcript abundances for putative Fe transporters in the centric and pennate diatoms T pseudonana and Phaeodactylum tricornutum, under conditions of steady state and rapidly changing iron availability.  Ferric reductases mRNA was much more abundant in low Fe cells for both species.  Metal inhibition of Fe uptake experiments suggest uptake in T. pseudonana is not mediated by divalent transporters, and a Cu dependency suggests Cu-containing ferroxidases are involved. The subsequent steps for Fe uptake in P. tricornutum are unclear, yet this species shows a pronounced ability to take up Fe under low Fe conditions that are lethal for T. pseudonana.