The influence of aqueous iron chemistry on the uptake of iron by the coastal diatom Thalassiosira weissflogii 1

Abstract

Iron uptake by the coastal diatom, Thalassiosira weissflogii, was measured by a technique that exploits the Fe(III) reducing property of ascorbate to dissolve filterable colloidal iron. For the well defined chemical conditions imposed by a large excess of the aminocarboxylate complexing agents, EDTA, CDTA, DTPA, and NTA, in the absence of light, the iron uptake rate was a function of the activity of Fe 3+. In the light, the uptake rate in the presence of EDTA and CDTA was enhanced by factors of 2 and 6 due to photodegradation of the Fe(III) complex yielding Fe(II). An even greater enhancement resulted from the addition of ascorbate, a Fe(III) reductant, to the medium. In the absence of chelators, iron uptake rates appeared to depend on the total iron concentration for both Fe(II) and Fe(III) additions to the medium. A saturated uptake rate (1.5 × 10−16 mol·cell−1·h−1) was observed in the presence of unchelated Fe(II) or colloidal Fe(III), as well as in the presence of chelators. Uptake rates measured in the presence of metabolic inhibitors are consistent with the hypothesis that a membrane‐bound metal binding complex (termed phytotransferrin) transfers iron across the membrane by a process not directly coupled to photosynthetic or respiratory activity. On the basis of all the experiments done, it is suggested that the iron uptake rate is determined by the extent of Fe binding to phytotransferrin, which is in turn controlled by pseudo‐equilibrium with free (reactive) iron in the medium, even when the medium is not at equilibrium due to (photo)reduction of Fe(III) or dissolution of ferric hydroxide.