Availability of well‐defined iron colloids to the marine diatom Thalassiosira weissflogii

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Abstract
Three different types of iron oxo‐hydroxo colloids with well‐defined sizes and chemistries have been synthesized by hydrolysis of Fe(III) in acidic and alkaline medium as well as by oxidative hydrolysis of Fe(II), resulting in solutions of β‐FeOOH (30 nm), a hexanuclear complex (HN; 2 nm), and a polynuclear oxidation product (OP; 3 nm). These colloids were tested for their availability to the coastal diatom Thalassiosira weissflogii. Iron‐limited cells were grown in synthetic seawater containing 0–0.5 µM of total iron in colloidal form as the only iron source. We found no evidence of direct uptake of colloids by the algae. The β‐FeOOH colloids, which dissolve exceedingly slowly in the light or in the dark, did not support diatom growth. In contrast, maximal growth rates and cell yields were observed in the presence of either 50 nM OP or HN. Photoreduction accounted for the availability of HN. In the case of OP, thermal (dark) dissolution of the colloid appears sufficient to satisfy the iron requirement of the diatom.