Abstract
We examined substrate‐saturated Fe(II) vs. Fe(III) uptake rates of Thalassiosira oceanica preconditioned to varying degrees of Fe limitation. Inorganic Fe(III) uptake rates by Fe‐sufficient T. oceanica were 2.4‐fold faster than the corresponding inorganic Fe(II) uptake rates. However, when cultures were severely Fe limited, the rate of Fe(II) uptake was upregulated 15‐fold, while that of Fe(III) uptake increased only fivefold. The interactions between substrate‐saturated uptake rates of inorganic Cd(II) and either Fe(II) or Fe(III) by Fe‐limited T. oceanica were also investigated. The addition of equimolar Cd(II) concentration to the Fe(II) uptake media resulted in a ~50% reduction of inorganic Fe(II) uptake rates compared with those in Cd‐free media. In turn, Cd uptake rates were inhibited ~36% in the presence of an equimolar Fe(II) concentration. In contrast to Fe(II), Fe(III) transport exhibited no interaction with Cd(II). T. oceanica thus has separate transporters for inorganic Fe(III) and Fe(II). Cadmium(II) and Fe(II) appear to enter the cell through a common putative divalent metal transporter that is upregulated under Fe deficiency. The interaction of Fe(II) and Cd(II) transport under Fe deficiency provides a plausible mechanism to explain some laboratory and field observations of higher Cd quotas in Fe‐limited phytoplankton.