Cadmium Uptake by Phytoplankton

Unlike Zn and Co, whose uptake rates can reach 60 to 90% of the maximum diffusion rates of unchelated Zn and Co to the cell surface, Cd uptake rate only reaches up to 20% of the maximum diffusion rate of unchelated Cd in E. huxleyi [37]. This may be because the dissociation of Cd from the strong chloride complexes in seawater slows down the binding to uptake molecules [37]. 

Both laboratory culture studies and shipboard incubation experiments have shown that cellular Cd concentrations are elevated in Fe-limited phytoplankton cells [60,63]. For example, the average estimated particulate Cd P ratio in samples from Fe-limited waters was 2.3-fold higher than that from other samples [64]. The effect of Fe-limitation can be partly explained by biodilution since cells often grow slower under Fe limitation while the Cd uptake rate remains unchanged [60]. Another possible mechanism is uptake of Cd via a putative divalent metal transporter that is up-regulated under Fe-limitation [65]. 

Colloid-bound Cd is taken up by phytoplankton either through dissolution of Cd or, possibly, through direct internalization of lipophihc colloid-bound Cd [66]. As is the case for other metals, Cd complexes with low molecular weight lipophilic organic ligands enter phytoplankton by passive diffusion across the plasma membrane [67]. 

3 Effect of pH pCO2 on and Role of Weak Ligands in Cadmium Uptake 

Since Cd is used in CDCA in diatoms and CDCA expression is regulated by the pH/pC02 of seawater (see Section 6.1), we might expect that Cd uptake should be 

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