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The effect of cell size on cellular Zn and Cd and Zn‐Cd‐CO2 colimitation of growth rate in marine diatoms

Cadmium (Cd) can serve as nutrient in marine diatoms by partially replacing zinc (Zn) in Zn‐limited cells. A major basis for this is the replacement of Cd for Zn in a Cd‐containing carbonic anhydrase (CDCA) needed for cellular acquisition of CO2. As a result of these interactions, cells can become colimited by Zn, Cd, and CO2. Large cell size should restrict the cellular uptake of Zn, Cd, and CO2 due to decreasing surface to volume ratios and limitation of diffusive flux of these nutrients to the cell surface. Large cells are thus more likely to become Zn‐Cd‐CO2 colimited than smaller ones. These predictions were confirmed in experiments with three marine diatoms: Thalassiosira pseudonana, Thalassiosira weissflogii, and Ditylum brightwellii, ranging in cell volume from 50 to 6000 fL. Cellular Zn : C ratios decreased by 10‐fold between the largest and smallest species, and consequently, the largest diatom was more readily growth‐limited than smaller ones by low external concentrations of bioavailable dissolved inorganic Zn (Zn′). Decreasing cellular Zn : C ratios were accompanied by large increases in cellular Cd uptake rates and Cd : C ratios. However, at the lowest Zn′ concentrations, Cd uptake plateaued, with the larger cells having lower cellular concentrations of both Zn and Cd. Cellular replacement of Zn by Cd was accompanied by increases in cellular CDCA expression. The largest diatom had higher combined carbonic anhydrase activities for a given Zn‐, Cd‐limited growth rate, supporting the hypothesis that these cells could be colimited by Zn, Cd, and CO2 at low external CO2 concentrations.