The effect of cell size on cellular Cd and Cd-Zn-CO2 co-limitation of growth rate in coastal diatoms

Cadmium serves as nutrient in marine diatoms by replacing zinc in Zn-limited cells. A major basis for this replacement is the use of a cadmium protein Cd-carbonic anhydrase (CDCA) in Zn-limited cells, which functionally replaces a Zn-containing carbonic anhydrase (ZnCA) needed for cellular uptake and fixation of CO₂. As a result of these interactions cells can become co-limited 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 in diffusive flux of these nutrients to the cell surface. So large cells are more likely to become Zn-Cd-CO₂ co-limited. These predictions were confirmed in experiments with three marine diatoms: Thalassiosira pseudonana, Thalassiosira weissflogii and Ditylum brightwellii, ranging in cell size from 50 to 6000 μm³. Cellular Zn:C ratios decreased by 10-fold between the largest and smallest species, and consequently, the largest diatom was more readily growth-limited by low external Zn concentrations than smaller ones. Decreasing cellular Zn:C ratios was accompanied by large increases in cellular Cd uptake rates and Cd:C ratios to facilitate Cd replacement of Zn. However, at the lowest external Zn concentrations, Cd uptake plateaued, with the larger cells having lower cellular concentrations of both Zn and Cd. Cellular replacement of Cd for Zn was accompanied by large increases in CdCA and CdCA:ZnCA ratios. The largest diatom had higher CA activities for a given Zn, Cd-limited growth rate, supporting the hypothesis that these cells could be co-limited by Zn, Cd, and CO₂.