SIZE SCALING OF EXTRACELLULAR CARBONIC ANHYDRASE ACTIVITY IN CENTRIC MARINE DIATOMS

Many microalgae have a surface-associated extracellular carbonic anhydrase (eCA) that converts HCO3- to CO2 for uptake and subsequent photosynthetic fixation. We investigated eCA activity and assessed its importance for photosynthetic CO2 supply in six centric diatom species spanning nearly the full range of cell sizes for centric diatoms (equivalent spherical radius 3-67 mu m). Since larger cells are more susceptible to diffusion limitation, we hypothesized that eCA activity would increase with cell size as would its importance for CO2 supply. eCA activity did increase with cell size, increasing with cell radius by a size-scaling exponent of 2.6 +/- 0.3. The rapid increase in eCA activity with cell radius keeps the absolute CO2 concentration difference between bulk seawater and the cell surface very low (