Diffusive boundary layers of the colony-forming plankton alga Phaeocystis sp - implications for nutrient uptake and cellular growth

The impact of colony formation on cellular nutrient supply was calculated f or Phaeocystis in a turbulent environment using a diffusion-reaction model. The model included diffusive boundary layer as predicted by Sherwood numbe rs in mass transfer to a sphere. Literature values for nutrient uptake (V-m ax, K-m) of single cells and colonies and the size dependence of cell numbe rs in colonies were used in the model. Colony formation was shown to decrea se nutrient uptake by Phaeocystis cells because of the presence of diffusiv e boundary layers with concentration gradients surrounding the colonies. At diffusion limitation, this concentration gradient was reflected by an appa rently higher half-saturation constants for nutrient uptake, K-M, for colon ial cells compared with that for single cells. The diffusion limited supply of inorganic nitrogen and orthophosphate from the bulk water phase with co ncentrations of 2 and 0.2 mu M, respectively, was sufficient to support nut rient demands for 1 cell doubling in colonies in 6-10 h, respectively, at a shear rate of 0.1 s(-1). The same nutrient concentration levels could theo retically support nutrient demands of single cells for one cell doubling wi thin 2-3 h. It was concluded that the lower grazing pressure in the size cl ass of colonies relative to that of single free-living cells may be more im portant for colony formation than nutrient concentrations.