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

Ploug, Helle, Willem Stolte, Bo Barker Jørgensen

Limnol. Oceanogr., 44(8), 1999, 1959-1967 | DOI: 10.4319/lo.1999.44.8.1959

ABSTRACT: The impact of colony formation on cellular nutrient supply was calculated for Phaeocystis in a turbulent environment using a diffusion-reaction model. The model included diffusive boundary layer as predicted by Sherwood numbers in mass transfer to a sphere. Literature values for nutrient uptake (Vmax , Km ) of single cells and colonies and the size dependence of cell numbers in colonies were used in the model. Colony formation was shown to decrease nutrient uptake by Phaeocystis cells because of the presence of diffusive boundary layers with concentration gradients surrounding the colonies. At diffusion limitation, this concentration gradient was reflected by an apparently higher half-saturation constants for nutrient uptake, KM , for colonial cells compared with that for single cells. The diffusion limited supply of inorganic nitrogen and orthophosphate from the bulk water phase with concentrations of 2 and 0.2 µM, respectively, was sufficient to support nutrient 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 theoretically support nutrient demands of single cells for one cell doubling within 2-3 h. It was concluded that the lower grazing pressure in the size class of colonies relative to that of single free-living cells may be more important for colony formation than nutrient concentrations.

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