Limnol. Oceanogr., 44(8), 1999, 1959–1967

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 mM, 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 s21. 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. Phaeocystis sp. is a common marine plankton alga that exists as free-living cells and as colonies with up to thousands of cells. The cells are primarily positioned at the surface of a mucus matrix, which can be several millimeters in diameter (Rousseau et al. 1994). The extracellular matrix mainly consists of exopolymeric gelatinous material, the synthesis of which was negatively correlated to the mineral nitrogen concentration in the Southern Bight of the North Sea (Lancelot 1983). Phaeocystis blooms often occur at ,0.5 mM orthophosphate in the sea (Veldhuis et al. 1986). Thus, colony formation appear to be of competitive advantage under nutrient limitation (Lancelot 1995). Uptake studies with 32P, however, have shown that the maximum uptake rate of orthophosphate is similar for single, free-living cells and colonial cells, whereas the half-saturation constant was 1 Present address: Marine Biological Laboratory, University of Copenhagen, Strandpromenaden 5, DK-3000 Hesingør, Denmark. 2 Present address: Department of Marine Sciences, Kalmar University, Box 905, S-39129 Kalmar, Sweden. Acknowledgments Ulf Riebesell, Thomas Kiørboe, and Christiane Lancelot are thanked for discussions. Richard Geider and two anonymous reviewers are thanked for their critical comments on earlier versions of the manuscript. This work was financed by the Environment Program of the European Commission (J:EV5V-CT94-0511) to Christiane Lancelot (organizer), Universite Libre Bruxelles, and by the Danish Natural Science Research Council (J:11-0557-1 PD), the Danish Research Academy (J: V930148), and the Max Planck Society (Germany) to H.P. one order of magnitude larger for colonial cells than for freeliving cells (Veldhuis et al. 1991). Colonial cells grow significantly slower than free-living cells under phosphate limitation (Veldhuis and Admiraal 1987). These results are consistent with limitation of nutrient uptake by the presence of a diffusive boundary layer (DBL) surrounding Phaeocystis colonies. DBLs around Phaeocystis colonies have been directly demonstrated by the detection of steep gradients of oxygen and pH levels in the DBL due to the photosynthetic activity of Phaeocystis cells in the mucus matrix (Lubbers et al. 1990; Ploug et al. in press). The measured DBL thickness surrounding sinking aggregates has shown to be in good agreement with the DBL thickness predicted by Sherwood numbers (Ploug et al. 1997; Ploug and Jørgensen 1999). Phaeocystis colonies, however, are often neutrally buoyant or sink at low velocities of a few meters a day. Turbulence with shear rates of 0.1–1.0 s21 decreases the DBL thickness and may thus increase nutrient uptake in Phaeocystis colonies in the natural environment (Ploug et al. in press). In the present study, we used a model to analyze the quantitative impact of DBLs on nutrient supply and limitation for colonial cells as compared with single free-living cells in a turbulent environment. Materials and methods Turbulence and shear—The impact of turbulence on mass transfer to a single free-living cell or a colony is dependent