This paper discusses structure and function of the Phaeocystis colony skin, and relates them to the specific impact of Phaeocystis colonies on ecology and biogeochemistry. The potential advantage of the recently discovered tough skin around the colonies of Phaeocystis globosa is discussed in context with the metabolic costs of this structure, and compared to potential functions of structures around other phytoplankton. It is further proposed that mainly small, fast-growing pathogens and predators will be deterred by the colony skin. It will be shown that these theoretical predictions are consistent with available data from the literature, and can explain the dominance of the colonial form in Phaeocystis blooms. Finally, the peculiar biogeochemistry of Phaeocystis colonies, especially the sedimentation of Phaeocystis-derived organic matter, is argued to be a function of the susceptibility of Phaeocystis colonies to certain grazers, which in turn is strongly determined by the architecture and function of the colony skin. During the exponential phase of the bloom, Phaeocystis-derived organic matter can efficiently sink in faecal material of large zooplankton, which actively feed on the colonies. However, the integrity of the colony skin, and consequently its protection for the cells therein, seems to be closely coupled to the phase of active growth. Accordingly, the cells are massively affected by small grazers and pathogens and thus rapidly disintegrate after the culmination of the bloom, so that sedimentation of Phaeocystis-derived organic matter becomes probably restricted to the more refractory extracellular components of the colonies. (C) 2000 Elsevier Science B.V.