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The bloom-forming prymnesiophyte Phaeocystis globosa forms hollow, spherical, mucilaginous colonies that vary from micrometres to millimetres in size. A recent paper gave the first empirical evidence that colony size increase in P. globosa is a defensive response against grazers, and knowing the signalling mechanism(s) behind this response will thus be a key to understanding the trophodynamics in systems dominated by this species. I conducted experiments with specially designed diffusion incubators, each of which consists of a non-grazing chamber (with P. globosa only) and a grazing chamber (grazers + phytoplankton) connected by 2 μm polycarbonate membrane filters. The results showed that physical contact with grazers was not required to initiate the defensive response; instead, P. globosa colony size increase was found to be stimulated by dissolved chemicals generated by ambient grazing activities. This signal was non-species specific, such that various combinations of three species of grazers and four species of phytoplankton in the grazing chambers all resulted in significant, but different extents of colony enlargement in P. globosa in the non-grazing chambers (30–300% larger than the ‘grazer-free’ control). High concentrations of ambient solitary P. globosa cells and other phytoplankton seemed to suppress colony enlargement in P. globosa, and grazers would help reduce this inhibition by removing the ambient solitary P. globosa cells and other phytoplankton. These non-species-specific mechanisms would allow P. globosa to regulate colony size development and defend itself in diverse planktonic systems, which may help to explain the global success of this species.

Grazing and colony size development in Phaeocystis globosa (Prymnesiophyceae): the role of a chemical signal