THE ECOPHYSIOLOGY OF PHAEOCYSTIS-GLOBOSA - A REVIEW

A straightforward ecophysiological characterization of Phaeocystis glo bosa is hampered by its complex polymorphic life cycle in which flagel lates and colonial cells express different physiological and morpholog ical properties. There is also increasing evidence that, besides the e xistence of different species, the most widespread species Phaeocystis globosa (Scherffel) has to be subdivided into at least five different ecotypes which again differ in their ecophysiological properties. Mos t research has been performed on the P. globosa ecotype North European (English Channel/North Sea isolates). From the available literature i t must be concluded that flagellate cells are better competitors for l ight and phosphate than colonial cells, due to their superior uptake c haracteristics. To a limited extent this phenomenon is compensated in colonial cells by their ability to continue growth and cell division i n the dark at the same rate as in the light, at the expense of extrace llular colonial mucus. In contrast with other algal species, colonial cells of P. globosa are better competitors for nitrogen than for phosp horus. Flagellates transform into vegetative cells and form colonies i n environments with irradiance levels of about 50 mu E . m(-2). s(-1) or more and an optimum phosphate concentration of 1 mu M. A solid subs trate and the presence of calcium are prerequisites for colony formati on. In environments where phosphorus is limiting no new colonies are f ormed. There is some evidence that nitrate stimulates colony formation , whereas high ammonium values (above 1 mu M) tend to suppress colony formation. Massive blooms of P. globosa colonies can be attributed to a combination of environmental conditions that induce colony formation and smaller grazing losses of colonial cells than of flagellates, rat her than to superior ecophysiological characteristics of colonial cell s.