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Flagellates of  Phaeocystis globosa  were observed inside colonies in North Sea samples in 1992. Field data suggest that low phosphate concentrations (≤0.5 μM) and, as a consequence of sedimentation, a 20- to &gt;100-fold reduction of daily irradiance (to &lt;10 W h m –2  day –1 ), triggered the transition of non-flagellate colonial cells to flagellate cells. The colonial flagellates appeared to be similar to cultured solitary  Phaeocystis  microflagellates; these flagellates do not produce pentagonal stars. Nevertheless, pentagonal stars were observed inside the colonies. This implies that two types of flagellates, ‘microflagellates’ and the larger pentagonal star-producing ‘mesoflagellates’, are formed simultaneously in  Phaeocystis  colonies. Therefore, this observation fills a missing link in the life cycle of  Phaeocystis . It is proposed that micro- and mesoflagellates are the ‘Mikrozoosporen’ described by Kornmann ( Helgol ä nder Wiss. Meeresunteruchungen  1955,  5 , 218–233) and that these flagellates are comparable to the small and large ‘zoids’ described by Parke  et al.  ( J. Mar. Biol. Assoc. UK  1971,  51 , 927–941). Thus, four different cell types can be distinguished in the life cycle of  P.globosa : (i) the haploid microflagellate (3.1 ± 0.1 μm), (ii) the haploid mesoflagellate (4.2 ± 0.1 μm), (iii) the diploid macroflagellate (6–7 μm) which is Kornmann's ( Kornmann, 1955 ) ‘Makrozoospore’, and (iv) the diploid non-flagellate colonial cell (6.1 ± 0.2 μm). A life cycle for  Phaeocystis  is presented, and the life-cycle transitions still to be tested are identified.

Observations of flagellates in colonies of Phaeocystis globosa (Prymnesiophyceae); a hypothesis for their position in the life cycle