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

Flagellates of Phaeocystis globosa were observed inside colonies in North Sea samples in 1992. Field data suggest that low phosphate concentrations (less than or equal to0.5 muM) and, as a consequence of sedimentation, a 20- to >100-fold reduction of daily irradiance (to <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 (Helgolander 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 m), (ii) the haploid mesoflagellate (4.2 +/- 0.1 mum), (iii) the diploid macroflagellate (6-7 mum) which is Kornmann's (Kornmann, 1955) 'Makrozoospore', and (iv) the diploid non-flagellate colonial cell (6.1 +/- 0.2 mum). A life cycle for Phaeocystis is presented, and the life-cycle transitions still to be tested are identified.