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

Abstract

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 >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 (Helgoländer 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.