MORPHOLOGY, PLOIDY, PIGMENT COMPOSITION, AND GENOME SIZE OF CULTURED STRAINS OF PHAEOCYSTIS (PRYMNESIOPHYCEAE)

We examined cell morphology, ploidy level, cell size, pigment composition, and genome size in 16 cultured strains of Phaeocystis Lagerheim. Two strains originated from the Antarctic, 3 from the tropical Western Atlantic, and 11 from temperate regions (Eastern Atlantic, English Channel, North Sea, and Mediterranean Sea). Thirteen strains made colonies morphologically similar to P. globosa Scherffel, whereas three never formed colonies under any circumstances. Five-rayed star-like structures with filaments were observed in 11 strains. In several strains, two ploidy levels were observed, one (haploid) linked to flagellates and one (diploid) linked to colonies. Cell size did not appear to be a very good criterion for distinguishing strains since size distributions overlapped. Pigment analysis by reversed-phase-high-performance liquid chromatography allowed the strains to be grouped into three clusters that differed from each other mainly by the relative proportions of three carotenoids: fucoxanthin, 19'-hexanoyloxyfucoxanthin, and diadinoxanthin. All strains contained low levels of 19'-butanoyloxyfucoxanthin. Differences in genome size measured by flow cytometry delimited at least five groups. On the basis of bath pigment composition and genome size, six clusters were defined, one corresponding to an Antarctic species (possibly P. antarctica), one to P. globosa, and the rest probably to several yet-undescribed species or subspecies. Two main conclusions emerge from this study. First, the taxonomy of the genus Phaeocystis needs to be clarified through a combination of morphological, biochemical, and molecular studies. Second, sexuality is a prevalent phenomenon in Phaeocystis, but controls of the sexual cycle are most likely strain-dependent,