Morphological and genetic characterization of Phaeocystis cordata and P-jahnii (Prymnesiophyceae), two new species from the Mediterranean Sea

Two new Phaeocystis species recently discovered in the Mediterranean Sea ar e described using light and electron microscopy, and their systematic posit ion is discussed on the basis of an analysis of their nuclear-encoded small -subunit ribosomal RNA gene (SSU rRNA) sequences. Phaeocystis cordata Zingo ne et Chretiennot-Dinet was observed only as flagellated unicells. Cells ar e heart shaped, with two flagella of slightly unequal length and a short ha ptonema. The cell body is covered with two layers of thin scales. The outer most layer scales are oval, with a faint radiating pattern, a raised rim, a nd a modest central knob. The inner-layer scales are smaller and have a fai nt radiate pattern and an inflexed rim. Cells swim with their flagella clos e together, obscuring the haptonema, pushing the cell, and causing it to ro tate about its longitudinal axis while moving forward. Phaeocystis jahnii Z ingone was isolated as a nonmotile colony. It forms loose aggregates of cel ls embedded in a mucilaginous, presumably polysaccharide matrix without a d efinite shape or visible external envelope. The flagellated stage has the f eatures typical of other Phaeocystis species. Cells are rounded in shape an d slightly larger than P. cordata. The cell body is covered with extremely thin scales of two different sizes with a very faint radiating pattern towa rd their margin. Swimming behavior is similar to that of P. cordata, with t he flagella in a posterior position as the cells swim. The SSU rRNA sequenc e analysis indicated that both species are distinct from other cultivated P haeocystis species sequenced to date. Regions previously identified as spec ific for the genus Phaeocystis are not found in P. jahnii, and new genus-sp ecific regions have been identified. P. cordata is more closely related to the colonial species P. globosa, P. antarctica, and P. pouchetii and has br anched prior to the divergence of the warm-water P. globosa species complex from the cold-water species P. antarctica and P. pouchetii. These results are discussed within a framework of the available data on the evolution of the world's oceans.