Nanoplankton protists from the western Mediterranean Sea. III. Morphological diversity of Phaeocystis unicells (Prymnesiophyceae = Prymnesiida p.p)

摘要

In phytoplankton samples collected during the spring-summer of 1998 in the Sea of Alboran and Barcelona coastal waters, several populations of Phaeocystis Lagerheim (Prymnesiophyceae = Prymnesiida) were found. Field-emission scanning electron microscopy showed that they belonged to six morphotypes. The flagellate unicell was the most abundant and widespread cell stage. No colonial stages were found, although single coccoid cells were occasionally seen; they had an external scale covering similar to that of the corresponding flagellate unicells. The features of the external scale covering were among the most informative diacritical characters. Morphotype 1 was assigned to Phaeocystis pouchetii sensu Jacobsen, morphotype 2 to Phaeocystis globosa sensu Peperzak et al. (mesoflagellate stage), and morphotype 3 to Phaeocystis scrobiculata. The remaining three differed from all known taxa. Morphotype 4 was the most widespread and abundant. It occurred as two forms of flagellate cells differing in the length of the flagella. Form-1 with shorter flagella was more frequent than form-2 with longer flagella. Single coccoid cells of morphotype 4 were found more rarely than either flagellate form. Morphotype 4 also had papillate small scales on the flagellate and the single coccoid cell stages alike. It is described informally as a species inedita. Morphotype 5 was similar to morphotype 1 owing to the scale morphology, possibly suggesting that it represented an alternative life-history stage. However, its larger scales were larger than those of morphotype 1 while the smaller ones appeared to be smaller, and the flagella and haptonema (when present) were much longer. Only one specimen of morphotype 6 - a flagellate cell with scales in the size range of the large scales of Phaeocystis scrobiculata - was seen. However, it appeared to lack smaller scales and it was unclear whether or not it possessed the characteristic 9-rayed exofilamentous structure of Phaeocystis scrobiculata. The concept of "morphotype" is advocated as a useful one for mapping the natural diversity of Phaeocystis. For any taxonomic revision of Phaeocystis to be successful, population genetics and life-cycle studies on cultured material will need to take the morphotype approach more and more into consideration.