The Studies Of Molecular Taxonomy Of Redtide-Caused Phaeocystis (Prymnesiophyte)

The genus Phaeocystis is one of key taxa in studying redtide-caused marine microalgae. The uniqueness of the genus Phaeocystis rests not only in the fabulous biomass reached by its blooms but also in its exceptional physiology and ecology. Due to its complex heteromorphic life cycle, great geographic differences, and minuteness of polymorphic motile cells, the taxonomy of Phaeocystis is extremely difficult and controversial. In order to determine the strains occurred in the coastal waters of southeast China, we studied morphological features and sequences of 18S rDNA and rDNA ITS. of three assumed Phaeocystis strains HK2,ST2 and ST3.Optical microscope observation showed that the species are difficult to be classified clearly, if not possible, based on polymorphic flagellated cells, colony-forming and colony morphology. 18S rDNA analyses indicated that the assumed Phaeocystis strains ST2 and ST3 isolated from Santou coastal waters are not Phaeocystis species. Compared to sequences from GenBank, ST3 and ST2 are closely related to Skeletonema and Isochrysis respectively. Pairwise divergence of 18S rDNA sequences of strain ST3 and two strains of Skeletonema costatum and S.Pseudocostatum are 0.055 and 0.053 respectively, and genetic distance of 18S rDNA sequences between strain ST2 and Isochrysis galvanaUIO102 is only 0.0016. Based on the phylogenetic analyses, 14 strains of Phaeocystis are clustered into 5 clades: P.PLY559-.Rjahnii, P.globosa, P.antarctica and P.pouchetii, P.cordata, of which P.pouchetii and P.antarctica cannot be separated, Hong Kong Strain HK2 is placed within P.globosa clade.Phylogenetic analysis of 25 rDNA ITS1 sequences and 12 rDNA ITS sequences showed that the colony-forming species could be classified as P.globosa, P.antarctica and P.pouchetii, Hong Kong Strain HK2 is P.globosa.18S rDNA is much more conserved than rDNA ITS does among Phaeocystis species. rDNA ITS exhibited substantial inter- and intra-specific sequence divergence and provided much finer resolution among Phaeocystis species. Hong Kong strain of Phaeocystis is P.globosa, but our results can't support the opinion that it is endemic and unique. As misidentification of redtide-caused microalgae is frequent, moleculartechniques will play an important role in redtide bloom related researches.