Description of the new phototrophic dinoflagellate Alexandrium pohangense sp. nov. from Korean coastal waters

• The phototrophic dinoflagellate Alexandrium pohangense n. sp. was described.
• Its LSU rDNA sequence was 14–17% different from that of the other Alexandrium spp.
• Wide rectangular 1′ plate whose left upper side is bent and touching the 2′ plate.
• Large 2′′′′ plate which area occupies ca. 21–34% of total area of the hypotheca.
• A. pohangense does not have the sxtA gene.

The planktonic phototrophic dinoflagellate Alexandrium pohangense sp. nov. isolated from the coastal waters off Korea is described from living and fixed cells by light and scanning electron microscopy (SEM). DNA sequence data were collected from the small subunit (SSU), the large subunit (LSU), internal transcribed spacer regions (ITS1 and ITS2), and 5.8S of the ribosomal DNA (rDNA). The SSU and LSU rDNA sequences of the new dinoflagellate were 4–7% and 14–17%, respectively, different from those of Alexandrium minutum, Alexandrium ostenfeldii, Alexandrium tamutum, Alexandrium margalefii, and Alexandrium pseudogonyaulax, the most closely related species. In addition, the 5.8S rDNA sequence of the new dinoflagellate was also 12% different from those of A. minutum, A. ostenfeldii, A. tamutum, and Alexandrium peruvianum. In a phylogenetic tree based on LSU rDNA sequences, A. pohangense formed a clade with A. margalefii, and this clade was clearly distinct from the clade of A. minutum, Alexandrium diversaporum, A. tamutum, Alexandrium leei, A. ostenfeldii, and Alexandirum andersoni. Moreover, in a phylogenetic tree based on SSU rDNA sequences, A. pohangense was positioned at the base of the clade containing A. leei and A. diversaporum. Morphological analysis showed that A. pohangense has a Kofoidian plate formula of Po, 4′, 6′′, 6c, 8s, 5′′′, and 2′′′′, which confirmed its assignment to the genus Alexandrium. This dinoflagellate has a wide rectangular 1′ plate, the upper left side of which is slightly bent, protruding, and touching the 2′ plate, unlike A. margalefii, which has a wide rectangular 1′ plate that does not touch the 2′ plate, or A. pseudogonyaulax and Alexandrium camurascutulum, which have a narrower elongated pentagonal 1′ plate that touches the 2′ plate. Furthermore, the 1′ plate of A. pohangense meets the 1′′ plate as a straight vertical line, whereas that of A. camurascutulum meets the 1′′ plate as an inclined line because it is lifted by the intrusion of the 1′′ plate. In addition, A. pohangense had a relatively small ventral pore whose majority was located on the 4′ plate, unlike A. margalefii or A. pseudogonyaulax, which have a relatively large ventral pore whose majority is located on the 1′ plate. Furthermore, A. pohangense had pores of two different sizes on the cell surface, unlike A. margalefii and A. pseudogonyaulax, which have similar pores of only one size. On the basis of morphological and phylogenetic criteria, it is proposed that this is a new species of the genus Alexandrium.