First report on okadaic acid production of a benthic dinoflagellate Prorocentrum cf. fukuyoi from Japan

The consumption of bivalves contaminated with diarrhetic shellfish toxins (DSTs) such as okadaic acid and dinophysistoxins has caused serious economic hardship to shellfish industries worldwide including Japan. Several benthic species of the genus Prorocentrum have been confirmed to produce DSTs. There have been no published studies of Japanese strains of Prorocentrum species that include a three‐way characterization of morphological characteristics, molecular phylogenetic data, and toxin profiles. In the present study, a benthic Prorocentrum strain isolated from Japanese temperate shallow water (depths < 3 m) was characterized using morphological, phylogenetic, and toxicological methods. Cell size of the Japanese Prorocentrum strain KSK4P was 27.2 ± 1.3 μm in length, 22.4 ± 0.9 μm in width, and had a length‐to‐width ratio of 1.22 ± 0.04. The thecal surface was smooth and covered with large and small thecal pores lying in rows, and the pores were lacking in the central area of the theca. The right theca had a deeply excavated periflagellar area that was narrow V‐shaped. Nine platelets and two wings were in the periflagellar area. These morphological features of the strain KSK4P are similar to those of P. fukuyoi. Phylogenetic trees based on the LSU rDNA D1/D3 sequences revealed that the strain KSK4P forms a new subclade F2f belonging to P. fukuyoi complex (subclade F2). The sequence analysis revealed that the strain KSK4P (subclade F2f) diverges from a subclade F2e to which the strain used for the description of P. fukuyoi belongs. Based on the results of morphological and phylogenetic analyses of the strain KSK4P, the strain was designated as P. cf. fukuyoi. The results of the LC/MS/MS analysis revealed that the strain KSK4P produced okadaic acid (1.5–2.7 fg cell⁻¹). This is the first report that a strain belonging to the P. fukuyoi complex can produce okadaic acid.