Fluorescence in situ hybridization using rRNA-targeted probes for simple and rapid identification of the toxic dinoflagellates Alexandrium tamarense and Alexandrium catenella

The toxic marine dinoflagellates Alexandrium tamarense (Lebor) Balech and A. catenella (Whedon and Kofoid) Taylor have been mainly responsible for paralytic shellfish poisoning in Japan. Rapid and precise identification of these algae has been difficult because this genus contains many morphologically similar toxic and nontoxic species. Here, we report a rapid, precise, and quantitative identification method using three fluorescent, rRNA-targeted, oligonucleotide probes for A. tamarense (Atm1), A. catenella (Act1), and the nontoxic A. affine (Inoue et Fukuyo; Aaf1). Each probe was species specific when applied using fluorescence in situ hybridization (FISH). None of the probes reacted with three other Alexandrium spp., A. lusitanicum Balech, A. ostenfeldii (Paulsen) Balech & Tangen, and A. insuetum Balech, or with eight other microalgae, including Gymnodinium mikimotoi Miyake et Kominami ex Oda and Heterosigma akashiwo (Hada) Hara et Chihara, suggesting that the species specificity of each probe was very high. Cells labeled with fluorescein 5-isothiocyanate-conjugated probes showed strong green fluorescence throughout the whole cell except for the nucleus. FISH could be completed within 1 h and largely eliminated the need for identifying species based on key morphological criteria. More than 80% of targeted cells of both species could be identified by microscopy and quantified during growth up to the early stationary phase; more than 70% of cells could be detected in the late stationary phase. The established FISH protocol was found to be a specific, rapid, precise, and quantitative method that might prove to be a useful tool to distinguish and quantify Alexandrium cells collected from Japanese coastal waters.