Bulletin of the Plankton Society of Japan
Online ISSN : 2434-0839
Print ISSN : 0387-8961
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Original Paper
  • Masafumi Natsuike, Makoto Kanamori, Akira Sugawara, Setsuko Sakamoto
    2021 Volume 68 Issue 1 Pages 1-9
    Published: February 25, 2021
    Released: March 06, 2021

    Some toxic dinoflagellates, including Alexandrium pacificum, contaminate bivalves with a toxin that causes paralytic shellfish poisoning (PSP). In Hokkaido, Japan, a one-time occurrence of A. pacificum (A. tamarense species complex Group IV) and the associated toxin contamination of cultured scallops was reported in Funka Bay during autumn in 1988. Little evidence of the occurrence of A. pacificum in Hokkaido has been reported since then. However, during the autumn of 2018, we observed cells of A. tamarense species complex collected in Hakodate and Funka Bays in southern Hokkaido by microscopy. A. tamarense species complex is rarely observed in autumn in Hokkaido; thus, we suspected them to be A. pacificum. Toxin profiles and growth responses to water temperature, salinity, and irradiance of A. pacificum unialgal strains isolated from the Hakodate and Funka Bays were examined to evaluate the possible risks of A. pacificum proliferation and PSP toxin contamination of bivalves in the region. Six A. pacificum axenic strains were established by isolating vegetative cells from seawater samples collected from the two bays in October and November 2018. All strains were identified by morphological observation and species-specific PCR for A. pacificum. PSP toxins were detected by HPLC analysis. Cellular toxin quantity was ranged from 10.1–65.8 fmol cell−1, and the dominant toxin in all strains was C2 (70.1–87.3 mol%). Growth rates of four of the six established A. pacificum strains (two isolated from Hakodate Bay and two from Funka Bay) were determined in 20 treatments that combined water temperatures of 5°C, 10°C, 15°C, 20°C, and 25°C and salinities of 20, 25, 30, and 35. Growth of these strains was also examined at different irradiances (10, 40, 70, 100, 150, and 200 µmol photons m−2 s−1). The four strains examined for growth responses grew at water temperatures of 15°C–25°C and salinities of 20–35. Optimal growth (0.50–0.54 divisions day−1) was noted at water temperatures of 20°C–25°C and salinities of 25–35. No growth of any of the four strains occurred at water temperatures of 5°C–10°C. Growth was observed at irradiances of 40–200 µmol photons m−2 s−1, and saturation occurred at 70–100 µmol photons m−2 s−1. Toxin profiles and growth responses to water temperature, salinity, and irradiance were similar to those determined for A. pacificum strains isolated from other areas of Japan. Optimal water temperature and salinity conditions for the growth of A. pacificum occur in Hakodate and Funka Bays during summer and autumn. Therefore, we suggest that the proliferation of the toxin-producing A. pacificum vegetative cells in southern Hokkaido occurs during summer and autumn. This species is unlikely to grow in the region during winter and spring. However, recent increases in seawater temperature in this region might cause changes in the period during which A. pacificum grows.

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Abstracts of Original Papers on Plankton & Benthos Research