Germination characteristics of naturally occurring cysts of Alexandrium tamarense (Dinophyceae) in Hiroshima Bay, Inland Sea of Japan

Abstract

In order to examine temporal changes in the germination ability, time to germination and autofluorescence properties of the resting cysts of the toxic dinoflagellate Alexandrium tamarense, a long-term investigation was conducted in Hiroshima Bay. In Hiroshima Bay, a spring bloom (March to May) of A. tamarense has been observed almost every year since 1992. Approximately 50 resting cysts were isolated monthly from the bottom sediment between June 1994 and June 1997. The cysts were incubated from the day of sampling onwards under in situ bottom water temperature conditions, and germination success and the emission of autofluorescence were checked every day. High germination success rates (>50%) were observed between December and April each year (bottom water temperature = 10.0–16.5°C), with an average germination time of 10.2 days (n = 455). Resting cysts were found to start to emit red autofluorescence a few days before germination (average duration = 3.1 days, n = 449), and germination time was nearly constant within the temperature range 10–20°C. From June to November, germination success rates were considerably lower (0–40%, bottom water temperature = 14.6–23.9°C). No germination at all was observed in September (bottom temperature = 23.6–23.9°C). The relationship between the incubation temperature and the rate of germination success indicates that the resting cysts have a temperature ‘window’ (c. 10–15°C) for germination, which controls the seasonal change in germination ability. The present results indicate that the germination characteristics of A. tamarense resting cysts are well adapted to the ambient water temperature rhythm in temperate shallow coastal environments, allowing A. tamarense to seed vegetative cell populations for the spring bloom.