The effects of temperature, growth medium and darkness on excystment and growth of the toxic dinoflagellate <i>Gymnodinium catenatum</i> from northwest Spain

Abstract

The chain-forming dinoflagellate Gynmodinium catenatum Graham causes recurrent outbreaks of paralytic shellfish poisoning (PSP) in the Galician Rias Bajas (northwest Spain). A sediment survey in Ria de Vigo in April 1986 indicated that the highest concentrations of cysts of this species were located in the middle sections of the ria, with maximum abundance of 310 cysts cm−3. The effects of temperature, growth medium composition and irradiance on the germination of laboratory-produced resting cysts were investigated. Newly formed cysts required very little time for maturation, as excystment was possible within 2 weeks of encystment. Growth media did not affect germination success. In contrast, the excystment rate was retarded signifiantly in darkness. Germination was also strongly affected by temperature, with ˜75% excystment success at 22–28°C and little or no germination below 11°C after 1 month of incubation. In culture, the optimum growth rate of vegetative cells was between 22 and 28°C, the highest rate being 0.53 divisions day−1 at 24°C. Growth did not occur at temperatures < 11°C or >30°C. These results are important with respect to the different hypotheses proposed to explain the initiation of G.catenatum blooms in the Galician Rias Bajas and Northern Portugal. The pattern of G.catenatum bloom development along this coast has been related to seasonal upwelling in the area, with major blooms occurring during the autumn as warmer offshore surface water is transported towards the coast when upwelling relaxes. The landward transport of established offshore populations of G.catenatum with the warm surface layer remains a viable explanation for the observed blooms within the rias, but alternatively, our data suggest that cysts within the rias can provide the inoculum population at times conducive to growth and bloom formation. Even though newly formed G.catenatum cysts have a very short maturation time and can germinate in darkness across a wide temperature range, bloom development will be significant only during the late summer and early autumn, since in other months light levels at the sediment surface and temperatures throughout the water column are too low for significant germination or growth.