ENCYSTMENT OF THE RED TIDE FLAGELLATE CHATTONELLA-ANTIQUA (RAPHIDOPHYCEAE) - CYST YIELD IN BATCH CULTURES AND CYST FLUX IN THE FIELD

In order to clarify the encystment conditions for the red tide flagellate. Chattonella antiqua, cyst yield in batch cultures under a variety of environmental treatments and cyst flux in natural populations were monitored. In laboratory culture experiments, attempts were made to form 'small cells' (gametes) and cysts under nutrient-replete conditions, but they could not be formed without N- or P-depletion. Once small cells were formed by nutrient depletion, encystment was affected by environmental conditions. Cyst production was highest under continuous darkness and decreased with increasing light intensity. The optimum temperature range for encystment was 21.6 to greater-than-or-equal-to 26.6-degrees-C, broader than that for maximum growth rate. Cyst production increased linearly with increase in motile cell concentration, indicating that the efficiency of encystment was independent of motile cell concentration. Re-addition of nutrients to N- or P-depleted cultures did not affect cyst production. In the field, cyst flux of Chattonella spp. together with environmental variables were monitored throughout the blooming period of C. antiqua in the Scto Inland Sea, Japan. Cysts were formed mainly below a depth of 15 m when nutrients were exhausted in the C. antiqua habitat (0 to 10 m) and the population was decreasing. Based on laboratory culture experiments and field observations, a simple model for encystment in the field was proposed. Following the development of the bloom, nutrients in the habitat of C. antiqua were exhausted and small cells were formed due to N- or P-depletion. Since small cells have a tendency to sink, they descended to the lower layer (> 15 m) where environmental conditions were more favorable for encystment than in the upper layer (i.e. lower irradiance, optimal temperature, and replete nutrients do not affect encystment), and cysts were formed through the fusion of small cells below 15 m. However, the possibility that small cells are formed without nutrient depletion cannot be completely ruled out, so the above model is not conclusive.