The effects of major environmental factors and nutrient limitation on growth and encystment of planktonic dinoflagellate Akashiwo sanguinea

•Optimal growth conditions of Akashiwo sanguinea were confirmed.•The cyst formation of A. sanguinea was studied, with encystment rate of 2.3–21.24% recorded in laboratory.•The effects of nitrate limitation on encystment were more obvious than phosphate and ammonium limitation.•Higher concentrations of ammonium induced low growth rates and abnormal larger cells of A. sanguinea.

The bloom-forming dinoflagellate Akashiwo sanguinea is commonly observed in estuarine and coastal waters around the world. Annually recurrent blooms of this species have been observed in the coastal waters of China, particularly in the Sishili Bay, Yantai since 2004. However, limited studies have been conducted on the recurrence mechanism of A. sanguinea other than periodical monitoring of its population dynamics and associated environmental variables. Thus, to further explore the bloom and succession mechanisms of A. sanguinea in the field, we studied the effects of major nutritional components on the growth and encystment of A. sanguinea, as well as the effects of key environmental factors on the growth of A. sanguinea through a series of laboratory trials. Our results indicated that A. sanguinea was able to grow well within the temperature range of 20–25 °C, salinity range of 20 - 30, with the maximum laboratory irradiance of 78.14 μE m−2 s−1, and was able to survive and grow in low nutrient. However, lower concentrations of nutrients (e.g., nitrate, phosphate) and higher ammonium exerted different degrees of limiting effects on the growth of A. sanguinea, and induced 2.3–21.24% of vegetative cells to form resting cysts simultaneously in laboratory cultures. On the other hand, very limited or no cyst formation was observed in nutrient-replete or extremely low nutrient cultures, indicating the threshold effect of nutritional stress on the encystment of A. sanguinea. The physiological strategy of encystment of A. sanguinea in nutrient-limiting environment facilitates the survival and succession of A. sanguinea species in fluctuating seawaters, and provides seed sources for reoccurring algal blooms under favorable environmental conditions.