Effects of temperature and salinity on rejuvenation of resting cells and subsequent vegetative growth of the harmful diatom Asteroplanus karianus

In the innermost region of the Ariake Sea, Japan, the diatom Asteroplanus karianus (Grunow) C. Gardner & R.M. Crawford frequently forms massive blooms in winter, causing bleaching of cultured nori ( Neopyropia ) by exhausting seawater nutrients. To understand the bloom-forming mechanism of A . karianus , the effects of temperature and salinity on the rejuvenation of resting cells and the growth of rejuvenated vegetative cells were examined in the laboratory. Surface sediments from the Ariake Sea were cultured at 6.5, 10, 15, 20, 25, 27.5, and 30°C in media with a salinity of 30 to determine the suitable temperatures for rejuvenation and subsequent growth. A suitable temperature range for the rejuvenation and subsequent growth of A . karianus was found to be 10–20°C, and 15°C was revealed to be the optimum temperature. The sediments were then cultured in media with salinities of 3, 7.5, 12, 16.5, 21, 25.5, and 30 at 15°C to determine suitable salinity for rejuvenation and subsequent growth. The suitable salinity range for the rejuvenation was found to be 16.5–30, and growth of the rejuvenated vegetative cells at salinities of 25.5 and 30 was higher than at 16.5 and 21. These results suggest that suitable temperatures and salinities for the rejuvenation and subsequent growth of rejuvenated vegetative cells were 10–20°C and 25.5–30, respectively. This study suggested that A . karianus can bloom only in winter because the low water temperatures in late autumn and winter allowed for active rejuvenation and subsequent growth of A . karianus . • Suitable conditions for Asteroplanus karianus rejuvenation and growth were studied. • Temperatures of 10–20°C and salinities of 25.5–30 promoted rejuvenation and growth. • Optimum temperature for rejuvenation and subsequent growth was 15°C. • Active rejuvenation and subsequent growth at low temperatures lead to winter blooms.