Effects of temperature, salinity, and photosynthetic photon flux density on the growth of the harmful diatom Asteroplanus karianus in the Ariake Sea, Japan

Abstract

Winter blooms of the diatom Asteroplanus karianus cause bleaching of cultivated Pyropia. We examined the effects of temperature, salinity, and photosynthetic photon flux density (PPFD) on the growth of A.karianus, using culture strains from the Ariake Sea. The maximum growth rate occurred at 20 °C and a salinity of 20, and growth rates at 15–25 °C and salinities of 15–33 were >70 % of this maximum. However, A. karianus could not survive at 30 °C. The threshold PPFD for growth was approximately 1 μmol photons m−2 s−1; this value was lower than that for other coastal diatoms. The half-saturation PPFD was 41.3 μmol photons m−2 s−1 at 15 °C and 30.5 μmol photons m−2 s−1 at 25 °C. The photosynthetic activity (Fv/Fm) of A. karianus was saturated at 50 μmol photons m−2 s−1, which is similar to or lower than surface PPFDs on cloudy days. We found that A. karianus is eurythermal and euryhaline and can photosynthesize and grow in very low light but not at high temperature. These physiological characteristics may partly explain why this diatom blooms in winter, when temperature and PPFD are at their lowest during the year in the Ariake Sea.