Feeding by the heterotrophic nanoflagellate Katablepharis remigera on algal prey and its nationwide distribution in Korea.

Heterotrophic nanoflagellates are ubiquitous in natural waters, and most heterotrophic nanoflagellates are known to grow on bacteria. Recently, the heterotrophic nanoflagellate Katablepharis japonica has been reported to be an effective predator of diverse toxic or harmful algal prey. To date, 7 Katablepharis species have been identified, and therefore important questions arise as to whether other Katablepharis species can feed on algal prey, and further whether the types of prey of other Katablepharis species differ from those of K. japonica. To answer these important questions, feeding by Katablepharis remigera on diverse algal prey was examined. Specific growth and ingestion rates of K. remigera feeding on the raphidophytes Heterosigma akashiwo and Chattonella subsalsa were determined. Furthermore, the abundance of K. remigera at 28 stations along the coastline of Korea from January 2015 to October 2017 was quantified using qPCR method and newly designed specific primer-probe sets. Among 25 potential algal prey tested, K. remigera fed on only H. akashiwo and C. subsalsa; however, it did not feed on a diatom, a prymnesiophyte, a prasinophyte, cryptophytes, dinoflagellates, Mesodinium rubrum, a mixotrophic ciliate, and another raphidophyte Fibrocapsa japonica. The number of prey types on which K. remigera could feed (2 species) was considerably smaller than that of K. japonica (14 species). With the increase in the mean prey concentration, the specific growth rates of K. remigera on H. akashiwo and C. subsalsa increased as well before becoming saturated. The maximum specific growth rates of K. remigera on H. akashiwo and C. subsalsa were 0.717 and 0.129 d-1 , respectively. In addition, the maximum ingestion rates of K. remigera on H. akashiwo and C. subsalsa were 0.333 and 0.661 ng C predator-1  d-1 (3.33 and 0.23 cells predator-1  d-1 ), respectively. The results of this study clearly indicate that K. remigera is an effective predator of 2 red tide-causing raphidophyte species, and additionally, the feeding activity of K. remigera differs greatly from that of K. japonica. The abundance of K. remigera was ≥0.1 cells mL-1 at 24 stations located in the East, West, and South Sea of Korea. Thus, K. remigera has a nationwide distribution in Korea. The highest abundance of K. remigera in Korean waters was 24.9 cells mL-1 in March 2017, when there was no red tide caused by H. akashiwo or Chattonella spp. In most regions where red tides caused by H. akashiwo or Chattonella spp. occurred in 2000-2017, K. remigera was detected. Thus, the abundance of K. remigera may increase during red tides caused by H. akashiwo and Chattonella spp.