Vitamin B12auxotrophy of the red tide dinoflagellate Heterocapsa rotundataand the effects of feeding on Synechococcusand vitamin B12availability upon phagotrophic activity

ABSTRACTVitamins are an important growth factor for the majority of microalgae. Although the effects of light and prey availability on the growth and feeding of mixotrophic dinoflagellates are relatively well understood, the effects of vitamins are currently poorly understood. We investigated the effects of vitamin conditions on feeding by the red tide-forming dinoflagellate Heterocapsa rotundataSIOHR01 on the cyanobacterium Synechococcussp. CC9311. Furthermore, we explored the growth rates of H. rotundatawith and without added prey under different vitamin conditions and found that H. rotundatais auxotrophic for vitamin B12. Heterocapsa rotundataSIOHR01 was able to ingest Synechococcussp. CC9311 with maximum ingestion and clearance rates, in f/2–Si seawater media, of 0.72 cells predator–1h–1and 28.08 nl predator–1h–1, respectively; these were considerably higher than those in f/2–Si seawater media without B vitamins, which showed a maximum of 0.15 cells predator–1h–1and 2.91 nl predator–1h–1, respectively. However, the mixotrophic growth rate of H. rotundataon Synechococcussp. under each vitamin condition was similar to that of its autotrophic growth rates without added prey under the same vitamin condition. Therefore, this study suggests that while the availability of prey did not enhance the growth of H. rotundata, the concentration of vitamin in the medium affected growth, apparently due to the auxotrophic requirement of the dinoflagellate for vitamin B12.