Mixotrophy in the sand-dwelling dinoflagellate Thecadinium kofoidii.

Thecadinium kofoidiiis a marine sand-dwelling dinoflagellate that sometimes forms dense blooms. This species was previously thought to be an exclusively autotrophic dinoflagellate, and its mixotrophic ability has not been explored yet. By investigating its ecophysiology, its trophic mode should be revealed. We explored the mixotrophic ability ofT. kofoidiiby examining its protoplasm under light and transmission electron microscopes with diverse algal prey species. Furthermore, the feeding mechanism ofT. kofoidiiand prey species on which it feeds were investigated. In addition, the growth and ingestion rates ofT. kofoidiias a function of prey concentration were determined when feeding on the benthic cryptophyteRhodomonas salina.Thecadinium kofoidiiwas able to feed onR. salinaand the dinoflagellateSymbiodinium voratum, which had equivalent spherical diameters (ESDs) ≤ 10.1 µm, while it did not feed on the benthic dinoflagellatesLevanderina fissa,Prorocentrum concavumorOstreopsiscf.ovata, which had ESDs ≥ 15 µm.Thecadinium kofoidiifed on the edible prey cells using the peduncle. The maximum ingestion rate ofT. kofoidiionR. salinawas 1.3 cells predator−1d−1. However, feeding onR. salinadid not significantly increase the growth rate ofT. kofoidii. The low ingestion rate ofT. kofoidiionR. salinamay have partially resulted in the lack of significant increase in its growth rate due to mixotrophy. The present study discovered predator–prey relationships betweenT. kofoidiiandR. salinaandS. voratum, which may change our view of the energy flow and carbon cycling in marine benthic food webs. [ABSTRACT FROM PUBLISHER]