Mixotrophy in the sand-dwelling dinoflagellate Thecadinium kofoidii

ABSTRACT

Thecadinium kofoidii is 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 of T. kofoidii by examining its protoplasm under light and transmission electron microscopes with diverse algal prey species. Furthermore, the feeding mechanism of T. kofoidii and prey species on which it feeds were investigated. In addition, the growth and ingestion rates of T. kofoidii as a function of prey concentration were determined when feeding on the benthic cryptophyte Rhodomonas salina. Thecadinium kofoidii was able to feed on R. salina and the dinoflagellate Symbiodinium voratum, which had equivalent spherical diameters (ESDs) ≤ 10.1 µm, while it did not feed on the benthic dinoflagellates Levanderina fissa, Prorocentrum concavum or Ostreopsis cf. ovata, which had ESDs ≥ 15 µm. Thecadinium kofoidii fed on the edible prey cells using the peduncle. The maximum ingestion rate of T. kofoidii on R. salina was 1.3 cells predator⁻¹ d⁻¹. However, feeding on R. salina did not significantly increase the growth rate of T. kofoidii. The low ingestion rate of T. kofoidii on R. salina may have partially resulted in the lack of significant increase in its growth rate due to mixotrophy. The present study discovered predator–prey relationships between T. kofoidii and R. salina and S. voratum, which may change our view of the energy flow and carbon cycling in marine benthic food webs.

SUBJECT EDITOR:

• Antje Boetius

KEYWORDS:

• Benthic
• growth
• ingestion
• bloom
• protist

Acknowledgement

We thank Keon Gang Jang for technical support.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This paper was supported by National Research Foundation (NRF) grants funded by the Korean government (MSIP) (NRF-2015-M1A5A1041808 and NRF-2016-R1C1B1007340) awarded to YD Yoo and the ‘Useful dinoflagellates’ Program of KIMST (Korea Institute of Marine Science and Technology) awarded to HJ Jeong and HS Kim.