Mixotrophy in the newly described dinoflagellate Yihiella yeosuensis: A small, fast dinoflagellate predator that grows mixotrophically, but not autotrophically.

To investigate tropical roles of the newly described Yihiella yeosuensis (ca. 8μm in cell size), one of the smallest phototrophic dinoflagellates in marine ecosystems, its trophic mode and the types of prey species that Y. yeosuensis can feed upon were explored. Growth and ingestion rates of Y. yeosuensis on its optimal prey, Pyramimonas sp. (Prasinophyceae), as a function of prey concentration were measured. Additionally, growth and ingestion rates of Y. yeosuensis on the other edible prey, Teleaulax sp. (Cryptophyceae), were also determined for a single prey concentration at which both these rates of Y. yeosuensis on Pyramimonas sp. were saturated. Among bacteria and diverse algal prey tested, Y. yeosuensis fed only on small Pyramimonas sp. and Teleaulax sp. (both cell sizes=5.6μm). With increasing mean prey concentrations, both specific growth and ingestion rates of Y. yeosuensis increased rapidly before saturating at a mean Pyramimonas concentration of 109ngCmL-1 (2725cellsmL-1). The maximum growth rate (mixotrophic growth) of Y. yeosuensis fed with Pyramimonas sp. at 20°C under a 14:10-h light-dark cycle of 20μEm-2s-1 was 1.32d-1, whereas the growth rate of Y. yeosuensis without added prey was 0.026d-1. The maximum ingestion rate of Y. yeosuensis fed with Pyramimonas sp. was 0.37ngCpredator-1d-1 (9.3cellspredator-1d-1). At a Teleaulax concentration of 1130ngCmL-1 (66,240cellsmL-1), growth and ingestion rates of Y. yeosuensis fed with Teleaulax sp. were 1.285d-1 and 0.38ngCpredator-1d-1 (22.4cellspredator-1d-1), respectively. Thus, Y. yeosuensis rarely grows without mixotrophy, and mixotrophy supports high growth rates in Y. yeosuensis. Y. yeosuensis has the highest maximum mixotrophic growth rate with the exception of Ansanella graniferaamong engulfment feeding mixotrophic dinoflagellates. However, the high swimming speed of Y. yeosuensis (1572μms-1), almost the highest among phototrophic dinoflagellates, may prevent autotrophic growth. This evidence suggests that Y. yeosuensis may be an effective mixotrophic dinoflagellate predator on Pyramimonas and Teleaulax, and occurs abundantly during or after blooms of these two prey species.