Five phototrophic <italic>Scrippsiella</italic> species lacking mixotrophic ability and the extended prey spectrum of <italic>Scrippsiella acuminata</italic> (Thoracosphaerales, Dinophyceae)

Abstract

Mixotrophic dinoflagellates act as primary producers, prey, and predators in marine planktonic food webs, whereas exclusively autotrophic dinoflagellates are primary producers and prey. Species of the dinoflagellate genus <i>Scrippsiella</i> are commonly found in marine ecosystems and sometimes cause harmful red tides. Among the 28 formally described <i>Scrippsiella</i> species, <i>S. acuminata</i> has been found to be mixotrophic and two unidentified species have been found to be mixotrophic. To determine whether the other species in this genus are similarly mixotrophic, the mixotrophic ability of <i>S. donghaiensis</i> SDGJ1703, <i>S. lachrymosa</i> SLBS1703, <i>S. masanensis</i> SSMS0908, <i>S. plana</i> SSSH1009A, and <i>S. ramonii</i> VGO1053 was explored using 15 potential prey items, including 2-µm fluorescently labeled microspheres (FLM) and heterotrophic bacteria (FLB), the cyanobacterium <i>Synechococcus</i> sp., and various microalgal prey species. The ability of <i>S. acuminata</i> to feed on FLM and FLB was also investigated. We found that <i>S. donghaiensis</i>, <i>S. lachrymosa</i>, <i>S. masanensis</i>, <i>S. plana</i>, and <i>S. ramonii</i> did not feed on any potential prey tested in this study, indicating a lack of mixotrophy. However, <i>S. acuminata</i> fed on both FLM and FLB, confirming its mixotrophic ability. These results lowered the proportion of mixotrophic species relative to the total number of tested <i>Scrippsiella</i> species for mixotrophy from 100% to 29–38%. Owing to its mixotrophic ability, <i>S. acuminata</i> occupies an ecological niche that is distinct from that of <i>S. donghaiensis</i>, <i>S. lachrymosa</i>, <i>S. masanensis</i>, <i>S. plana</i>, and <i>S. ramonii</i>.