Morphology, behavior, and phylogenomics of Oxytoxum lohmannii

Dinoflagellates are an abundant and diverse group of protists representing
a wealth of unique biology and ecology. While many dinoflagellates are
photosynthetic or mixotrophic, many taxa are heterotrophs, often with
complex feeding strategies. Compared to their photosynthetic counterparts,
heterotrophic dinoflagellates remain understudied, as they are difficult
to culture. One exception, a long-cultured isolate originally classified
as Amphidinium but recently reclassified as Oxytoxum, has been the subject
of a number of feeding, growth, and chemosensory studies. This lineage was
recently determined to be closely related to Prorocentrum using
phylogenetics of ribosomal RNA gene sequences, but the exact nature of
this relationship remains unresolved. Using transcriptomes sequenced from
culture and three single cells from the environment, we produce a robust
phylogeny of 242 genes, revealing Oxytoxum is likely sister to the
Prorocentrum clade, rather than nested within it. Molecular investigations
uncover evidence of a reduced, non-photosynthetic plastid and
proteorhodopsin, a photoactive proton pump acquired horizontally from
bacteria. We describe the ultrastructure of O. lohmannii, including
densely packed trichocysts, and a new type of mucocyst. We observe that O.
lohmannii feeds preferentially on cryptophytes using myzocytosis, but can
also feed on various phytoflagellates using conventional phagocytosis. O.
lohmannii is amenable to culture, providing an opportunity to better study
heterotrophic dinoflagellate biology and feeding ecology.