Morphology, behavior, and phylogenomics of Oxytoxum lohmannii

<b>Abstract</b><br/><p>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 <em>Amphidinium </em>but recently reclassified as <em>Oxytoxum, </em>has been the subject of a number of feeding, growth, and chemosensory studies. This lineage was recently determined to be closely related to <em>Prorocentrum </em>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 <em>Oxytoxum </em>is likely sister to the <em>Prorocentrum </em>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<em> </em>ultrastructure of <em>O. lohmannii</em>,<em> </em>including densely packed trichocysts, and a new type of mucocyst. We observe that <em>O. lohmannii</em> feeds preferentially on cryptophytes using myzocytosis, but can also feed on various phytoflagellates using conventional phagocytosis. <em>O. lohmannii</em> is amenable to culture, providing an opportunity to better study heterotrophic dinoflagellate biology and feeding ecology.</p>