Unveiling the hidden genetic diversity and chloroplast type of marine benthic ciliate Mesodinium species

Ciliate Mesodinium species are commonly distributed in diverse aquatic systems worldwide. Among Mesodinium species, M. rubrum is closely associated with microbial food webs and red tide formation and is known to acquire chloroplasts from its cryptophyte prey for use in photosynthesis. For these reasons, Mesodinium has long received much attention in terms of ecophysiology and chloroplast evolution. Mesodinium cells are easily identifiable from other organisms owing to their unique morphology comprising two hemispheres, but a clear distinction among species is difficult under a microscope. Recent taxonomic studies of Mesodinium have been conducted largely in parallel with molecular sequence analysis, and the results have shown that the best-known planktonic M. rubrum in fact comprises eight genetic clades of a M. rubrum/M. major complex. However, unlike the planktonic Mesodinium species, little is known of the genetic diversity of benthic Mesodinium species, and to our knowledge, the present study is the first to explore this. A total of ten genetic clades, including two clades composed of M. chamaeleon and M. coatsi, were found in marine sandy sediments, eight of which were clades newly discovered through this study. We report the updated phylogenetic relationship within the genus Mesodinium comprising heterotrophic/mixotrophic as well as planktonic/benthic species. Furthermore, we unveiled the wide variety of chloroplasts of benthic Mesodinium, which were related to the green cryptophyte Chroomonas/Hemiselmis and the red cryptophyte Rhodomonas/Storeatula/Teleaulax groups.