Phylogenetic Analyses of Three Genes of Pedinomonas noctilucae, the Green Endosymbiont of the Marine Dinoflagellate Noctiluca scintillans, Reveal its Affiliation to the Order Marsupiomonadales (Chlorophyta, Pedinophyceae) under the Reinstated Name Protoeuglena noctilucae

Abstract

In the last decade, field studies in the northern Arabian Sea showed a drastic shift from diatom-dominated phytoplankton blooms to thick and widespread blooms of the green dinoflagellate, Noctiluca scintillans. Unlike the exclusively heterotrophic red form, which occurs widely in tropical to temperate coastal waters, the green Noctiluca contains a large number of endosymbiotic algal cells that can perform photosynthesis. These symbiotic microalgae were first described under the genus Protoeuglena Subrahmanyan and further transferred to Pedinomonas as P. noctilucae Sweeney. In this study, we used the 18S rDNA, rbcL and chloroplast 16S rDNA as gene markers, in combination with the previously reported morphological features, to re-examine the phylogenetic position of this endosymbiotic algal species. Phylogenetic trees inferred from these genes consistently indicated that P. noctilucae is distantly related to the type species of Pedinomonas. The sequences formed a monophyletic clade sister to the clade of Marsupiomonas necessitating the placement of the algal symbionts as an independent genus within the family Marsupiomonadaceae. Based on the phylogenetic affiliation and ecological characteristics of this alga as well as the priority rule of nomenclature, we reinstate the genus Protoeuglena and reclassify the endosymbiont as Protoeuglena noctilucae.

Keywords: Arabian Sea.; Noctiluca; Pedinomonas noctilucae; Protoeuglena; flagellate; molecular phylogeny; symbiosis.