Rhodelphis edaphicus sp. nov.—a new lineage of predatory archaeplastids from agricultural soil

Predatory flagellated protists remain extremely poorly studied but often represent the most important deep-branching lineages of eukaryotic evolution. One of the most enigmatic and poorly studied predatory protist groups is Rhodelphidia. These are heterotrophic flagellates, yet belong to the primarily photosynthetic supergroup Archaeplastida and are related to red algae in particular. Here, we describe a new rhodelphid species and the first known soil representative of this group, Rhodelphis edaphicus sp. nov., which was isolated into a clonal culture from agricultural soil. The novel species actively phagocytoses the cells of other protists and bacteria. Using light and electron microscopy, we examined its morphology and identified several novel features, including complex tripartite mastigonemes—not previously reported for this taxon—which link rhodelphids with Cryptista. We expanded a previous 18S rRNA dataset for rhodelphids with environmental sequences and revealed the presence of a soil clade to which the new species belongs. A search of soil metabarcoding data yielded several unknown rhodelphid lineages. Analysis of the distribution of known species and environmental DNA data revealed that rhodelphids inhabit diverse geographic locations worldwide and are found in a variety of habitats, including marine and fresh waters, soils, and, most likely, anaerobic bottom sediments near fumaroles. The diversity of modern rhodelphid habitats, including soil ecosystems, highlights the different environments in which early stages of Archaeplastida evolution may have occurred. The identification and comprehensive study of new lineages of basal archaeplastids provides new insights into the complex evolutionary processes associated with early eukaryotic radiation, the emergence of photosynthesis and plastid evolution that gave rise to the diversification of numerous forms of algae and land plants.