19.11.2025
A research team from the Faculty of Science at Charles University, led by Professor Ivan Čepička and his doctoral student Marek Valt, has discovered a rare single-celled organism with a unique cellular structure, named Solarion arienae. In a study published in the prestigious journal Nature, the scientists show that this species, along with several other little-known protist lineages, forms a new eukaryotic supergroup (kingdom) called Disparia. Solarion retains features previously thought to have been lost long ago, providing an unprecedented insight into the biology of early eukaryotic cells.
Black-and-white illustration of a cross-section of a Solarion cell
The microscopic protist Solarion arienae — inconspicuous, barely mobile, and only a few micrometers in size — was discovered in a laboratory culture of marine ciliates at the Department of Zoology, Faculty of Science, Charles University. Due to its tiny size, it was overlooked for a long time until the larger ciliates in the culture died. This fortunate coincidence revealed Solarion, and its unexpected discovery proved to be scientifically groundbreaking.
Scientists from Charles University and collaborators in the United States used genome sequencing and extensive phylogenomic analyses to determine that Solarion does not belong to any previously discovered major eukaryotic lineage. Only a single close relative is known, the unusual protist Meteora sporadica. Together they form a new phylum, Caelestes, which, along with protists from the phyla Provora and Hemimastigophora, constitutes a new eukaryotic supergroup named Disparia. Although only a few Disparia species are known today, it is an ancient evolutionary lineage, with current representatives potentially being relics of long-extinct diversity.
Illustration of the eukaryotic tree of life, with the new supergroup Disparia in turquoise
Solarion is almost absent from global environmental DNA databases, despite searches through more than 1.8 petabytes of metagenomic data. Nevertheless, it has been detected several times in sequence data from marine sediment samples from different parts of the world. This suggests it is an evolutionary lineage that is globally distributed but extremely rare everywhere. The discovery highlights the importance of cultivation-based methods in studying microbial eukaryotes and the need to explore previously little-studied environments.
Solarion is striking due to its unique appearance, if observed given its tiny size. It most often appears as immobile “sun-shaped” cells with stalked celestiosomes protruding in all directions. These unique organelles capture bacterial prey, and their structure was visualized in detail using 3D electron microscopy. During its life cycle, Solarion also produces much rarer motile flagellated cells, which have a distinct appearance and cannot be confused with any other eukaryotic organism.
The most remarkable feature of Solarion is its mitochondria — cell organelles derived from the ancient engulfment of a prokaryotic ancestor that generate energy for the cell. Solarion retains the secA gene, a rare remnant of an ancient protein transport system inherited from the bacterial ancestor of mitochondria. This gene has been lost in almost all modern eukaryotes, making Solarion one of the few living eukaryotic organisms that still retain this original molecular machinery. Its mitochondria also contain a unique combination of genes, providing a rare opportunity to reconstruct the metabolic capacities of the mitochondria of the last common ancestor of eukaryotes.
Structure of the SecA protein from Solarion arienae
The lead authors of the study, Professor Ivan Čepička and Mgr. Marek Valt of Charles University’s Faculty of Science, emphasize the significance of the discovery: “Solarion is a remarkable reminder of how little we still know about microbial diversity. The discovery of such an evolutionarily deep lineage — essentially a living fossil — shows that key parts of the eukaryotic story remain hidden in places we rarely study. This organism allows us to glimpse a very ancient chapter of cellular evolution that we previously could only reconstruct indirectly.”
