A research team from the Faculty of Science, Charles University in Prague has discovered Solarion arienae, an extremely rare and morphologically unique unicellular eukaryote that sheds new light on early eukaryotic evolution. The study, published in Nature, reveals that Solarion represents a newly defined phylum and, together with a few enigmatic protist lineages, forms an entirely new eukaryotic supergroup (kingdom) named Disparia. Remarkably, Solarion preserves ancestral mitochondrial features previously thought to have long disappeared, offering an unprecedented insight into the biology of early eukaryotic cells.
The microscopic organism Solarion arienae – inconspicuous, barely motile, and only a few micrometers in size – was found by chance in a long-term laboratory culture of marine ciliates at the Department of Zoology, Faculty of Science, Charles University. Because of its minute size, it remained overlooked for years until the larger ciliates in the culture unexpectedly died. This fortunate accident brought Solarion into view, and its discovery proved to be scientifically groundbreaking.
Through genomic sequencing and extensive phylogenomic analyses, scientists from Charles University and their collaborators in the USA demonstrated that Solarion does not belong to any previously recognized major lineage of eukaryotes. Only one close relative is known so far – the peculiar protist Meteora sporadica. Together they form a new phylum, Caelestes, which, along with the protist phyla Provora and Hemimastigophora, constitutes the newly defined eukaryotic supergroup (kingdom) Disparia. Although only a few species of Disparia are currently known, this represents an ancient evolutionary lineage whose surviving members may be relics of a once much greater diversity.
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 collected in different parts of the world. This suggests that Solarion represents a lineage that is globally distributed but everywhere extremely rare. Its discovery underscores the importance of cultivation-based methods in protist research and the need to explore poorly studied environments.
Solarion also attracts attention with its distinctive appearance – if its tiny size allows it to be observed at all. Most often it occurs in immobile “sun-like” cells, from which stalked organelles called celestiosomes radiate in all directions. These unique structures serve to capture bacterial prey and were visualized in detail using 3D electron microscopy. In its life cycle, Solarion also produces rarer flagellated cells with a striking morphology that cannot be confused with any other known eukaryotic organism.
Perhaps the most remarkable aspect of Solarion lies in its mitochondria, the energy-producing organelles that originated from an ancient bacterial ancestor. Solarion has retained the gene secA, a rare remnant of a protein translocation system inherited from the alphaproteobacterial progenitor of mitochondria. This gene has disappeared from nearly all modern eukaryotes, making Solarion one of the few living eukaryotic organisms that still possess this ancestral molecular toolkit. Its mitochondria also contain an exceptional combination of genes that provide a unique opportunity to reconstruct the metabolic capacities of the mitochondria of the last common ancestor of eukaryotes.
Prof. Ivan Čepička and Marek Valt, the main authors of the study, emphasize the significance of the discovery: “Solarion is a remarkable reminder of how little we still know about the diversity of microbial life. 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 explore. This organism allows us to look into a very ancient chapter of cellular evolution that we previously could reconstruct only indirectly.”
Reference:
Valt, M., Pánek, T., Mirzoyan, S., Tice, A. K., Jones, R. E., Dohnálek, V., Doležal, P., Mikšátko, J., Rotterová, J., Hrubá, P., Brown, M. W., & Čepička, I. (2025). Rare microbial relict sheds light on an ancient eukaryotic supergroup. Nature. https://doi.org/10.1038/s41586-025-09750-0