Tiny 'Sun-Like' Microbe, Solarion arienae, May Represent a Whole New Branch of Eukaryotic Life

Biologists have identified an extremely small, previously overlooked microbe that could add an entirely new branch to the eukaryotic tree of life. The organism, named Solarion arienae, was revealed only after a long-term laboratory culture of marine ciliates—collected from Croatian coastal waters in 2011—collapsed and the tiny cells became visible.

Lead researchers Ivan Čepička and Marek Valt of Charles University describe the discovery as a rare opportunity to examine an ancient chapter of cellular evolution directly. The cells are very small and only weakly motile, which explains why they went unnoticed in the mixed ciliate culture for years. The team cautions that Solarion would likely remain hidden in routine environmental samples because of its subtle appearance and behavior.

“This organism allows us to look into a very ancient chapter of cellular evolution that we previously could reconstruct only indirectly,” the researchers say.

Structurally, Solarion is a single-celled eukaryote with a membrane-bound nucleus and mitochondria—the organelles that convert fats and carbohydrates into chemical energy. Genetic analyses show Solarion does not fit into any currently defined eukaryotic group. The authors place it in a newly defined phylum alongside the unusual protist Meteora sporadica, and that phylum is nested within a newly proposed kingdom that also includes the distantly related protists Provora and Hemimastigophora.

Most strikingly, Solarion's mitochondria retain an unusual gene, secA, which is typically associated with bacterial protein-transport machinery. Scientists widely accept that mitochondria evolved from free-living bacteria that entered into symbiosis with a host cell; modern mitochondrial genomes usually retain only a greatly reduced remnant of that ancestry. The presence of secA in Solarion's mitochondria offers a rare, more direct molecular glimpse of the ancestral state and how an ancestral symbiont may once have functioned.

That retained genetic trace strengthens support for the endosymbiont theory and gives researchers new material to reconstruct how early eukaryotic lineages merged and evolved. The find also underscores how much microbial diversity remains undiscovered, even in samples scientists have studied for years.

“Solarion is a remarkable reminder of how little we still know about the diversity of microbial life,” Čepička and Valt say. “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.”

The study describing Solarion arienae has been published in the journal Nature.