The first cells with a nucleus probably arose from a symbiosis between archaea and bacteria. In microbial communities in Australia, researchers have now discovered a previously unknown archaeon that lives in close contact with a bacterium. The interactions provide new insights into how the foundations of complex life may have emerged more than two billion years ago.
Humans, animals, plants and fungi are all made up of cells with a nucleus called eukaryotes. These cells, which form the basis of all complex life, were created around two billion years ago. “Eukaryotes are thought to be the result of a merger of an original archaeal host cell and an endosymbiotic bacterium,” explains a team led by Stephanie-Jane Nobs from the University of New South Wales in Sydney. The mitochondria, the power plants of the cells, developed from the bacteria absorbed into the archaeal cell.
Ancestor of eukaryotes
“What exactly the host cell was was long considered one of the biggest mysteries in evolutionary biology, but more and more evidence suggests that they were so-called Asgard archaea,” explain Nobs and her colleagues. These microbes, first discovered in hydrothermal vents in the deep sea, are most closely related to eukaryotes and already possess many of the genes typical for this cell type that carries nuclei. However, because they are difficult to cultivate in the laboratory, the processes that could have led to the creation of cells with a nucleus have so far been difficult to understand.
But now Nobs and her team have identified a new type of Asgard archaea and, after years of attempts, have successfully cultivated it. They discovered the new species, which they named Nerearchaeum marumarumayae, in microbial mats in Western Australia’s Shark Bay. “Microbial mats are thick, layered biofilms in which different organisms interact and exchange substances to obtain energy and nutrients, developing metabolic interactions that are essential for their survival,” explains the research team. Between 2.5 and 0.5 billion years ago, such mats were widespread and probably formed important centers of evolution. Today they are only found in environments where they are little disturbed by eukaryotes – including the very saline shallow water of Shark Bay.
Symbiosis between archaeon and bacteria
All attempts to cultivate N. marumarumayae in isolation failed. “The fact that we have never been able to bring these organisms into pure culture is probably because they always rely on other organisms to survive,” says Nob’s colleague Brendan Burns. But using high-resolution electron cryotomography, the researchers identified the partner of the newly discovered Asgard archaeon: a bacterium called Stromatodesulfovibrio nilemahensis. Together they were able to cultivate the two microorganisms and examine their interactions in more detail.
As the team discovered using electron microscopic observations and genetic analyses, archaeon and bacterium actively exchange vital substances with each other. N. marumarumayae produces hydrogen, acetate, formate and sulfite while the bacterium synthesizes amino acids and vitamins. These substances are transported back and forth between the two partners by chains of vesicles and tiny nanotubes. “These interactions may reflect an early step in the symbiotic evolution of eukaryotic cells,” write Nobs and her colleagues. Archaeon and bacteria thus provide a model for how complex cells could have developed from simple microbial life forms two billion years ago.
Source: Stephanie-Jane Nobs (University of New South Wales, Sydney, Australia) et al., Current Biology, doi: 10.1016/j.cub.2026.03.041