Capturing our closest archaeal relatives offers clues to the origin of eukaryotes

A research team from Japan's Agency for Marine-Earth Science and Technology, in collaboration with Kyoto University, has isolated two strains of Heimdallarchaeia, a group of archaea closely related to eukaryotes. These findings, published in Nature, shed light on the enigmatic origins of complex eukaryotic cells. The leading hypothesis suggests that eukaryotic cells emerged when an archaeal cell partnered with a bacterium, which eventually became the mitochondrion.

In 2020, the team first cultivated a member of this group, strain MK-D1, from deep-sea sediment. By successfully cultivating and examining two new strains of Heimdallarchaeia, the researchers found that these simple archaea lacked membrane-bound organelles like a nucleus. Instead, they possessed complex surface structures with multiple long, branching membrane protrusions.

These protrusions enabled the archaea to attach to other microorganisms and form symbiotic partnerships. Although anaerobic (oxygen-independent), one strain could tolerate small amounts of oxygen when aerobic bacteria were present. These findings support the idea that the archaeal ancestor of eukaryotes remained simple internally but used surface structure-mediated symbiosis to develop complex cellular interactions, similar to those seen in eukaryotes.

This suggests that early partnerships between archaea and bacteria, possibly involving the bacterial ancestor of mitochondria, may have laid the foundation for the evolution of complex eukaryotic cells.