A research team at the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) has isolated and grown two strains of Heimdallarchaeia, an archaeal group considered the closest known relatives of eukaryotes — the lineage that includes animals, plants and fungi. The work, led by Hiroyuki Imachi and Masaru K. Nobu with Kyoto University's Satoshi Nakagawa, was carried out in Kyoto, Japan, and reported by Mirage News. It marks the first time members of this group have been cultivated in the laboratory, allowing scientists to study their cells and lifestyles directly rather than inferring them from environmental DNA.
Why it matters
How the complex cells of eukaryotes came to be is one of biology's longest-standing questions. A leading hypothesis holds that a eukaryotic cell arose when an archaeon from the group Promethearchaeota entered into a close partnership with a bacterium that later became the mitochondrion. Until now, ideas about the archaeal side of that partnership rested largely on genetic material recovered from the environment. Having living cultures changes that, according to Mirage News, giving researchers the clearest picture yet of what the archaeal ancestor of eukaryotes may have looked like and how it lived.
The JAMSTEC group has history here. In 2020 it cultivated strain MK-D1, a member of Promethearchaeota taken from deep-sea sediment, and used its cell structure and metabolism to propose the E3 model for eukaryotic origins. Later studies of environmental DNA pointed to Heimdallarchaeia, a lineage within Promethearchaeota, as the nearest archaeal relatives of eukaryotes. Some researchers suggested these organisms might already possess complex internal structures or the ability to use oxygen, much as eukaryotes do. Without laboratory cultures, neither claim could be checked.
Simple inside, elaborate outside
The two new strains did not match those expectations. Their interiors were plain, with no membrane-bound organelles such as a nucleus. The complexity sat on the outside instead: the cell surface carried numerous long, branching membrane protrusions. These extensions let the cells attach to other microorganisms and form symbiotic relationships.
Both strains were anaerobic, meaning they do not depend on oxygen. One of them, however, carried several systems for protecting itself against oxygen and could grow when small amounts of the gas were present at the start, provided that oxygen-using bacteria shared the environment.
Mirage News reports that the findings point in two directions. First, the archaeal ancestor of eukaryotes may have been simple on the inside. Second, the team identified symbioses mediated by surface structures — described as the first biological link between structural complexity and symbiosis in archaea. That supports the view that eukaryotic internal complexity arose through surface-mediated partnerships with the bacterial ancestor of mitochondria.
The results also suggest where that partnership may have started: in surroundings with low oxygen, where the bacterial partner could have helped the anaerobic archaeon cope with the gas. One such archaeon may eventually have taken the mitochondrial ancestor inside its cell, setting off sweeping changes to its structure and metabolism and, in time, producing an ancient eukaryote.
The cultivated Heimdallarchaeia now serve as experimental models for probing eukaryotic origins. Further work on their cell architecture and their interactions with microbial partners is expected to clarify how a simple archaeal cell became the complex cells of eukaryotes.
