A Window Into Our Deepest Ancestry
Researchers at the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) have successfully isolated and cultivated two strains of Heimdallarchaeia, offering the first laboratory-grown look at the closest known archaeal relatives of eukaryotes. By observing these microorganisms directly, the team confirmed that these ancestors likely lacked complex internal organelles, such as a nucleus, despite their close evolutionary link to the cells that make up animals, plants, and fungi.
Extracting Life from the Chiba Gas Fields
The research team, led by Hiroyuki Imachi and Masaru K. Nobu of JAMSTEC alongside Satoshi Nakagawa of Kyoto University, obtained the samples from a gas production well in the Southern Kanto gas field in Chiba Prefecture, Japan. The biofilm samples were collected from a separator discharge outlet at depths of 20 to 40 centimeters below the water surface following brine eruptions in 2019, 2020, and 2023.
This milestone follows the team’s 2020 success in cultivating strain MK-D1, a member of the Promethearchaeota group. While prior studies of environmental DNA suggested that Heimdallarchaeia might possess complex internal structures or oxygen-utilizing capabilities similar to eukaryotes, the laboratory cultures revealed a different reality. The researchers found that the internal structure of both strains remains simple, lacking the membrane-bound compartments that define eukaryotic life.
Surface Protrusions Facilitate Symbiotic Partnerships in Archaea
Direct observation revealed that while the cells are internally simple, their exterior is highly specialized. The archaea extend long, branching membrane protrusions that facilitate attachment to other microorganisms. The study identifies these surface structures as the primary mechanism for forming symbiotic partnerships, providing the first biological link between structural complexity and symbiosis in archaea.
The researchers propose that this surface-mediated symbiosis may have been a precursor to the internal complexity found in eukaryotes. The findings suggest that an ancestral archaeon may have formed an early partnership with a bacterium in a low-oxygen environment. This partner likely assisted the anaerobic archaea in managing oxygen exposure, eventually leading to the integration of the bacterial ancestor of mitochondria into the archaeal cell.
Challenging Theories of Cellular Evolution
Heimdallarchaeia are considered the closest known archaeal relatives to eukaryotes, the domain that encompasses humans, animals, plants, and fungi. Studying them allows scientists to test hypotheses about how simple, single-celled ancestors evolved into the complex life forms seen today.
The team conducted laboratory growth experiments showing that the strains are oxygen-independent. However, they observed that one strain possessed protective systems allowing it to survive in the presence of small amounts of oxygen, provided that oxygen-utilizing bacteria were also present in the culture. These results support the idea that the archaeal ancestor of eukaryotes remained internally simple before developing complex structures, suggesting the transition was driven by external symbiosis rather than inherent internal complexity.
Imaging Techniques Verify Physical Characteristics of Strains
To verify the physical characteristics of the strains, the team utilized a variety of imaging techniques. When movement caused by Brownian motion obscured the cells, researchers used agarose-coated glass slides to stabilize samples for phase-contrast microscopy.
For more detailed visualization, the team employed Scanning Electron Microscopy (SEM) and Quick-Freeze Deep-Etch Electron Microscopy (QFDE-EM). These methods allowed for the documentation of membrane vesicles and thin filaments on the surface of cell protrusions. The researchers also validated their findings using specific antibodies created to target the lokiactin sequences of the two strains, ensuring the accuracy of their observations regarding the cellular architecture.
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