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Archaea—one of the three primary domains of life alongside bacteria and eukaryotes—are often overlooked and sometimes mistaken for bacteria due to their single-celled nature and lack of a nucleus. Yet, archaea are found across diverse environments, from oceanic plankton to the human microbiome.

Despite their superficial similarity to bacteria, their genetic makeup has long suggested a closer evolutionary relationship with eukaryotes, the domain encompassing plants and animals. This new research uncovers a remarkable capacity within archaea to organize beyond their single-celled existence under specific physical conditions.

Intrigued by the unique combination of genetic and structural traits in archaeal cells—particularly their proteinaceous surface layer instead of a rigid cell wall—researchers from Brandeis University, the MRC Laboratory of Molecular Biology in Cambridge, and the Max Planck Institute for Biology in Tübingen sought to explore the mechanobiology of these ancient organisms.

Lead researcher Alex Bisson from Brandeis University explains, "The absence of a covalent-bound cell wall suggests a more dynamic, but less rigid structure, leading to the hypothesis that archaea might be 'squishy' and sensitive to mechanical stimuli." This initial curiosity led to an unexpected and significant discovery.

Their research resulted in the accidental identification of multicellularity across all three domains of life and demonstrated the importance of mechanical forces in shaping archaeal tissues. "Our work shows that the emergence of complexity in life isn't limited to a few special branches on the tree of life—it's a deeper property, present even in lineages we've long overlooked," noted Vikram Alva, co-lead author from Max Planck Institute for Biology Tübingen.

Pedro Escudeiro, a postdoctoral researcher in the Alva group, added, "This work also underscores the power of combining comparative genomics with observable traits to uncover genes behind novel behaviors—an approach that has long driven discoveries in plants and animals." The study is in the journal Science.

The role of mechanical forces in multicellularity

Working with Haloferax volcanii, a resilient archaeon that thrives in extreme environments like salt flats, the team observed an astonishing transformation. Instead of undergoing typical cell division, when subjected to mechanical compression, the cells grew larger and organized in tissue-like arrangements containing multiple sets of genetic material.

Their study describes how the flexible outer protein layer contributes to adaptive growth strategies. "It was Theopi Rados, the first author leading the project, who first observed and described this remarkable behavior," said Bisson. "As Olivia Leland, co-first author, aptly put it—it's as if the cells were squished down and then encouraged to grow wider and taller, more like a rising sourdough loaf than traditional cell division," explained Bisson.

As cells were subjected to specific pressures, they transitioned from solitary organisms to interconnected cellular communities. "That such behavior can be triggered by a simple physical constraint and involves cytoskeletal remodeling, and coordinated cellularization suggests that the capacity for structural organization runs deeper in biology than previously thought," remarked Rados.

"The fact that archaea can orchestrate complex from tissue-like structures suggests that nature can emerge complex traits from seemingly unsophisticated raw materials," adds Bisson. "By revealing just a fraction of natural diversity, we could advance our intellectual and medical needs."

Tanmay Bharat, a co-lead author from the MRC Laboratory of Molecular Biology in Cambridge, underscores the broader implications of research in archaea on multicellularity: "We found that mechanical compression induces multicellularity, a surprising finding, to say the least." He further suggests that this discovery raises questions about whether other unicellular organisms might possess a similar latent ability to develop multicellularity in response to environmental cues.

Although it is common knowledge that archaea don't like to be confined, likely because their cell envelope structure is more fragile than that of other microbes, archaeal tissues have now added another facet to our understanding of multicellularity. This research encourages other scientists to explore whether applying similar stimuli could prompt other ordinarily unicellular organisms to transition to multicellularity.