Tiny microbes may have given rise to complex life

Ten years ago, the scientific community had no idea that a remarkable group of microbes called Asgard archaea even existed.

That changed in 2015 when researchers studying deep-sea sediments stumbled upon mysterious gene fragments. With the help of computer software, they pieced these fragments together and discovered something entirely new – a previously unknown group of microbes.

These microbes are part of a group of organisms known as the archaea, and are single-celled organisms similar to bacteria but genetically distinct.

Archaea differ from bacteria in important ways, such as how their cell envelopes are built and how they process energy.

This newly identified group was so unique that scientists gave it its own category: Asgard archaea.

The name, inspired by Norse mythology, acknowledges their initial discovery near Loki’s Castle, a hydrothermal vent in the mid-Atlantic Ocean.

From tiny microbes to complex life

What made this discovery even more exciting was the realization that Asgard archaea may be the closest living relatives of eukaryotes – organisms whose cells contain a nucleus, including humans, animals, and plants.

This revelation challenged the long-standing model that life falls into three domains: bacteria, archaea, and eukaryotes. Some scientists now argue that eukaryotes may actually have evolved from within the Asgard archaea, which would reduce the tree of life to just two early domains: archaea (including eukaryotes) and bacteria.

At ETH Zurich, scientists have been exploring these microbes for several years. In a 2022 study, researchers focused on a species that is found in the sediments of a brackish water channel in Slovenia called Lokiarchaeum ossiferum.

The species was isolated by a research team from the University of Vienna. The team found that Lokiarchaeum ossiferum shares key features with eukaryotic cells.

“We found an actin protein in that species that appears very similar to the protein found in eukaryotes – and occurs in almost all Asgard archaea discovered to date,” explained Professor Martin Pilhofer.

A skeleton of tentacles

To study this further, the researchers used a variety of advanced microscopes. They discovered that this actin protein, named Lokiactin, forms long, thin filaments, which are especially visible in the many tentacle-like projections of the microbes.

“They appear to form the skeleton for the complex cell architecture of Asgard archaea,” added Florian Wollweber.

Eukaryotic cells don’t just have actin filaments – they also rely on structures called microtubules, which are built from proteins known as tubulins. Microtubules help move materials around inside cells and play a key role during cell division.

For a long time, scientists didn’t know where microtubules originally came from. That mystery may now be closer to being solved.

The ETH Zurich team found similar structures in Asgard archaea. Although the Asgard versions are smaller, they share striking similarities with the microtubules found in eukaryotes.

Rare, but important structures

Interestingly, only a small number of Lokiarchaeum cells produced these microtubule-like structures.

Unlike the widely present actin proteins, tubulins only appear in a few known Asgard species. It’s still unclear why they are so rare or what precise function they serve. In eukaryotic cells, microtubules act like highways, allowing motor proteins to transport materials.

So far, the ETH Zurich researchers have not seen these types of motor proteins in Asgard archaea. But they did observe something telling.

“We have shown, however, that the tubes formed from these tubulins grow at one end. We therefore suspect that they perform similar transport functions as the microtubules in eukaryotes,” explained Jingwei Xu, co-first author of the study.

Xu produced the tubulin proteins using insect cells and examined their shape and behavior.

The study brought together experts in microbiology, biochemistry, cell biology, and structural biology. “We would never have progressed so far without this interdisciplinary approach,” stated Pilhofer.

Building blocks of complex life

Could the cytoskeleton – the structural framework inside cells – have played a vital role in the rise of complex life? The researchers think so.

They believe that long ago, an Asgard archaea may have used its tentacle-like appendages to grab a bacterium.

Over time, this type of bacterium became a permanent resident inside archaeans, and evolved into the mitochondrion – the energy center of today’s cells. From there, other features like the nucleus eventually developed, giving rise to the eukaryotic cell.

“This remarkable cytoskeleton was probably at the beginning of this development. It could have enabled Asgard archaea to form appendages, thereby allowing them to interact with, and then seize and engulf a bacterium,” said Pilhofer.

Fishing for elusive microbes

The ETH Zurich team isn’t stopping here. They’re now shifting focus to understand the role of actin and tubulin in Asgard archaea. They also plan to study the unique proteins that appear on the microbes’ outer surfaces.

One goal is to create antibodies that can recognize these surface proteins. This would let scientists “fish” for Asgard archaea within complex microbial communities.

“We still have a lot of unanswered questions about Asgard archaea, especially regarding their relation to eukaryotes and their unusual cell biology,” said Pilhofer. “Tracking down the secrets of these microbes is fascinating.”

The search for answers continues, but one thing is clear: these tiny organisms may hold the key to one of life’s biggest mysteries.

The full study was published in the journal Cell.

Image Credit: Margot Riggi, Max Planck Institute of Biochemistry

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