Beep, beep. An archaeal Road Runner.

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Archaea are small, non-pathogenic, relatively simple single-celled microorganisms. A list of characteristics that would make them seem not very appealing. Indeed, many people do not even know that they exist. The public is much more acquainted with the other unicellular cousins of archaea, the bacteria. We often hear about bacteria as “the bad guys” that make us ill, or “the good guys” that help us stay healthy (especially our digestive system). But we hardly ever hear about archaea!

Nonetheless, Archaea are one of the three domains of life (together with Bacteria and Eukarya) and, in my opinion, they are just as “cool” as bacteria. Many archaea live in extreme environments, where temperature, pH, and salinity would kill most bacterial and eukaryotic life forms. Would you choose to live in the middle of a geyser? Well, some archaea do, happily! But we do not have to go very far to find archaea. In fact, they are everywhere, on us, inside us. But no need to panic: no harmful archaea have been discovered yet.

The ubiquity of archaea means that they have conquered very diverse environments all around the globe. Their striking success as colonizers is – at least in part – down to their ability to swim by using a molecular propeller that we call an “archaellum”. You can think of it as a whip that is attached to a tiny rotary engine. This engine is located in the cell envelope. When the engine rotates, the extracellular whip rotates too, propelling the cell.

In this study, we investigated the archaellum of Methanocaldococcus villosus. In Latin, villosus means hairy. An adjective aptly chosen, because the many archaella make M. villosus cells appear hairy under the microscope. In nature, M. villosus has chosen as its home a shallow volcanic system under the sea near Iceland. Here, the sea water rises to temperatures of about 80°C, and in these waters M. villosus swims at a speed of ~500 bps (body lengths per second). Considering that the tiny cell is only ~1 micrometre in size, this means half a millimetre in one second. At first glance, this does not seem much. But in comparison, a cheetah achieves only 20 bps. If an M. villosus cell had the size of a cheetah, it would swim at approximately 3,000 km/h! The incredible speed that this archaeon can achieve makes it one of the fastest organisms on the planet.

Scanning electron micrograph of M. villosus cells showing a large number of archaella. Kindly provided by Dr. Annett Bellack and Prof. Dr. Gerhard Wanner.

Intrigued by the question of how the archaellum allows M. villosus to swim through almost boiling sea water at such amazing speed, we decided to investigate its structure. To do this, we employed cryogenic electron microscopy (cryoEM), a microscopy technique that uses a beam of electrons to visualise the sample. One of the advantages of cryoEM is that we can visualise cells at much higher resolution than light microscopy, thereby capturing molecular details that would not be visible otherwise. 

We found that the M. villosus archaellum consists of thousands of copies of two protein subunits (ArlB1 and ArlB2), which alternate along the filament to form a helix. While genetic and molecular biology studies have previously shown that some archaea can produce archaella that are made of more than one protein homologue, researchers never visualised them before. By being able to investigate the structural details of this archaellum at the molecular level, we observed how ArlB1 and ArlB2 move and interact with each other. Based on these observations, we proposed that the particular arrangement of ArlB1 and ArlB2 along the archaellum is crucial to maintain its structure, thereby propelling the cell at such high speed.

Surface representation (centre) of a small portion of the M. villosus archaellum, showing the alternating subunits ArlB1 (orange) and ArlB2 (golden). The two subunits are shown in ribbon representation, with glycan moieties in ball-stick.

Many archaea use archaella to swim. Researchers have shown that archaea can change the composition of their archaella based on their environmental conditions. I imagine, just as we choose smart shoes, trainers or boots depending on the weather and where we are going, archaea too choose the archaellum that best suits their needs.

Although this study has shed light on the structure and function of archaella, at the back of my mind, I still have an unanswered question: why is M. villosus in such a hurry?!

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