TLDR; After researching two unusual forms of bacterial locomotion, I've begun to rethink my assumption that rotary motors are the best way for microbes to get around. Perhaps the dominant forms of microbial life on other planets will resort to other, far stranger ways of traveling through watery and gel-like environments.
As many of you are probably aware, bacteria make use of a wide range of motility mechanisms. Some use their pili to crawl along surfaces, many control their buoyancy through internal gas vacuoles, and others make implement several forms of gliding which result from a variety of different mechanisms. However, the most dominant form of bacterial locomotion relies on the flagellum; a long, filamentous appendage powered by an internal motor. This motor rotates 360 degrees and is powered by the flow of protons from one side of the membrane to the other, much like how the flow of water causes a turbine to spin in a hydroelectric dam.
This spinning generates a tremendous amount of torque and enables bacteria to propel themselves forwards (and occasionally backwards) through the water. Some bacteria even have bundles of flagella which interweave to generate more thrust. These bundles can also disperse when rotated clockwise, allowing the bacterium to randomly spin and travel in a different direction. This form of locomotion is known as "running and tumbling" and when coupled with chemotaxis or phototaxis, can steer bacteria towards and away from harmful stimuli.
(Top) A 3D model of the flagellar motor found in bacteria (Bottom) A diagram demonstrating
how "running and tumbling" directs bacteria towards favorable environments.
In more unusual cases, the flagella will form bundles within the cell that become sandwiched between two membranes. These "axial filaments" rapidly spin within the organism and cause it to vibrate. These vibrations, coupled with the helical-shape of Spirochetes (a successful group of anaerobic and microaerophilic bacteria), is what enables them to rotate in a corkscrew-like fashion and to burrow through viscous substances (such as the mucus that lines your stomach or the extracellular matrix that gives biofilms their structure).
*NOTE: This motion can also be used quite effectively for swimming (Check out the video down below).
(Top) A cross-section of a Spirochete. (Bottom) A microcolony of Spirochetes
viewed under a scanning electron microscope.
However, in researching bacterial movement, I came across something absolutely extraordinary; bacteria that swim without flagella!
The first one I came across might be familiar to some of you if you've watched "Journey to The Microcosmos", "PBS Eons", or just have an interest in marine biology. It's a tiny, highly-successful Cyanobacterium by the name of Synechococcus.
Ever since the 80's, researchers have noted their peculiar form of swimming and how it doesn't seem to result from any external motor or means of propulsion. While the mechanism that drives this form of movement is still the subject of debate, recent models suggest that the cells makes use of a helical track system underneath their cell walls. This track system is likely composed of cytoskeletal filaments and would transport tiny "cargo proteins". As they do so, the movement of the cargo proteins causes deformations in the cell wall, creating wave-like contractions that run from pole to pole, generating thrust.
Interestingly, a similar system of tracks can be found across many distantly-related gliding bacteria, including Myxobacteria and possibly filamentous Cyanobacteria related to Synechococcus (although hard evidence has yet to be found for its existence in the latter). In these cases, the bacteria secrete "slime" (a loose tangle of polysaccharides) through specialized pores. This slime expands like shaving cream and propels them forward. Meanwhile, the contractions along the cell's body help to anchor them into the trail of slime and push off from it, generating even more thrust and enabling the bacterium to more effectively glide across surfaces.
(Top) The "Gliding System" observed within a number of diverse, non-flagellated bacteria (possibly including Synechococcus). (Bottom) A putative model of Synechococcus, demonstrating the wave-like bulges that extend from the leading end of the cell to the other (Note that propulsion only occurs in one direction.)
However, such a mechanism has never been observed aiding an organism in swimming. In fact, it was once thought that this system of movement would be impractical for traveling through water, since it likely wouldn't displace enough water molecules to propel the cell forward.
But in a study conducted back in 1996, it was proposed that specialized S-layer proteins on the surface of the Synechococcus may aid in motility by bulging outward when the cargo proteins pass underneath them. In theory, this would cause a larger ripple to form on the organism's surface that can displace even more water. With this, it was believed that these tiny Cyanobacteria were able to generate the necessary thrust, traveling at speeds of roughly 5 to 25 microns per second.
Soon after this proposal was made, mutant strains were created that lacked this protein. As a result, they were unable to swim and simply spun around in circles, confirming the role of the S-layer proteins as "little oars" and that they were being pushed by some unseen mechanism beneath the cell wall.
(Above) An illustration, showing how the S-Layer proteins (shown in green) would lift up
as the cargo protein (shown in blue) passes underneath them.
As for the affect motility has on the ecology of Synechococcus, it doesn't appear to be all that useful at first glance, given how slow it is in comparison to some other forms of locomotion (some prokaryotes can travel as fast as 500 microns per second!) However, one study found that the strain WH8102 was capable of preforming chemotaxis towards sources of nitrogen, suggesting that swimming might play a vital role in allowing the Cyanobacterium to forage for metabolites in the nutrient-poor waters of the open ocean (With that said though, it still remains unclear as to how the cells change their direction beyond simply bumping into surfaces.)
"Serpentine-Swimmers"
Meanwhile, in contrast to Synechococcus, a more versatile form of swimming can be observed in another bacterium known as Spiroplasma. During which, the cell's corkscrew-shaped body undergoes wave-like undulations that cause the organism to bend back and forth, much like a snake as it slithers along the ground. This serpentine locomotion enables it to rapidly navigate through viscous environments within the circulatory systems of insects. Also, unlike the Synechococcus, Spiroplasma's form of locomotion can propel the cell along a number of different paths by altering the way in which the leading end of the cell bends or by reversing the direction of its undulations. As a result, Spiroplasma is able to preform it's own method of chemotaxis.
(Top) A biofilm containing Spiroplasma cells (The helical, spirochete-like organisms). (Bottom) While we still don't understand the exact mechanism behind Spiroplasma's form of locomotion, this is a model of what might be happening on the interior of the cell. In this diagram, it's shown how specialized proteins within the organism form a semi-rigid "backbone", with each protein either consolidating or pushing away from one another when ATP is introduced. This presumably results in the the "relaxed side" developing a convex curve and the "contracted side" developing a concave curve. These undulations would then travel along the organism's length from pole to pole, resulting in their characteristic "slithering" motion.
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"Wave-Makers" on Leeuwenhoek
After reading about these unusual forms of locomotion, it got me thinking of whether similar strategies could become far more widespread and versatile on Leeuwenhoek, perhaps even partially replacing spinning propellers as the dominant means of propulsion. While I'm still under the impression that rotating motors are likely to convergently evolve and reappear on Leeuwenhoek (given the fact that they evolved twice independently on Earth within bacteria and archaea respectively), I imagine that wave-like contractions could still play an important role in enabling cells to travel within environments of either high or low viscosity.
As a result, Leeuwenhoek now hosts a three primary forms of swimming. The most common and versatile is based around a propeller, much like the proton motive force-based flagellum found in bacteria. Similarly, the "moneral flagella" is only found within one domain, roughly comparable to bacteria in terms of ecology, habitat range, membrane morphology, etc.
In contrast, Synechoccus-like swimmers would be dominant within a domain analogous to archaea, having evolved from an ancestral gliding lineage. However, unlike Synechococcus, this form of swimming is reversible (much like how many gliding bacteria on Earth can reverse the movement of their cargo proteins to change their orientation in the environment). In addition, this system of tracks and cargo would often be accompanied by another group of cytoskeletal filaments which aid in changing the cell's shape.
By flexing, swimming filaments and rods can alter their direction and engage in a crude form of steering. If the curve is highly pronounced, the cell will begin to spin randomly, allowing them to preform a unique form of "running and tumbling".
Furthermore, some Synechococcus-like swimmers have larger surface proteins which enable more efficient water displacement and faster movement. As a result, speed can vary considerably ranging anywhere from 5-50 or so microns per second. (While I don't imagine that this would make them the fastest organisms on Leeuwenhoek, it would definitely enable more rapid responses to environmental changes.)
Lastly, some Synechococcus-like swimmers may abandon the wave-like contractions altogether in favor of a system of locomotion that relies more heavily on rapid longitudinal flexing. These Spiroplasma-like organisms would be especially prevalent in viscous environments (i.e. microbial mats, biofilms, organic-rich sediments, etc.) and would fill the ecological role of Spirochetes on Leeuwenhoek.
*NOTE: I'm still undecided on whether or not I will describe their locomotion mechanism in detail or leave it unexplored when I include them in the book. However, there is a good chance they might make use of an undulating backbone that is similar to the one in the model above, or possibly a highly-simplified mechanism that is analogous to the one that is present within eukaryotic flagella (check out the video down below).
For now, these three forms of motility don't have names, but feel free to recommend some to me through discord! (either through the official microcosmos discord server or my channel on the official spec evo server.)
Supplemental Material :
Footage of Spirochetes :
https://www.youtube.com/watch?v=cXYfT5hSLoQ
An Old German Documentary about Myxobacteria (a commonly-studied predatory bacterium that moves through gliding and flexing) :
https://www.youtube.com/watch?v=ZHGEi2JzXso
Grainy, "found-footage" style clips of synechococcus swimming :
https://www.youtube.com/watch?v=u4jql9hgaE8
https://www.youtube.com/watch?v=OEKIK-H6Ty8
Footage of Spiroplasma :
https://www.youtube.com/watch?v=VQzILpQ3RpU
3D Model of The Eukaryotic Flagellum :
https://www.youtube.com/watch?v=9nZYlyFGm50
Citations :
- https://www.caister.com/backlist/jmmb/v/v1/v1n1/09.pdf
- https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0036081
- https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2021.706426/full
- https://www.cell.com/current-biology/pdf/S0960-9822(13)01593-5.pdf
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