Self-Organization of Living Polymeric Matter
What is “living polymeric matter”? Imagine a bowl of pasta coming to life, each spaghetti gliding along its contour through the entangled mess of its siblings. What we actually study isn’t pasta, but looks similar when observed under a microscope: filamentous cyanobacteria. These are truly fascinating organisms that form long filaments, indeed a bit like tiny green spaghetti that move on top of each other. And they’re alive, actually since over two billion years, as far as we can tell from their fossil record. That means they have survived all the climates that our planet went through, from snowball earth to the hot phases when dinosaurs felt comfy. Even more, they “made” climate because of their photosynthetic activity, using light to convert carbon dioxide into biomass and oxygen. This caused the great oxygenation event, the biggest climate catastrophe in the history of our planet, over two billion years ago.
Here, “catastrophe” is from cyanobacteria’s point of view; we actually breathe the oxygen they made. But we also burn their remainders, fossil fuels, causing the exact inverse, a “great oxidation event” (another climate catastrophe, this time, from our point of view). But they survived their catastrophe, so we can learn from them how they managed. And that’s exactly what we want to find out. The key message is simple: Together they were strong! Individual cells could not have survived such vastly different conditions, but as a colony of millions of filaments, they could adapt, simply by changing the shape of their colony. A compact sphere with minimal surface when facing too much light, but spreading out and collecting the very last photons when volcanoes darkened the skies. What we don’t understand is how this works: Since they are simple organisms, not capable of “thinking”, they must rely on the principles of self-organization i.e., what emerges naturally in an ensemble of simple, yet responsive filaments.
This is where our research starts: we quantify their active and passive mechanical properties and the responses that individual filaments show to stimuli, under varying culture conditions. At the same time, we observe the colony architectures and their macroscopic physico-chemical properties that form in these varying culture conditions. To learn about the hierarchy of mechanisms that leads from the small end to the large end, we use an old trick: We place them into artificial environments, reducing their freedom to move and organize, and thereby isolate the mechanisms at play, to study them one by one. Apart from the beauty of their ballet, we might actually get a better clue how to use and tame them, to cope with the blooms they form in our lakes and oceans, and use them as a renewable energy source.
Meanwhile, we made some first steps in understanding these organisms. One of the most fundamental question, which is debated for at least a century, is how they self-propel. Filamentous cyanobacteria can't swim, but they "glide" over surfaces. The magic is: They don't have feet or any other appendages that look useful for self-propulsion! That has led to many hypotheses and speculations, but apparently, no one ever asked how strong the propulsion forces actually are. That's where we came in: We noticed that they are strong enough to bend themselves when hitting an obstacle, but only the long ones. That type of deformation is known as buckling, or, to be precise, self-buckling. Leonhard Euler probably was first (toward the end of the 18th century) to investigate such processes, asking how high a tree could grow (it would eventually buckle under the action of gravity). In collaboration with Oliver Bäumchen from the University of Bayreuth, we first measured the stiffness of single filaments. Knowing the stiffness, the critical self-buckling length and the evolution of the buckling profile can be analyzed to obtain the propulsion force. Have a look at [Kurjahn et al., eLife (2023)]!