Structural mechanics and collective self-organisation in filamentous cyanobacteria

Filamentous cyanobacteria, one of the earliest types of organisms to have evolved on Earth, are photoautotrophs made of single cells joined together in long filaments. They are ubiquitous, living in water, soil, rocks and extreme environments like hot springs. Their oxygen production is believed to have led to the evolution of oxygen-dependent organisms like us. They live in colonies forming biomats, and are associated with stromatolites, which are important for understanding the evolution of early life. Commercially, filamentous cyanobacteria are used for biofuel production, food supplements, cosmetics and medicines. In order to maximise the usage of these microorganisms, we must understand how individual filaments interact and form collective structures. This thesis, therefore, focuses on quantifying the mechanical properties and collective organisation of filamentous cyanobacteria. First, the structural and mechanical properties of the filaments, such as the bending stiffness, are quantified. The mechanical properties are linked to their shapes, to predict the magnitude of internally generated active forces. These results can be used to model cyanobacteria motion and self-organisation. Next, this thesis looks at the behaviour of filaments in isolation and when interacting with other filaments or walls. These results provide parameters such as filament speed, angular drift and curvature that are then used by collaborators for modelling and predicting the collective behaviour of the cyanobacteria. The last part of this thesis provides experimental evidence of how self-organisation occurs for filamentous cyanobacteria in an open space and in confinement. A density-dependent phase transition was found, between disordered and nematically ordered patterns of filamentous cyanobacteria. Finally, in confinement studies, it was observed that certain chamber geometries, e.g. circular, promote unequal filament distribution. The results here are applicable in areas such as the study of stromatolites and the evolution of early life, and in the production of algae-based biofuels

Active spaghetti: collective organization in cyanobacteria

Filamentous cyanobacteria can show fascinating examples of nonequilibrium self-organization, which however are not well-understood from a physical perspective. We investigate the motility and collective organization of colonies of these simple multicellular lifeforms. As their area density increases, linear chains of cells gliding on a substrate show a transition from an isotropic distribution to bundles of filaments arranged in a reticulate pattern. Based on our experimental observations of individual behavior and pairwise interactions, we introduce a nonreciprocal model accounting for the filaments' large aspect ratio, fluctuations in curvature, motility, and nematic interactions. This minimal model of active filaments recapitulates the observations, and rationalizes the appearance of a characteristic lengthscale in the system, based on the Péclet number of the cyanobacteria filaments

Structural mechanics of filamentous cyanobacteria

Filamentous cyanobacteria, forming long strands of connected cells, are one of the earliest and most successful forms of life on Earth. They exhibit self-organised behaviour, forming large-scale patterns in structures like biomats and stromatolites. The mechanical properties of these rigid structures have contributed to their biological success and are important to applications like algae-based biofuel production. For active polymers like these cyanobacteria, one of the most important mechanical properties is the bending modulus, or flexural rigidity. Here, we quantify the bending stiffness of three species of filamentous cyanobacteria, of order Oscillatoriales, using a microfluidic flow device where single filaments are deflected by fluid flow. This is complemented by measurements of the Young's modulus of the cell wall, via nanoindentation, and the cell wall thickness. We find that the stiffness of the cyanobacteria is well-captured by a simple model of a flexible rod, with most stress carried by a rigid outer wall. Finally, we connect these results to the curved shapes that these cyanobacteria naturally take while gliding, and quantify the forces generated internally to maintain this shape. The measurements can be used to model interactions between cyanobacteria, or with their environment, and how their collective behaviour emerges from such interactions