Abstract
Morphological transformations in primitive organisms have long been observed; however, its biomechanical roles are largely unexplored. In this study, we investigate the structural advantages of dimorphism in Arthrospira platensis, a filamentous multicellular cyanobacterium. We report that helical trichomes, the default shape, have a higher persistence length (Lp), indicating a higher resistance to bending or a large value of flexural rigidity (kf), the product of the local cell stiffness (E) and the moment of inertia of the trichomes’ cross-section (I). Through Atomic Force Microscopy (AFM), we determined that the E of straight and helical trichomes were the same. In contrast, our computational model shows that I is greatly dependent on helical radii, implying that trichome morphology is the major contributor to kf variation. According to our estimation, increasing the helical radii alone can increase kf by 2 orders of magnitude. We also observe that straight trichomes have improved gliding ability, due to its structure and lower kf. Our study shows that dimorphism provides mechanical adjustability to the organism and may allow it to thrive in different environmental conditions. The higher kf provides helical trichomes a better nutrient uptake through advection in aquatic environments. On the other hand, the lower kf improves the gliding ability of straight trichomes in aquatic environments, enabling it to chemotactically relocate to more favorable territories when it encounters certain environmental stresses. When more optimal conditions are encountered, straight trichomes can revert to their original helical form. Our study is one of the first to highlight the biomechanical role of an overall-shape transformation in cyanobacteria.
Highlights
The shape of a living organism is a result of its functional adaptation
The mid-lines were selected because we found that collecting force-distance curves from positions not perpendicular to the Atomic Force Microscopy (AFM) tip increased the probability of measurement artifacts due to a high aspect ratio of the lateral sides (~ 8–10 μm height) compared to the tip
The data indicates that helical trichomes are stiffer than straight trichomes according to Eq (2)
Summary
Driven by certain selective pressures such as nutrient acquisition, cell division and predation, an adapted shape equips an organism with specific utilities [1]. This can be seen clearly in the case of the helix. In Campylobacter jejuni, a helical shape gives the bacterium increased motility in the viscous intestinal mucus of its host and the ability to cause disease [4]. Morphological adaptions of primitive organisms are generally discussed, an overall-shape transformation, which in some case causes a nearly permanent loss of an adapted morphology, is minimally mentioned, especially in the context of biomechanical roles
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