An international research team, led by Hokkaido University, has characterized the mechanics enabling the shelled, single-celled amoeba Arcella to move across various surfaces. Their findings, published in the Proceedings of the Japan Academy, Series B, detail how this microorganism maintains mechanical balance during locomotion by coordinating multiple pseudopodia.
Arcella, commonly found in peatlands and freshwater, is a testate amoeba, meaning its body is primarily enclosed within a dome-shaped shell, or "test." This shell is hypothesized to provide protection from predators, parasites, and environmental stressors like desiccation.
Through an opening on its underside, Arcella forms finger-like projections called pseudopodia for movement and capturing prey. While amoeboid motility, which uses pseudopodia, is common in living organisms (e.g., human immune cells, cancer cells), Arcella's locomotion is distinct due to its rigid shell. Researchers observed that its movement involves the coordinated use of multiple pseudopodia simultaneously.
Researchers discovered that Arcella samples from Hokkaido University's Tomakomai Experimental Forest generate traction stress on surfaces, facilitating grip and migration. To quantify these forces, fluorescent beads were embedded in the underlying surface and their displacement tracked. This method allowed for the measurement of traction stress and clarified how Arcella orientates itself. Differences in stress distribution between the front and back influenced straight-line movement, while left-right differences affected sideways movement.
Arcella's locomotion varied based on surface stiffness. On hard surfaces (e.g., glass, firm gel), the amoeba moved more actively, covering up to three times the distance compared to soft gel surfaces within the same timeframe. On hard surfaces, pseudopodia were primarily extended in the direction of movement. Conversely, on soft gel, movement was less directed, with pseudopodia extending in various directions.
This surface-dependent locomotion strategy may represent Arcella's adaptive mechanism for diverse natural environments, potentially crucial for its survival. It enables rapid, efficient migration on hard surfaces and a slower, more exploratory approach on soft surfaces.
Understanding shelled amoebae contributes to insights into their ecological functions, environmental responses, and biodiversity, given their role in food webs. Additionally, the pseudopodial strategies observed in Arcella could inform innovations in soft microrobotics.