This Tiny Organism Contracts 100 Body Lengths a Second — Scientists Now Know How

The single-celled aquatic organism Spirostomum ambiguum can shrink to one quarter of its body length in less than five milliseconds — about 100 body lengths per second, hundreds of times faster than a human blink. Researchers have now traced that speed to an unusual calcium-activated protein network arranged like a fishnet, a discovery that could offer clues for building faster artificial muscles and synthetic cellular machinery.

Spirostomum is a giant single-celled ciliate, a type of organism covered with hair-like cilia used for swimming, and it can repeat its extreme contraction almost immediately. Human muscle fibers can shorten by a comparable proportion, but the process takes about 10 times longer. "The difference between what Spirostomum can do and what we can do comes down to what is powering the contraction, and what the machinery behind it looks like," said Mary Elting, an associate professor of biophysics at North Carolina State University and co-corresponding author of the study, published in the Proceedings of the National Academy of Sciences.

Using electron and immunofluorescence microscopy, the researchers found that calcium ions trigger the movement, while a fishnet-shaped web of fibrous structures called myonemes — built from the calcium-binding proteins centrin and Sfi1 — rapidly tightens across the organism's exterior, pulling the cell inward before springing back. This geometry lets Spirostomum contract uniformly, protecting its internal organelles even at such speed. Elting said the Sfi1 protein can shift from stiff to flexible: in the presence of calcium ions, it loses stiffness and clumps up "like a ball of wet spaghetti," pulling the fishnet tight.

Human muscle contraction depends on adenosine triphosphate (ATP), a molecule that is chemically "burned up" like gasoline to release energy. Spirostomum instead responds directly to calcium ions, more like an electrical current, though researchers still do not know what generates the signal that starts the contraction or how the system resets to allow it to happen again and again. The team is now investigating what initiates the calcium signal and how the organism restores itself after each contraction, which could be key to building an ATP-independent artificial muscle.