Spirostomum uses a calcium-activated protein network to contract at remarkable speed without conventional muscle fibers.
In less than five milliseconds, the aquatic single-celled organism Spirostomum ambiguum can shrink to one quarter of its body length, a movement 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 mechanism that could offer clues for building faster artificial muscles and synthetic cellular machinery.
Spirostomum ambiguum is a giant single-celled ciliate, a type of organism covered with a fringe of hair-like cilia that it uses for swimming. Among ciliates, it stands out for its ability to contract at roughly 100 body lengths per second and repeat the movement almost immediately. That speed may help it escape predators or communicate with other ciliates.
Human muscle fibers can shorten by comparable proportions, but the process takes about 10 times longer. That difference has made Spirostomum especially interesting to researchers trying to understand how biological systems can generate rapid movement without relying on conventional muscle.
“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,” says Mary Elting, associate professor of biophysics at North Carolina State University and co-corresponding author of the work. “If we can understand those processes, it could help us build synthetic systems that mimic the speed and power of this single-celled organism.”
A fishnet network powers extreme speed
To uncover the machinery behind the contraction, the researchers examined Spirostomum with electron and immunofluorescence microscopy. Their observations showed that calcium ions trigger the movement, while a distinctive fishnet-like structure inside the cell generates the contraction itself.
Unlike humans, single-celled organisms such as Spirostomum do not have muscle fibers. Instead, they contain myonemes, fibrous structures built from the calcium-binding proteins centrin and Sfi1. In Spirostomum, those myonemes form a fishnet-shaped web across the organism’s exterior.
When contraction begins, that network rapidly tightens, pulling the cell inward before springing back to its original configuration. Its geometry allows the organism to shorten across its body rather than collapsing unevenly.
“The fishnet geometry is unique because it lets Spirostomum contract uniformly, which protects is internal organelles (single cells’ versions of organs) while it moves so quickly,” Elting says. “It works because the Sfi1 protein in the myoneme can shift from stiff to flexible. In the presence of calcium ions, Sfi1 loses its stiffness and clumps up like a ball of wet spaghetti, which causes the fishnet to pull tight, shrinking the organism.”
The ability of Sfi1 to change its physical state provides a mechanical explanation for how the network can contract so rapidly. Rather than depending on the same energy system that powers human muscle, Spirostomum responds directly to calcium.
Calcium replaces the usual muscle fuel
Human muscle contraction depends on adenosine triphosphate, or ATP, a molecule that stores and releases energy inside cells. Spirostomum uses a fundamentally different system.
“Comparing the way our muscles contract to the way Spirostomum works is like comparing gas to electric power,” Elting says. “ATP undergoes a chemical change and gets ‘burned up,’ like gasoline, whereas calcium ions act like an electrical current, although we still don’t know what produces the voltage that starts the current, or how it gets ‘reset’ so contraction can happen again.”
That reset remains one of the central unanswered questions. Calcium-triggered reactions would normally be expected to work only once without some mechanism restoring the system, yet Spirostomum can contract repeatedly at high speed.
The researchers are now investigating what initiates the calcium signal and how the organism restores the system after each contraction.
“We would expect calcium-triggered reactions to be ‘one shot,’ but Spirostomum can do it repeatedly,” Elting says. “Understanding those aspects of its motion are the keys to building a fast-moving, ATP-independent artificial muscle.”
Reference: “A centrin–Sfi1 myoneme fishnet powers ultrafast calcium-triggered contraction in the giant ciliate Spirostomum ambiguum” by Joseph Lannan, Carlos Floyd, L. X. Xu, Peter M. Thompson, Connie Yan, Wallace F. Marshall, Suriyanarayanan Vaikuntanathan, Aaron R. Dinner, Jerry E. Honts, Saad Bhamla and Mary Williard Elting, 29 May 2026, Proceedings of the National Academy of Sciences.
DOI: 10.1073/pnas.2601408123
Supported by the National Science Foundation under award numbers 1935260, 2313722, 2313724, 1935262, 1817334, 2313727 and 2313725, and by NIGMS of the National Institutes of Health under award numbers R35GM130327 and R35GM142588.
Never miss a breakthrough: Join the SciTechDaily newsletter.
Follow us on Google and Google News.