[Ube Kosen] Microbes also "cannot stop suddenly"!?

Figure 1: The optical lever measurement system developed in this study (left) and the actual dynamic data at the time of collision (right). The slight, gentle or steep deflection when Volvox swimming in a water tank collides with a microscopic cantilever is precisely detected by the change in the reflection angle of laser light. As the data (right) shows, standard-sized Volvox (top right) smoothly transmit force even after collision, but only in large Volvox (bottom right) is a clear "inertial collision force" spike (steep change) detected at the moment of impact, significantly exceeding the forward propulsion force. This data shows the core findings of this research.

Katsuya Shimabukuro, Associate Professor at the Department of Materials Science and Engineering, Ube National College of Technology (Yamaguchi Prefecture, Ube City; President: Junhiro Kawamura; hereinafter referred to as "Ube Kosen"), along with a joint research group including Professor Noritaka Mitsudome from the Faculty of Education, Tokoha University (Shizuoka Prefecture, Shizuoka City), has succeeded in directly measuring the underwater collision force of multicellular algae Volvox while swimming in millisecond units.

Recent research (Ueki & Wakabayashi, 2024) has reported that as Volvox bodies become larger and their "Reynolds number" (Note 1) increases during evolution, their ciliary response to light stimuli changes dramatically, suggesting a relationship with the fluid environment. This research group, through high-sensitivity mechanical measurements, has demonstrated the "inertial collision force" during swimming in larger species, explaining the physical necessity of braking behavior. Furthermore, computational simulations (Note 2) revealed that their own mass contributes to swimming stability. These results were published in the international academic journal "Physical Review Research" on June 30, 2026 (local time).

[Notes]

Note 1: Reynolds number: An index indicating the ratio of "inertial force (force to continue moving forward)" to "viscous force (braking due to fluid viscosity)" when an object moves in a fluid (such as water). Bacteria and small microorganisms have very small Reynolds numbers (low Reynolds number environment) and swim bound by water with almost no influence from inertia. However, as size increases, the Reynolds number rises, and the influence of inertia becomes non-negligible, as in the case of the large Volvox in this study.

Note 2: Computational simulation: In this study, a mathematical model based on Newton's equations of motion, considering the organism's mass, size, pulsating force generated by cilia, and water resistance, was constructed, and numerical calculations were performed on the swimming trajectory and velocity fluctuations (stability) of larger species.

1. Background of the Research: The "Underwater World" Changes as Bodies Grow Larger

From algae like Volvox, to protozoa like Paramecium, and even to the cells in human airways that prevent foreign substances from entering the lungs, the "cilia" (or flagella) found in eukaryotic organisms are universal biological motors (fluid-driving devices) that generate water flow for movement and material transport. In this microscopic world observed under a microscope, the viscosity of the surrounding water usually dominates, so when an organism stops moving, it instantly stops in place without moving forward due to inertia. Therefore, the swimming speed and trajectory of microorganisms have been thought to directly reflect the propulsion force generated by the cells in real-time.

However, in the process of evolving from single-celled to multicellular organisms (like Volvox), as the body becomes larger and heavier, the environment changes such that the influence of "inertial force (force to continue moving forward)" becomes non-negligible compared to the water's viscosity.

In relation to this, recent research (2024) by Ueki (Hosei University) and Wakabayashi (Kyoto Sangyo University), who are also co-researchers in this study, comprehensively analyzed nearly 30 species of Volvox. They discovered that while standard-sized species temporarily stop their cilia upon receiving light stimulation, large Volvox (such as Volvox ferrisii) with a diameter exceeding 0.8 mm exhibit a special behavior of reversing the direction of water flow by changing the beating direction of their cilia (Figure 2), suggesting a close relationship with the fluid environment (Reynolds number).

It was speculated that large species might require such reverse water flow control because they cannot stop suddenly due to their own inertia, but there had been no direct measurement and verification of the "inertial force (impact force)" generated during actual swimming. Therefore, Professor Ueki and Professor Wakabayashi, who proposed this hypothesis, joined the research team, and Associate Professor Shimabukuro (Ube Kosen), Professor Mitsudome (Tokoha University), and others took the lead in attempting verification through a physical approach.

2. Research Achievements: Collision Experiments and Fluid Simulations Reveal Physical Constraints

The research group applied the "optical lever method" (Note 3) of scanning probe microscopy to construct an experimental system that directly measures the force when Volvox swimming freely in water collides with a microscopic cantilever with a very high time resolution of 4 milliseconds (0.004 seconds) (Figure 1, left).

In the experiment, measurements were taken on two species: Volvox carteri (V. carteri, approximately 0.2 mm in diameter), which is a standard size within the Volvox genus and served as a comparison in this study, and the large species Volvox ferrisii (V. ferrisii, approximately 0.8 mm in diameter).

[Notes]

Note 3: Optical lever method: A technique for precisely measuring extremely small forces by irradiating a microscopic cantilever with laser light and detecting the change in the reflection angle of the light when the cantilever bends slightly due to force. This enables the measurement of forces on the order of nanonewtons (one billionth of a newton) experienced by swimming microorganisms in real-time and with high time resolution.

"Impact Force at Collision" Appearing Only in Large Species

In the standard-sized V. carteri, only periodic pulsations of propulsion force were observed during collision. However, in the large V. ferrisii, a steep "inertial collision force" spike signal (orange region in the lower right of Figure 1) was clearly captured at the moment of collision, significantly exceeding the subsequent propulsion force. This is empirical evidence that large species possess significant inertia (momentum) during swimming and are literally colliding with a "thud!"

Physical Constraints Preventing Sudden Stops Due to Own Weight, and Swimming Stability

This physically demonstrates that "large Volvox live in an environment where they cannot stop suddenly" due to their large weight (mass). It has been successfully explained that simply stopping the cilia temporarily would result in continued forward motion due to residual inertia, thus necessitating active reversal of water flow as a brake to avoid danger. Furthermore, this study focused on how large species, by averaging the force of thousands of cilia, and their own weight, act as a "smoothing filter" (averaging role) that cancels out fine motion fluctuations and smooths them out. When this mechanism was quantified through fluid dynamics simulations (Note 2), it was confirmed that their own mass suppresses swimming fluctuations, achieving very high stability.

3. Future Prospects: The Principles of Nature Unfolding Beyond the Microscope

This research presents achievements from both experimental and computational perspectives, demonstrating how the surrounding hydrodynamic environment influenced the mechanisms of motion control as organisms increased in size through multicellularity.

Volvox species, ranging from single-celled to large species with tens of thousands of cells, coexist on Earth today, each with a different cell count. This is thought to be the result of each species having settled on a cell count suitable for its survival and corresponding fluid control mechanisms.

What is important in studying organisms is to concretely imagine the world behind how these microscopic lives, observed through the lens of a microscope, have survived in the actual natural environment. In the future, in addition to precise mechanical measurements in the laboratory, it will be necessary to approach the elucidation of further mysteries from the perspective of nature's original form by directly engaging with the natural environments in which they live.

How the universal fluid-driving device called "cilia," widely used from algae to humans, changes its control principles in response to changes in size and environment. The current achievement serves as a new foothold for hydrodynamically unraveling the mechanical responses of microorganisms in their habitats from mechanical data obtained under the microscope.

Figure 2: Differences in body size and braking methods (conceptual illustration). When exposed to light stimulation, standard-sized Volvox (top: V. carteri) can stop in place simply by "stopping" their cilia, but large Volvox (bottom: V. ferrisii), due to their large weight (inertia), need to apply the brake by "reversing" the beating direction of their cilia.

Contact Information

Regarding research content:

Katsuya Shimabukuro, Associate Professor, Department of Materials Science and Engineering, Ube National College of Technology E-mail: [email protected]

Noritaka Mitsudome, Professor, Faculty of Education, Tokoha University E-mail: [email protected]

[Reference URLs]

・This research paper (2026): Physical Review Research

https://journals.aps.org/prresearch/abstract/10.1103/2b1s-z7t6

・Preceding research paper (2024): BMC Ecology and Evolution Official paper URL: https://bmcecolevol.biomedcentral.com/articles/10.1186/s12862-024-02302-6

・Easy-to-understand explanation of preceding research (Kyoto Sangyo University Press Release): URL:

https://www.kyoto-su.ac.jp/wr-news/20241004_345_release_ka01.html

About Ube National College of Technology

Ube National College of Technology, established in 1962 as one of the first national colleges of technology, is one of the oldest Kosen in Japan. Its curriculum, which balances general subjects with specialized subjects, fosters rich general knowledge and systematic specialized knowledge necessary for engineers.

Currently, the five-year "Undergraduate Program" has five departments (Department of Mechanical Engineering, Department of Electrical and Systems Engineering, Department of Control Engineering, Department of Materials Science and Engineering, Department of Management and Information Science), and the subsequent two-year "Advanced Program" has three majors (Department of Production Systems Engineering, Department of Materials Science and Engineering, Department of Management and Information Engineering).

In September Reiwa 8 (2026), a new information education building will be constructed, creating an environment that promotes information education in collaboration with universities and companies. Ube Kosen aims to cultivate human resources with "advanced specialized skills" and "information technology skills" who can play active roles in society.

Exterior view of Ube Kosen

[School Overview]

School Name: Ube National College of Technology, National Institute of Technology Organization

Location: 2-14-1 Tokiwadai, Ube City, Yamaguchi Prefecture

President: Junhiro Kawamura

Established: 1962

URL: https://www.ube-k.ac.jp/

Business Activities: Colleges of Technology / Higher Education Institutions