3D Recordings of Swimming Algae
Swimming microorganisms set up complex fluid flows that affect their ability to feed and communicate. Using advanced holographic methods, researchers have now imaged the entire 3D flow field around a swimming alga, revealing vortex rings that help propel the organism [1]. The researchers hope that the experiments will lead to improvements in measuring the energy expenditure and swimming strategies of a wide range of microorganisms.
The single-cell alga Chlamydomonas reinhardtii swims in a “breaststroke” style by beating its flagella—two hair-like appendages located at the front of its body—cycling 50 times per second. The flagella propel the organism forward while creating a surrounding fluid flow field that influences nutrient uptake and allows the organism to detect predators or mates. “The flow field generated by a swimming microorganism is one of its most fundamental characteristics,” says Xiang Cheng of the University of Minnesota. But he says that previous experiments have only captured partial details of this field, such as vortices to the left and right of the swimming organism. Researchers have speculated that these vortices might be connected in a larger coherent 3D flow pattern, but experiments have been unable to resolve such structures.
Now Cheng and colleagues have adapted high-speed digital holographic microscopy to produce a more complete picture. Following this technique, they used a laser to illuminate the alga in a suspension of tiny tracer particles and captured the interference patterns created as light scattered off the tracers. The holography technique allowed them to use these patterns to reconstruct the full 3D flow environment around the swimming alga.
The results show several striking features in the flow field. Foremost is the presence of micrometer-scale vortex rings—fluid flow structures that resemble smoke rings—surrounding the swimming alga. In previous 2D images, researchers had seen several vortices flanking the cell. The new data reveal that these 2D vortices were actually slices through the newly discovered vortex rings.
The data also show how these vortex rings evolve during the cycle of the flagella. After the flagella execute the first half of their breaststroke-style motion—moving backward to pull the alga forward (the power stroke)—vortex rings on either side of the cell break apart and then reconnect with new rings forming in front of and behind the cell. These new vortex rings persist as the flagella move forward to their initial positions (the recovery stroke). Cheng says that this continuous creation of new vortex rings is reminiscent of a phenomenon observed in larger-scale flows generated by flying birds and by swimming fish known as vortex shedding: As the flow moves across the animal’s body, vortices drift away and are constantly recreated.
Cheng and his colleagues ran simulations of the flows generated by the swimming microbe. The results convinced them that the vortex-ring evolution allows the alga to efficiently switch between the power stroke and the recovery stroke in each cycle. This kind of efficiency gain was previously seen only in much larger organisms like fish or insects.
“These are the first measurements of the fully three-dimensional flow around a freely swimming microorganism,” says Raymond Goldstein of Cambridge University in the UK, a mathematician and expert in biological physics. “This flow field should make possible more accurate estimates of the energy dissipation rate associated with the flagellar beating.”
Cheng expects other researchers to take advantage of this new adaptation of the holography technique. “We anticipate that our experimental approach can be applied to visualize the 3D fluid environment around a variety of microorganisms with different swimming gaits and flagellar or ciliary dynamics,” Cheng says.
–Mark Buchanan
Mark Buchanan is a freelance science writer who splits his time between Abergavenny, UK, and Notre Dame de Courson, France.
References
- G. Pradipta et al., “Seeing new depths: Three-dimensional flow of a free-swimming alga,” Phys. Rev. X 16, 021019 (2026).