New concept explains how tiny particles navigate water layers, with implications for marine conservation

A new UBC study published recently in Proceedings of the National Academy of Science has unveiled insights into how microscopic organisms such as marine plankton move through water with different density layers.

Researchers Gwynn Elfring and Vaseem Shaik found that density layers, created by variations in temperature or salinity, influence the swimming direction and speed of tiny particles navigating a liquid.

Pushers and pullers

"There are two different types of microscopic swimmers—pullers and pushers—and they navigate density gradients differently," explained Dr. Elfring, a professor at UBC's faculty of applied science who studies fluid mechanics.

"Pullers create thrust at the front of their bodies and align their swimming direction with the density gradient, moving parallel to it. In contrast, pushers, generating thrust at the back, swim perpendicular to these gradients—navigating through the layers rather than along them."

Dr. Elfring compares the concept to someone swimming in a pool: "Imagine swimming where the top layer of water is warm, and the bottom layer is cold. Just as you might feel different resistance and buoyancy at different depths, tiny swimmers in natural water bodies experience similar effects due to changes in water density—the 'density gradient' changes the way organisms move through the water."

Densitaxis and ocean migrations

The researchers named this dynamic densitaxis—a portmanteau of "density" and the ancient Greek word taxis, which means arrangement.

Dr. Elfring believes densitaxis can help scientists understand how organisms of different sizes move through their environment.

Marine biologists, for example, may gain insights into the movements of various marine organisms, from tiny plankton to larger marine animals. Ocean water density changes with depth due to variations in temperature and salinity, affecting how organisms find food, avoid predators, and migrate.

Some marine organisms, like krill and plankton, perform vertical migrations in search of food. The study suggests that pullers might find it easier to navigate these density layers, aiding their vertical movement. Conversely, pushers might face more challenges, potentially affecting their feeding and migration patterns.

Climate change impact

Researchers hope the study informs future work aimed at predicting changes in marine ecosystems due to climate change.

"Global warming has increased density stratification in oceans, creating more pronounced layers. This change affects how marine organisms move and behave, potentially disrupting feeding patterns and migration routes. By studying how tiny swimmers interact with these density gradients, scientists can better predict the effects of climate change on marine ecosystems and develop appropriate conservation strategies," said Dr. Elfring.

The findings also have practical applications in technology and industry. The ability to manipulate density gradients could be used to sort and organize tiny particles or organisms in laboratory settings. This could be valuable for scientific research, medical applications, and industrial processes where precise control of particle movement is essential.

"As a fluid mechanics researcher, I'm confident that understanding the mechanics of fluids is valuable for understanding the movement of living organisms. I hope the current paper contributes to new insights in biological work and technology development," said Dr. Elfring.