The slowing effect of invisible comet tails of mucus on sinking flakes of ‘marine snow’

New observations suggest that the presence of goop-like mucus surrounding tiny detritus flakes in the ocean slows their descent. Reported by Rahul Chajwa, a physicist at Stanford University, at the American Physical Society’s Division of Fluid Dynamics meeting on November 19, this discovery might impact our understanding of carbon sequestration processes in the ocean and consequently, global climate change.

The unseen mucus forms "comet tails" that envelop each detritus flake, thereby reducing the speed at which these flakes sink. This phenomenon is a part of "marine snow", but until now, scientists had been unable to quantify its impact on the sinking speed.

The term "marine snow" refers to the mix of decaying organic matter, dead and living phytoplankton, bacteria, feces, and other aquatic debris, bound by mucus secreted by the organisms, mirroring the sticky substance known to obstruct the respiratory tract during viral seasons. The mucus is classified as a viscoelastic fluid, exhibiting properties of both liquids and solids.

Studying this underwater phenomenon is particularly challenging - the particles sink rapidly and become invisible to the naked eye when in the ocean, while bringing them onshore for further inspection compromises their integrity and can cause harm to the living organisms within the marine snow.

To address this problem, Chajwa and his team set up a unique physics lab at sea. They collected marine snow particles from traps positioned 80 meters below the surface of the water in the Gulf of Maine. These particles were then observed falling within a specially designed apparatus aboard their research vessel.

The apparatus, known as "the gravity machine", is a fluid-filled wheel that's continuously rotated to keep a single flake within the view of a camera — akin to a hamster wheel for sinking debris. Each rotation of the wheel effectively offsets the sinking snow playing out in reverse, enabling an indefinite study of the snowfall. The gravity machine is mounted on a gimbal to counter the swaying motion of the ship.

Anupam Sengupta, a biophysicist at the University of Luxembourg who wasn't part of the study, praised the innovative approach of combining both lab and field conditions to study marine snow.

To measure fluid flow around the sinking particles, the researchers added tiny beads to the fluid within the gravity machine. The speed of the fluid was significantly reduced within a comet tail-shaped region around the particle, indicative of the sunken invisible mucus surrounding it.

The team discovered that the particles could sink up to 200 meters per day, with the mucus affecting the pace significantly — Chajwa noted, "The more the mucus, the slower the particles sink". The team found that, on average, the mucus presence makes marine snow particles linger twice as long as they would in the top 100 meters of the ocean.

Marine snow that sinks deep enough can effectively sequester carbon from the atmosphere. Phytoplankton in marine snow absorb carbon dioxide and emit oxygen. If flakes manage to reach the ocean floor, they can sequester carbon over extended periods. Therefore, understanding how fast these particles sink is crucial for calculating the impact of oceans on the Earth's climate.

Oceans, being significant contributors to the planetary carbon cycle, have absorbed an estimated 30% of the carbon dioxide released since industrialization. The research team hopes that their findings will help improve climate model accuracy, which currently do not account for the presence of mucus.

Highlighting the crucial role of this microscopic mucus, coauthor of the study and Stanford physicist Manu Prakash emphasizes, '“We’re talking about microscopic physics, but multiply that by the volume of the ocean … that’s what gives you the scale of the problem.'"