Hidden Bacteria in Marine Snow May Be Dissolving Ocean Shells — and Disrupting Carbon Storage

Anyone who has ever dove in the ocean has seen tiny white specks drifting through the water like snow. This so-called marine snow is made up of sinking debris, fragments of dead plankton, pieces of organic matter and tiny mineral particles drifting in the water column. Marine snow also contains calcium carbonate, the mineral that forms the shells of many marine organisms.

In theory, this mineral should dissolve primarily in the deep ocean, where colder, more acidic waters break it down. But ocean measurements showed that calcium carbonate disappeared much closer to the surface.

A new study published in Proceedings of the National Academy of Sciences suggests that bacteria living in marine snow could be responsible.

This discovery could reshape how researchers understand the biological carbon pump, one of the ocean’s most important climate-regulating processes.

“Think of marine particles as the megacities of the ocean,” Benedict Borer, lead author of the study, said in a press release. “In these tiny spaces, there are enormous amounts of microbial activity. This is where the calcium carbonate dissolves.”

Learn more: 99.999 percent of the ocean’s depths are unexplored — its secrets are essential to understanding our planet

The role of Marine Snow in ocean carbon storage

On the ocean surface, microscopic algae called phytoplankton absorb carbon dioxide from the atmosphere during photosynthesis.

When these organisms die, their remains sink into the water column as marine snow, carrying carbon downward. Billions of tonnes of organic and mineral carbon pass through the ocean each year.

The deeper these particles go before dissolving or decomposing, the longer the carbon remains stored away from the atmosphere.

Eventually, the calcium carbonate dissolves in deeper waters, releasing carbon dioxide into the ocean and continuing the cycle.

But observations have shown that a surprising amount of this mineral is disappearing much sooner than expected, prompting researchers to search for an explanation.

Inside the marine snow particles

To investigate the mystery, Borer and his colleagues created an experiment designed to mimic what happens inside sinking marine particles.

Using a specially designed microfluidic chip, the team recreated miniature ocean conditions in the laboratory. The device allowed them to study marine particles containing calcite – a crystalline form of calcium carbonate – as well as the bacteria that naturally grow on these particles.

Artificial seawater was circulated through the system while researchers monitored temperature, oxygen levels and bacterial abundance.

As the bacteria grew, they breathed in carbon dioxide. This process made the small environment around the particles more acidic, which accelerated the dissolution of the calcite.

In other words, the microbes helped dissolve the minerals from the shells within the marine snow itself.

Bacteria could slow the sinking of carbon in the ocean

The discovery could have implications beyond these microscopic particles.

Calcium carbonate helps weigh down sinking debris, acting like tiny weights that pull marine snow toward the ocean depths. If bacteria dissolve this mineral earlier in the journey, the particles can flow more slowly.

Slower sinking means more time for carbon to be recycled near the surface instead of being stored in deep water.

Since bacterial respiration also releases carbon dioxide, the process could potentially return some of that carbon to the atmosphere.

“Oceanographers often think on the macro scale, but in this case, what happens in the microscopic particles controls the entire ocean,” Borer said in the press release.

The researchers say additional work will be needed to confirm the extent of this mechanism in the ocean. But the study highlights how processes taking place inside tiny drifting particles could influence how the ocean stores carbon and how much of it ends up returning to the atmosphere.

Learn more: The Atlantic Ocean could have its own Grand Canyon – and it could be even bigger

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