In the ocean’s marine ‘snow,’ a scientist seeks clues to future climate

(Press-News.org) As any diver knows, oceans can be cloudy places. Even on sunny days, snow-like particles drift through the water column, obscuring the aquatic world below.

Scientists have long known that this “marine snow” carries inorganic calcium carbonate – the building block of shells – but couldn’t explain how the mineral dissolves in the upper part of the ocean.

New research from Rutgers University-New Brunswick points to the culprit: bacteria.

“Think of marine particles as the megacities of the ocean,” said Benedict Borer, an assistant professor of marine and coastal sciences at the Rutgers School of Environmental and Biological Sciences and lead author of the study published in the journal Proceedings of the National Academy of Sciences. “Within these tiny spaces, there are huge amounts of microbial activity. It’s here where calcium carbonate dissolves.”

The findings could reshape how climate scientists model carbon sequestration – the natural or engineered process by which carbon dioxide gas is removed from the atmosphere – and ocean carbon cycling (the exchange of carbon between the atmosphere and the ocean), Borer said.

“Oceanographers often think about the macro-scale, but in this instance, what’s happening in microscopic particles is controlling the entire ocean,” he said. 

Oceans are central to the planet's biological carbon pump. At the surface, microscopic algae called phytoplankton absorb carbon dioxide from the atmosphere – including that released by the burning of fossil fuels – and convert it into biomass and, in the case of a phytoplankton called coccolithophores, calcium carbonate shells.

When marine organisms die and sink, billions of tons of organic and inorganic carbon are carried downward each year. The deeper the carbon sinks, the longer it is stored. Eventually, in the cold, acidic depths, calcium carbonate dissolves, carbon dioxide is released, and the cycle continues.

However, while oceanographers have long known that calcium carbonate dissolves in the upper few thousand meters of the ocean, they could not explain the mechanism. The chemistry doesn’t favor it, Borer said.

Recent studies have provided clues, showing that acidic microenvironments in the guts of zooplankton enhance calcium carbonate dissolution, and suggesting that the interiors of marine snow particles may be additional hotspots for calcite dissolution, the crystalline form of calcium carbonate.

To test this theory, Borer and colleagues at the Massachusetts Institute of Technology and Woods Hole Oceanographic Institution studied how the chemistry of marine snow behaves in shallow seas.

In the lab, Borer built a three-layer microfluidic chip to mimic marine snow sinking through the water column. The middle layer held marine particles with calcite and marine bacteria. The top and bottom layers sealed the system, while artificial seawater flowed through the narrow channel between them, simulating particle sinking.

By controlling gas pressure, temperature, oxygen, and bacterial abundance, the team recreated the conditions within a sinking particle and measured how bacterial growth affected calcite.

As particles settled, bacterial respiration increased acidity around them, accelerating calcite dissolution. As a critical consequence, less calcite acting as ballast means that particles sink more slowly.

The results suggest that microbially driven changes in marine snow may dissolve enough calcite near the surface to slow sinking rates and reduce the efficiency of carbon sequestration. And because growing bacteria release carbon dioxide as a byproduct, the process may accelerate the return of heat-trapping gases to the atmosphere, Borer said.

More work is needed to confirm the findings in the open ocean, but the discovery clarifies bacteria’s role in carbon cycling and could improve future climate models and inform geoengineering approaches, he said.

“Our results provide a critical first step to decipher the influence of microbial-enhanced calcite dissolution in marine snow particles, and how it impacts the ocean's ability to sequester carbon at the global scale,” Borer said.

He added: “The question now is how the biological carbon pump will change in the future. Will the transport of carbon to depth become more efficient, or will bacteria respire the carbon more quickly, releasing carbon dioxide back into the atmosphere? To predict this, we need to understand all mechanisms that impact carbon transport to depth, such as the microbially enhanced dissolution of ballasting calcite. What I find quite scary, honestly, is that this process could go either way.”

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