Bacteria riding marine snow dissolve the ballast that helps the ocean store carbon

Proceedings of the National Academy of Sciences, March 2026

Billions of tons of carbon drift downward through the ocean each year, carried by a constant shower of dead organisms and organic debris that oceanographers call marine snow. If these particles sink deep enough, the carbon they carry gets locked away for centuries. The whole system works, in part, because dense minerals like calcium carbonate weigh the particles down, acting as ballast.

But something has been dissolving that ballast in waters where it should remain intact. Scientists have observed dissolved calcium carbonate in the upper ocean for years without a clear explanation. The chemistry of shallow seawater should not dissolve it. Now, a team at MIT and collaborating institutions has identified the culprit: bacteria.

Cheeseburgers for microbes

The study, published in the Proceedings of the National Academy of Sciences, demonstrates that bacteria hitching rides on marine snow particles actively erode the calcium carbonate that helps those particles sink. The mechanism is straightforward: as bacteria feed on the organic material in the particles, they produce acidic waste products. Those acids dissolve the surrounding calcium carbonate, stripping away the ballast that enables sinking.

Associate professor Andrew Babbin of MIT's Department of Earth, Atmospheric and Planetary Sciences offered a vivid analogy: the ocean is a dilute medium with respect to organic matter, and particles of marine snow are like cheeseburgers for bacteria. The microbes actively seek out and colonize these particles, creating localized acidic microenvironments that dissolve minerals even when the surrounding seawater chemistry says they should not.

This microscale process, occurring within individual particles, appears to control what scientists had previously tried to explain through macroscale ocean chemistry. The disconnect between expected and observed calcium carbonate dissolution in shallow waters had been a persistent puzzle. The bacterial explanation resolves it.

A sweet spot for destruction

To study the process in controlled conditions, lead author Benedict Borer (then an MIT postdoc, now at Rutgers University) and colleagues built a microfluidic chip that simulated a sinking marine snow particle. They synthesized particles containing varying concentrations of calcium carbonate and bacteria, then flowed seawater across them at different rates to simulate different sinking speeds.

The results revealed a counterintuitive relationship between sinking speed and calcium carbonate dissolution. Very slow sinking limited dissolution because the bacteria were not getting enough oxygen from the surrounding water. Very fast sinking also limited dissolution because acidic waste products were flushed away before they could do damage.

At intermediate speeds, conditions were optimal for bacterial destruction of ballast. The bacteria received enough oxygen to stay active, and acidic waste accumulated around the particle long enough to dissolve significant calcium carbonate. This sweet spot corresponds to the sinking speeds of many real marine snow particles in the upper ocean.

Carbon sequestration at risk

The implications for climate science are significant. Marine snow is the primary vehicle of the ocean's biological carbon pump, the process that moves carbon from the surface, where phytoplankton absorb it from the atmosphere, to the deep ocean, where it can be stored for hundreds to thousands of years. Anything that slows the sinking of marine snow particles reduces the efficiency of this pump.

By dissolving calcium carbonate ballast, bacteria make particles lighter and slower. Slower particles spend more time in shallow water, where they are more likely to be decomposed entirely, releasing their carbon back into the upper ocean and potentially back into the atmosphere. The biological carbon pump, in effect, springs a leak.

This matters particularly as researchers explore climate solutions that involve enhancing the ocean's capacity to sequester carbon. If bacteria are working against the biological pump more aggressively than models assume, the effectiveness of ocean-based carbon removal strategies could be overestimated.

Open questions in open water

The study was conducted in laboratory conditions using synthetic particles and controlled flows. Whether the effects scale up to match real-world ocean conditions remains to be confirmed through field studies. The ocean's complexity, including variations in temperature, pressure, bacterial communities, and particle composition, could amplify or dampen the laboratory findings.

Borer, now establishing his research program at Rutgers, noted a troubling uncertainty: the biological carbon pump could become either more or less efficient in future climate scenarios. Whether warming oceans increase bacterial activity enough to accelerate ballast dissolution, or whether other factors compensate, is not yet clear.