Bacteria riding on sinking ocean particles can erode the mineral ballast that helps those particles descend, slowing the delivery of carbon to the deep sea and potentially weakening one of the planet’s largest natural carbon sinks. A new study published in the Proceedings of the National Academy of Sciences details how microbial respiration creates acidic microzones around marine snow, dissolving calcium carbonate shells even in waters where that mineral should remain stable. The findings challenge standard assumptions built into climate models about how efficiently the ocean buries atmospheric carbon.
How Bacteria Strip the Ballast From Sinking Particles
Marine snow, the constant rain of dead plankton, fecal pellets, and organic debris drifting from sunlit surface waters toward the seafloor, depends on dense mineral components to sink quickly. Calcium carbonate shells from organisms like coccolithophores act as ballast, weighting down these fragile aggregates so they can carry organic carbon past the upper ocean before microbes consume it all. The new PNAS paper used a microfluidic platform to simulate a sinking particle loaded with calcite and live bacteria. According to the study, bacterial respiration drives local acidification that dissolves calcite even in supersaturated upper-ocean conditions, where chemistry alone would not break down the mineral.
That distinction matters. Standard ocean chemistry predicts that calcite should dissolve only below a certain depth, known as the lysocline, where surrounding water becomes corrosive enough. Yet oceanographers have long observed significant calcium carbonate loss well above that boundary. The Rutgers University team behind the microfluidic experiments framed this as an oceanographic mystery: calcite dissolving in waters that should preserve it. Their lab work points to biology, not just chemistry, as the explanation. When bacteria colonize a particle and respire, they release CO2 into the thin film of water surrounding the grain, dropping pH locally and eating away at the protective shell.
The experiments also highlight just how small-scale these processes are. The acidic microzones form within tens to hundreds of micrometers around individual grains of calcite embedded in organic matter. From the perspective of large-scale carbon models, that level of detail is invisible, yet it can determine whether a given particle retains enough ballast to punch through the upper ocean or instead lingers where remineralization is most intense.
Independent work on microbial ecology supports this picture of tightly coupled interactions between minerals and microbes. A review of particle-associated microbes emphasizes that bacteria living on aggregates experience very different chemical conditions from those in the surrounding seawater, including steep gradients in oxygen and pH. The PNAS study effectively turns that conceptual framework into a quantifiable mechanism for enhanced calcite dissolution.
Mucus, Biogels, and the Drag Problem
Calcite erosion is not the only way microbes slow the descent of carbon-rich particles. A separate line of research shows that biological coatings physically increase drag on sinking aggregates. Experiments with marine snow analogs demonstrate that biofilm-forming bacteria and the sticky biogels they produce reduce sinking speeds by altering particle shape and surface properties. As bacterial communities grow on the surface, they exude polymers that turn compact clumps into more irregular, fluffy structures.
These sticky exudates do double duty. They help bacteria adhere to particles and access organic substrates, but they also increase the effective cross-sectional area facing the flow as particles fall, boosting drag. In some experiments, the presence of microbial biogels slowed sinking by tens of percent compared with sterile controls. Over the hundreds of meters that separate the surface from the twilight zone, that slowdown translates into many extra days of exposure to hungry microbes and zooplankton.
Observations in the open ocean echo these laboratory findings. Imaging of natural aggregates has revealed marine snow dressed in mucus and trailing filamentous material, which deforms into tail-like structures as particles sink. Those tails act like parachutes, further retarding descent. The net effect is that biology not only strips away ballast but also reshapes the remaining organic matter into less streamlined forms.
The combined effect is a kind of double brake. Bacteria remove dense mineral components that make particles heavy, while biogels add drag that keeps the lighter aggregates suspended longer. Both mechanisms extend the time a particle spends in the upper water column, where heterotrophic bacteria consume organic matter and convert it back to dissolved CO2. Microbial scavengers form an invisible cloud that follows marine snow as it moves through the water column, breaking it into ever smaller carbon-containing pieces and increasing the total surface area available for further degradation.
Pressure Adds Another Layer of Complexity
The story does not end in the sunlit zone. As particles sink deeper, rising hydrostatic pressure reshapes microbial communities and their metabolic behavior. Experiments that exposed natural aggregates to in situ pressures found that compression alters microbial respiration and degradation processes on marine snow, changing how much carbon is remineralized versus exported to the deep ocean. Some taxa become more active under high pressure, while others are inhibited, shifting the balance of metabolic pathways.
A related study of sinking organic particles showed that marine snow begins to leak dissolved carbon and nitrogen once it reaches depths where pressure is intense, providing deep-sea microbes with an unexpected energy boost. Rather than arriving as sealed packages of particulate carbon, many aggregates are partially unpacked en route, bleeding dissolved organic compounds that can be consumed long before the particles reach the seafloor.
This means that even particles heavy enough to escape the upper ocean face continued attrition on the way down. Particulate organic matter is the major form in which carbon fixed at the surface reaches the deep, but microbial activity at every depth horizon chips away at that cargo. A modeling study in Nature Communications showed that heterotrophic microbial dynamics on sinking particles shape how particulate organic carbon flux attenuates with depth, linking microscale processes to large-scale export patterns. In that framework, small changes in microbial growth rates or attachment behavior can substantially alter how much carbon ultimately escapes back to the atmosphere on climate-relevant timescales.
Why Current Carbon Models May Be Too Optimistic
Most Earth system models treat the biological carbon pump as a largely physical and chemical conveyor belt: particles form, they sink, and gravity does the work. Microbial degradation is typically represented as a simple decay term that reduces particulate carbon with depth, without explicitly capturing the feedback loops now emerging from laboratory and field studies.
The new findings suggest that this approach may systematically overestimate how efficiently the ocean sequesters carbon. If bacteria actively dissolve the very mineral that makes particles sink fast, and if biological coatings reshape aggregates in ways that slow their descent, then the effective transfer of carbon to the deep sea is more fragile than many models assume. Instead of a straightforward rain of organic matter, the biological pump looks more like a gauntlet of microbial filters that progressively strip away carbon at each stage of the journey.
Incorporating these mechanisms into global models will not be simple. Processes such as micro-scale acidification around calcite grains, formation of drag-enhancing biogels, and pressure-dependent shifts in microbial metabolism all operate at scales far smaller than the grid cells of climate simulations. Yet their cumulative impact over vast ocean basins and over decades to centuries could be large.
One path forward is to treat particle-associated microbes and their interactions with minerals as an explicit sub-grid component, constrained by studies that quantify how factors like respiration rate, particle size, and mineral content influence dissolution and sinking speed. Another is to use emerging observations from autonomous platforms and high-resolution imaging systems to benchmark how quickly particulate carbon actually disappears with depth in different ocean regions, then adjust model parameterizations accordingly.
For policymakers and planners, the message is not that the ocean will suddenly stop absorbing human-generated CO2, but that the capacity of the biological pump is more dynamic, and more sensitive to ecological change, than previously appreciated. Shifts in plankton communities, nutrient supply, or ocean stratification could reverberate through the microbial machinery that governs particle sinking, altering the long-term balance between carbon storage in the deep sea and return to the atmosphere. Understanding and modeling those links will be essential for reliable projections of future climate.
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*This article was researched with the help of AI, with human editors creating the final content.
