Deep below the ocean’s surface, in cold, dark waters where pressure can crush the unprepared, a surprising contributor to Earth’s carbon cycle has been quietly at work. New research confirms that deep-sea fish—specifically, the blackbelly rosefish—are not only surviving in these harsh conditions but are also producing carbonate minerals at rates comparable to their shallow-water cousins.
The finding provides the first direct evidence that mesopelagic fish, which make up as much as 94% of the world’s fish biomass, excrete carbonate material through their guts, helping regulate ocean chemistry on a global scale.
“This study is the first to confirm that they do and that the mechanisms and characteristics of ichthyocarbonate formation are remarkably consistent across depths,” says Martin Grosell, a marine biologist at the University of Miami and lead author of the new work.
For years, scientists had suspected that fish played a major role in the production of marine carbonate—solid particles that affect everything from ocean acidity to the formation of deep-sea sediments. But while the role of surface-dwelling species was established, it was unclear whether the same could be said for the fish that inhabit the deep middle layer of the ocean known as the mesopelagic zone.
These deep-sea dwellers face extreme pressures, frigid temperatures, and limited food. They are also notoriously difficult to study. Most mesopelagic species are small, delicate, and unable to survive capture. That’s where the blackbelly rosefish came in. This hardy species lacks a swim bladder, which makes it unusually tolerant of sudden pressure changes, allowing scientists to collect live specimens from 400 meters down and keep them alive for weeks in 6°C seawater.
“It fills a major gap in our understanding of ocean chemistry and carbon cycling,” says Amanda Oehlert, a co-author of the study.
In the lab, the researchers measured how much carbonate the rosefish excreted. The answer: about 5 milligrams per kilogram per hour—almost exactly what metabolic scaling models had predicted based on temperature and body size. And the carbonate itself, known as ichthyocarbonate, turned out to be chemically and structurally similar to that produced by shallower fish, despite being formed under high-pressure, low-temperature conditions.
In other words, the rules that govern carbonate production in tropical reef fish also seem to apply to those living in the ocean twilight.
“This validates previous global models of fish-derived carbonate production,” Grosell says. “Mesopelagic fish aren’t just prey; they’re chemical engineers of the ocean.”
The discovery carries broad implications. Carbonate particles produced in fish guts sink through the ocean, influencing both the global carbon cycle and the ocean’s ability to store carbon long-term. Understanding how much carbonate is produced, and how fast it dissolves at different depths, is key to predicting how oceans will respond to climate change.
Until now, estimates of deep-sea ichthyocarbonate production were based mostly on assumptions. This new data strengthens the case that mesopelagic fish contribute significantly to carbonate flux—possibly enough to affect models of Earth’s carbon budget.
The researchers also found that the mineral particles produced by the rosefish were more soluble than those from warmer, shallower species. This means the carbonate might dissolve sooner as it sinks, altering where in the ocean it delivers carbon.
Despite their tiny size, mesopelagic fish represent a vast, underexplored biomass. Many species perform daily vertical migrations—rising at night to feed near the surface, then diving again by day—which may further influence how they contribute to carbon cycling.
The new study, the first to isolate and characterize ichthyocarbonate from mesopelagic depths, opens the door to more accurate modeling of ocean carbon dynamics. It may also prompt scientists to look more closely at the overlooked ecosystems of the deep ocean.
“These results offer strong support for global models… which had assumed—but not verified—that mesopelagic species contribute at similar rates,” Grosell says.
As researchers expand their efforts to study the deep sea, the blackbelly rosefish reminds us that some of the most important players in Earth’s climate system may be swimming far below the radar.
The study has been published in the Journal of Experimental Biology.
