Blackbelly rosefish have been kept alive under conditions that resemble the deep Atlantic in a lab at the University of Miami’s Rosenstiel School, a research facility situated along the edge of Biscayne Bay, where the water changes from green to deep blue within a few miles. 6 degrees Celsius is cold water. elevated pressure. Almost dark. The fish, which were extracted from depths of 350 to 430 meters, appear unremarkable. Spiny, reddish, and visually unremarkable. However, what scientists discovered within them is subtly changing the way ocean chemists view the planet’s carbon budget.
Carbonate minerals are expelled from the fish’s intestines. It has long been known that shallow-water species undergo this process, known as ichthyocarbonate production, but deep-sea fish were thought to either not engage in it or to do so at rates too low to be significant. The blackbelly rosefish excreted about 5 milligrams of ichthyocarbonate per kilogram per hour, according to the Miami study, which was published in the Journal of Experimental Biology in July 2025. That nearly perfectly aligns with predictions from global metabolic models. It turns out that depth doesn’t alter the chemistry in the way that was anticipated. In the dark, the fish simply continue to produce in silence.
This finding’s implied scale is what makes it truly unusual and significant. Up to 94% of the world’s fish biomass is made up of mesopelagic fish, a broad category of deep-dwelling species that includes these rosefish. It’s worth stopping to consider that number. The majority of fish in the ocean reside in the midwater zone, where they are mostly uncounted, infrequently studied, and virtually absent from the climate models used by governments and academic institutions to forecast future atmospheric carbon levels. According to Martin Grosell, the lead author of the study and chair of Rosenstiel’s Marine Biology and Ecology department, these fish are more than just prey—they are the ocean’s chemical engineers. Up until now, that framing had been mostly absent from the discussion.
Beyond the production rate, the mechanism is important. Because ichthyocarbonate is denser than other biogenic carbonates, it dissolves more slowly in colder, deeper water and sinks more quickly. Because of this, it has a longer half-life in the carbon sequestration process than lighter carbonate materials. Some estimates suggest fish-produced carbonates capture a meaningfully higher proportion of dietary carbon than other marine sources — locking it away in deep-sea sediments rather than letting it cycle back toward the surface. Fish guts are contributing to the ocean’s alkalinity, or its ability to buffer against atmospheric CO2, which has not been taken into account by conventional Earth system models. There is a gap there. An actual one.
The implications might go even farther. A different team’s study, which examined the biological pump more generally, discovered that since 1950, commercial fishing has cut fish carbon sequestration by almost half. Fish removal affects more than just protein in a food web, especially for migratory mesopelagic species that move vertically through the water column. A portion of the carbon transport system is eliminated. Their respiration, excretions, and the transfer of carbon from the surface to the depth all decrease. Scientists were unaware that the fishing industry was unintentionally destroying a climate mechanism.
After reading this research, one feels that the ocean has been subtly undervalued for a very long time. Not in the obvious ways, as everyone is already aware of how much CO2 the ocean absorbs. However, in particular, fine-grained, biological ways. The USC Dornsife team published complementary work earlier this year showing that marine microbes, similarly overlooked, operate through functional metabolic strategies that current climate models don’t properly capture. Around the same time, two distinct lines of research emerged that both pointed to the same issue: too many workers are missing from the models. Fish. Microbes. Things living in the dark, doing chemistry on a planetary scale, largely invisible to the instruments scientists have been relying on. It’s still unclear how quickly Earth system models can be updated to incorporate these findings, or whether the adjustments will change near-term climate projections in any meaningful way. But the direction of the correction seems clear enough. The ocean has more going on than the models have been showing — and the fish, of all things, are part of the reason why.
