E+E Leader Team
Marine fish may be among the most overlooked contributors to the ocean’s carbon cycle. A new study led by researchers at the University of Miami offers the first direct evidence that fish living at mesopelagic depths—200 to 1,000 meters below the ocean surface—excrete biogenic carbonate, a key component of the marine inorganic carbon cycle.
Published in the Journal of Experimental Biology, the research focuses on the blackbelly rosefish (Helicolenus dactylopterus), a deep-sea species that survives capture and lab acclimation. Scientists found that this fish, residing at roughly 400 meters depth, produces intestinal carbonate at rates similar to those of shallow-water species, confirming a long-standing but untested assumption in ocean carbon modeling.
Fish Carbonate: A Deep Dive Into Ichthyocarbonate
Ichthyocarbonate—carbonate precipitated in the guts of marine fish as part of their osmoregulation—is critical for absorbing seawater and eliminating excess ions. These particles, expelled into the ocean, sink and dissolve, influencing both surface and deep ocean chemistry.
The study found that blackbelly rosefish excrete ichthyocarbonate at a rate of approximately 5 mg/kg/hour—consistent with predictions based on fish size and temperature. Their metabolic rates, however, were lower than anticipated, suggesting that even low-energy deep-sea fish may contribute more to global carbonate budgets than previously thought.
Summarized by Dr. Martin Grosell, Chair of Ichthyology at the University of Miami,
Our observations support assumptions of ichthyocarbonate excretion by mesopelagic fishes, and suggest that thermal and allometric relationships for shallow water species extend to deep populations.
Implications for the Global Carbon Cycle
Mesopelagic fishes make up an estimated 94% of global fish biomass. Until now, their contribution to carbonate production—let alone its composition, structure, and fate—had been largely speculative. With confirmation that fish at these depths produce and excrete carbonate similar in morphology and mineral content to shallow species, a major uncertainty in carbon cycle modeling has been addressed.
Notably:
- Composition: The carbonate was high-magnesium calcite and amorphous carbonate, matching those found in warmer, shallow species.
- Stability: The carbonate dissolved faster at cold temperatures, meaning more is likely to reach deeper ocean layers where long-term carbon sequestration occurs.
- Origin: Up to 52% of the carbonate’s carbon came from dietary sources, transporting organic carbon to deeper waters—a key part of the biological pump.
This discovery elevates the role of mesopelagic fish in transferring both inorganic and organic carbon from the surface to depths where it can be locked away for centuries.
Challenges and Next Steps
Capturing and studying deep-sea fish without compromising their physiology remains difficult. The blackbelly rosefish lacks a swim bladder, enabling successful retrieval and lab monitoring. But it’s a benthic (non-migratory) species, and most mesopelagic fish migrate vertically by hundreds of meters each day, likely increasing their carbonate production through elevated metabolic activity.
Future research is needed to:
- Measure ichthyocarbonate production in vertically migrating species
- Determine whether fish excrete carbonate at the shallow or deep end of their migrations
- Refine global carbon cycle models with these new insights
Why This Matters
Carbonate produced by marine organisms—including corals, plankton, and now fish—helps buffer ocean acidity, influence deep-sea alkalinity, and control how much CO₂ the ocean can store. This study shows that fish—often overlooked in climate models—are potentially major drivers of oceanic carbon dynamics, particularly at depth.
For climate scientists, oceanographers, and environmental modelers, incorporating this revised understanding of biogenic carbonate production into global carbon budgets could improve projections of carbon sequestration and ocean acidification resilience.