Deep Fish Reveal Ocean Carbon Secrets

Massive Deep-Sea Populations Mimic Shallow Species in Crucial Carbon Process

A groundbreaking study has provided the first direct evidence that fish inhabiting the ocean’s twilight zone, the mesopelagic zone, contribute to carbonate mineral excretion at rates previously only observed in shallower waters. This finding validates global models underscoring the significant role of marine fish in producing biogenic carbonates.

Unveiling Mesopelagic Ichthyocarbonate

Researchers from the University of Miami Rosenstiel School of Marine, Atmospheric, and Earth Science focused on the blackbelly rosefish (Helicolenus dactylopterus), a species found at depths between 350 and 430 meters. Their investigation aimed to determine if these deep-dwellers produce and excrete intestinal carbonates, a process known as ichthyocarbonate. This physiological function is vital for maintaining internal salt and water balance in marine fish and plays a critical role in the ocean’s carbon cycle.

“Mesopelagic fish live in deep, cold, high-pressure environments, and until now, it was unclear if they produced carbonate like shallow water fish do — or at what rate,” stated Martin Grosell, the lead author and chair of the Department of Marine Biology and Ecology at the Rosenstiel School. “This study is the first to confirm that they do and that the mechanisms and characteristics of ichthyocarbonate formation are remarkably consistent across depths.”

Consistent Carbon Excretion Across Depths

The blackbelly rosefish proved an ideal subject due to its lack of a swim bladder, allowing for survival during capture and laboratory acclimation. Maintained in conditions mimicking their natural habitat at 6 degrees Celsius, these fish excreted approximately 5 milligrams of ichthyocarbonate per kilogram per hour. This rate aligns with predictions from thermal and metabolic scaling models.

Filling a Critical Gap in Ocean Chemistry Understanding

“This research fills a major gap in our understanding of ocean chemistry and carbon cycling,” commented co-author Amanda Oehlert, an assistant professor in the Department of Marine Geosciences. “With mesopelagic fish playing such a significant role, their contribution to carbonate flux — and how it might change with warming oceans — deserves greater attention.”

Key Study Outcomes

• Deep-sea blackbelly rosefish exhibit carbonate production rates and compositions similar to their shallow-water counterparts, confirming that environmental depth and pressure do not impede ichthyocarbonate formation.
• These results reinforce global estimates of fish-generated carbonate production, establishing mesopelagic species as substantial contributors to the ocean’s overall carbonate budget.
• The uniformity of ichthyocarbonate composition, irrespective of formation depth, offers insights into its ocean storage and dissolution patterns.

Martin Grosell further emphasized the study’s significance, noting, “These results offer strong support for global models of fish-derived carbonate production, which had assumed — but not verified — that mesopelagic species contribute at similar rates. Mesopelagic fish aren’t just prey; they’re chemical engineers of the ocean.” The study highlights the crucial, yet often overlooked, contribution of the vast, largely unexplored mesopelagic biomass to the ocean carbon cycle through ichthyocarbonate.

The research opens new avenues for investigating deep-sea carbon dynamics and could enhance the accuracy of Earth system models. These sophisticated models integrate the complex interplay of physical, chemical, and biological processes influencing carbon production and export.

Published on July 15, 2025, in the *Journal of Experimental Biology*, the study, titled “Osmoregulation by the gastro-intestinal tract of marine fish at depth — implications for the global carbon cycle,” was authored by Martin Grosell, Bret Marek, Sarah Wells, Carolyn Pope, Cameron Sam, Rachael M. Heuer, and Amanda M. Oehlert, all affiliated with the University of Miami Rosenstiel School of Marine, Atmospheric, and Earth Science.

Financial backing for this research was provided by the National Science Foundation’s Chemical Oceanography Program and Earth Sciences Instrumentation and Facilities, alongside contributions from the University of Miami Rosenstiel School’s Departments of Marine Biology and Ecology and Marine Geosciences.