Mesopelagic fish, living hundreds of meters below the surface, play a quiet but powerful role in carbon cycling.
A new study confirms that deep-sea fish excrete carbonate minerals, just like their shallow-water relatives. Their massive biomass and physiological traits make them a critical but overlooked part of ocean chemistry.
Researchers from the University of Miami Rosenstiel School studied the blackbelly rosefish, a deep-sea species that tolerates lab conditions. They confirmed it produces ichthyocarbonate – a mineral excreted by the intestines to maintain salt and water balance.
“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,” said Martin Grosell, lead author of the study.
“This study is the first to confirm that they do and that the mechanisms and characteristics of ichthyocarbonate formation are remarkably consistent across depths.”
Deep-sea fish produce carbonate
At 6°C, mirroring its native depth, the blackbelly rosefish excreted carbonate at about 5 mg/kg/hour. This aligns with predictions from previous models that connect metabolism, temperature, and depth.
The study also tested how carbonate formation resists change. Even when held in laboratory settings far from their original pressure conditions, the fish continued producing carbonate. This suggests the process is robust and not sensitive to depth-induced pressure changes.
Carbonate excretion comes from the gastrointestinal tract. It’s driven by ion transporters that move bicarbonate into the gut. There, it reacts with calcium and magnesium to form solid minerals. These are expelled into seawater and can later dissolve or sink.
Deep fish and carbon cycling
The researchers examined the carbonate composition of the blackbelly rosefish. They found that it matched the mineral makeup found in shallow-water species. Magnesium-rich calcite dominated, with minor aragonite and other forms. These materials dissolve at different rates, affecting their fate in the water column.
The uniformity suggests that regardless of where fish live, their carbonate waste behaves similarly once expelled. This means deep-sea fish can contribute to upper-ocean carbonate chemistry even if they dwell much deeper.
“This research fills a major gap in our understanding of ocean chemistry and carbon cycling,” said Amanda Oehlert, co-author and assistant professor.
“With mesopelagic fish playing such a significant role, their contribution to carbonate flux – and how it might change with warming oceans – deserves greater attention.”
Chemical engineers of the ocean
By verifying carbonate production in mesopelagic fish, this work supports broader carbon flux models. Until now, those models included deep fish as contributors, but lacked direct measurements. This new data gives them firmer footing.
“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,” Grosell said. “Mesopelagic fish aren’t just prey; they’re chemical engineers of the ocean.”
The study also suggests that mesopelagic fish could influence carbon export. Their carbonates may either dissolve near the surface or fall deeper, adding to seafloor sediments.
Implications for carbon budgets
Carbonates affect seawater alkalinity and pH buffering. Fish-sourced minerals, especially from deep waters, could impact how oceans respond to acidification and warming.
These particles interact with dissolved carbon and influence chemical stability at depth. The study’s results could help refine Earth system models used to forecast these changes.
The researchers stress that ichthyocarbonate is not trivial waste. It is a steady mineral output with wide-scale consequences. It travels, dissolves, or settles, shaping carbon flow through ocean layers. With up to 94% of global fish biomass living in mesopelagic zones, their contributions matter.
Understanding when and where this mineral is released is key. It could improve predictions of long-term carbon storage.
“Ichthyocarbonate release by individual fish is episodic and under sophisticated endocrine control, but we know very little about the timing and frequency of release, offering an important area for future research,” concluded the study authors.
The study was funded by the National Science Foundation and the University of Miami. The findings are published in the Journal of Experimental Biology.
—–
Like what you read? Subscribe to our newsletter for engaging articles, exclusive content, and the latest updates.
Check us out on EarthSnap, a free app brought to you by Eric Ralls and Earth.com.
—–
