Mesopelagic Fish: Key Players in Oceanic Carbon Cycling Unveiled

In a groundbreaking study published in the Journal of Experimental Biology on July 28, 2025, researchers from the University of Miami's Rosenstiel School have revealed that mesopelagic fish, residing at depths of hundreds of meters in the ocean, play a significant yet often overlooked role in carbon cycling. This research specifically focused on the blackbelly rosefish, a species known for its resilience in laboratory conditions, confirming that these deep-sea fish excrete carbonate minerals, akin to their shallow-water counterparts.

The study's lead author, Dr. Martin Grosell, a professor of marine biology at the University of Miami, emphasized the importance of this discovery. "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," he stated. The findings indicate that the blackbelly rosefish excretes carbonate at a rate of approximately 5 mg/kg/hour at a temperature of 6°C, which mirrors its natural habitat. This aligns with predictions from earlier models that link metabolism, temperature, and ocean depth.

The research also assessed the resistance of carbonate formation to environmental changes, finding that even in laboratory settings far removed from their natural pressure conditions, these fish continued to produce carbonate. This robustness suggests that the carbonate excretion process is not sensitive to variations in pressure, which is crucial for understanding the dynamics of carbon cycling in the ocean.

Following a detailed analysis of the carbonate composition of the blackbelly rosefish, the researchers discovered that it matched the mineral makeup found in shallow-water species. The dominant component was magnesium-rich calcite, with smaller amounts of aragonite and other forms. The uniformity of these findings suggests that regardless of their habitat, the carbonate waste produced by these fish behaves similarly once released into the water column.

Co-author Dr. Amanda Oehlert, an assistant professor at the University of Miami, highlighted the implications of this research for our understanding of ocean chemistry and carbon cycling. "With mesopelagic fish playing such a significant role, their contribution to carbonate flux—and how it might change with warming oceans—deserves greater attention," she remarked.

This research not only fills a critical gap in knowledge regarding ocean chemistry but also supports broader carbon flux models, which until now had included deep-sea fish as contributors without direct measurements. According to Dr. Grosell, the results provide strong support for global models of fish-derived carbonate production, previously assumed but not verified.

Moreover, the study suggests that mesopelagic fish could influence carbon export in the ocean. Their carbonate excretion may either dissolve near the surface or settle deeper, contributing to seafloor sediments. This process has significant implications for carbon budgets as carbonates can affect seawater alkalinity and pH buffering, influencing the ocean's response to acidification and warming.

The researchers stress that ichthyocarbonate, the mineral produced by these fish, is not merely waste. It represents a steady output with far-reaching consequences for oceanic carbon flow. With an estimated 94% of global fish biomass residing in mesopelagic zones, understanding the timing and conditions under which this mineral is released is crucial for improving predictions of long-term carbon storage in the oceans.

Dr. Grosell concluded, "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." The study was funded by the National Science Foundation and the University of Miami, highlighting the collaborative effort to enhance our understanding of deep-sea ecosystems and their roles in global carbon cycles.