Revealing New Insights into the Toxicity of Bluefin Tuna

In a groundbreaking study that deepens our understanding of mercury detoxification in marine predators, researchers at the European Synchrotron Radiation Facility (ESRF), in collaboration with CNRS, ENS Lyon, and the Institute of Marine Research in Norway, have unveiled the biochemical pathways by which Atlantic Bluefin tuna neutralize one of the most insidious environmental toxins: methylmercury. Published in the prestigious journal Environmental Science & Technology, this research sheds new light on how these apex predators transform toxic mercury species into chemically stable and biologically inert forms, potentially reshaping our approach to seafood safety assessments.

Mercury contamination remains a critical global health issue, largely due to the heavy biomagnification of toxic organic mercury species through aquatic food webs. The element mercury (Hg), found naturally in volcanic eruptions and forest fires, is also released extensively through anthropogenic activities such as coal combustion, artisanal gold mining, and the incineration of industrial wastes. In aquatic ecosystems, inorganic mercury is converted by microbial action into methylmercury, a potent neurotoxin that readily accumulates in organisms, particularly top-level predators like the Atlantic Bluefin tuna. These massive fish, renowned for their migratory prowess and economic value, act as reservoirs of accumulated mercury due to their lengthier lifespans and high trophic positions.

However, this new study challenges the conventional notion that all mercury present in fish tissue is harmful, highlighting the critical importance of mercury speciation in assessing risk. Alain Manceau, a senior researcher at CNRS/ENS Lyon and ESRF scientist, explains that measuring total mercury alone may lead to an overestimation of toxicity risk in seafood. “Our findings demonstrate that a significant fraction of the mercury present in Bluefin tuna muscle exists as mercury–selenium complexes, which are far less toxic or potentially inert,” he notes. This insight has profound implications for regulatory agencies and public health guidelines that have traditionally relied on gross mercury levels to recommend fish consumption limits.

Central to the research was the use of advanced synchrotron-based techniques, particularly high-energy-resolution X-ray absorption spectroscopy, enabling the authors to probe mercury’s chemical environment at unprecedented resolution. This powerful method illuminates the precise molecular interactions and chemical forms of mercury within biological tissues, unraveling the complex pathways that govern mercury detoxification. Remarkably, the study reveals that unlike other marine apex predators such as toothed whales and seabirds, which primarily detoxify methylmercury in the liver, Atlantic Bluefin tuna employ the spleen as their principal detoxification organ.

The detoxification mechanism hinges on the biochemical interplay between mercury and selenium—a micronutrient abundant in seawater. Through a cascade of redox reactions mediated by a specialized selenium-containing protein known as selenoprotein P, selenium binds to mercury, converting it into stable mercury–selenium complexes. One of these, identified as a tetraselenolate complex (Hg(Sec)_4), was detected within the edible muscle tissue and appears to be a precursor to inert mercury selenide deposits formed in the spleen. Such complexes markedly reduce mercury’s bioavailability and toxicity, effectively diminishing its harm to the organism and, by extension, to humans who consume the tuna.

This spleen-centric detoxification pathway suggests previously underestimated biological adaptations in large pelagic fish. The researchers hypothesize that the formation of mercury selenide, a virtually inert mineral phase within spleen tissues, serves as a long-term sink preventing methylmercury’s harmful effects. Furthermore, the relatively low concentration of mercury in muscle tissue explains the absence of mercury selenide there, reflecting a compartmentalized detoxification system that immobilizes and isolates toxic compounds away from edible tissues.

Samples analyzed in the study were obtained from Atlantic Bluefin tuna caught along the Norwegian coast, presenting a rare opportunity to study large specimens—often up to 300 kilograms—that act as essential bioindicators for assessing mercury cycling in marine environments. Martin Wiech of the Institute of Marine Research underscores the significance of these specimens as “key model organisms” due to their elevated trophic levels and substantial mercury loads, which reflect both environmental health and the complexities of trophic transfer in oceanic food webs.

Critically, the study emphasizes that these conclusions cannot be generalized to all tuna species. While the Atlantic Bluefin and Bigeye tuna share high trophic positions and corresponding bioaccumulation patterns, smaller, lower trophic-level tunas such as albacore and skipjack exhibit substantially lower mercury levels. These species, commonly found canned and consumed worldwide, reportedly contain markedly less mercury, and the detoxification dynamics are presumably different, necessitating species-specific investigations.

By parsing the nuanced chemistry of mercury in tuna, the study advocates for a paradigm shift in seafood safety frameworks, proposing that regulations and advisories should be based explicitly on methylmercury content rather than total mercury. Since methylmercury concentration directly correlates with toxicity, while other mercury complexes pose little to no risk, this approach promises more accurate risk assessments and finer consumer guidance. Notably, the research indicates that up to 25% of mercury in Bluefin tuna muscle exists as less harmful species, and astonishingly, this figure increases to 90% in related species like marlin (makaire), underscoring the complexity underlying mercury bioavailability.

This revelation also deepens our understanding of the geochemical and biochemical cycling of mercury in marine ecosystems, demonstrating how elemental interactions, enzymatic pathways, and biological compartmentalization coalesce to mitigate toxicity in apex predators. Beyond providing a scientific foundation for improved consumer safety, the results open new avenues for ecological and toxicological research aimed at unraveling the interplay between nutrients and pollutants in oceanic food chains.

In conclusion, the pioneering work by Manceau, Glatzel, and colleagues represents a significant advance in environmental chemistry and marine toxicology. Employing cutting-edge synchrotron spectroscopy, the researchers not only uncovered a novel demethylation pathway concentrated in the spleen of Atlantic Bluefin tuna but also revealed the vital role of selenium in neutralizing mercury toxicity. These findings hold promise for refining seafood consumption recommendations globally and highlight the importance of chemical speciation in evaluating the risks associated with environmental contaminants.

News Publication Date: 18-Sep-2025

Web References: 10.1021/acs.est.5c08815

References:
Manceau, A., et al., Demethylation Pathway of Methylmercury in the Spleen and Peripheral Organs of Bluefin Tuna – Implications for Fish Consumers, Environmental Science & Technology, 18 September 2025.