Ocean Warming Triggers Methylmercury Production in Marine Life
Ocean warming is reshaping not only the planet's climate but also the foundation of marine life. Reduced oxygen levels in water can turn seafood into a source of toxins even without increased mercury emissions. A study by Umeå University shows that historical changes in the Black Sea once triggered the growth of deadly compounds -- a scenario that could happen again.
Researchers found that thousands of years ago, periods of deoxygenation in the Black Sea activated microbes that converted harmless mercury into methylmercury, a neurotoxin. This compound accumulates in organisms and moves up the food chain -- from plankton to fish, and from fish to humans.
"Methylmercury climbs the marine food chain and ends up on our plates," explained Umeå University researcher Andrei Ionescu.
These microbes thrive in low-oxygen environments. Today, warm oceans increasingly suffer from similar zones: as water warms and stratifies, deep layers no longer mix with surface layers, creating ideal conditions for toxin formation.
The research team extracted ancient DNA from Black Sea sediments dating back 13,500 years. Their focus was the hgcA gene, a key marker for bacteria capable of producing methylmercury.
Analysis revealed that gene activity sharply increased during periods of climate warming and oxygen depletion, particularly between 9000 and 5500 BCE. This natural "experiment" confirmed that ecosystems can trigger dangerous chemical processes even without human influence.
The Black Sea acted as a natural laboratory: oxygen-rich surface layers overlay vast oxygen-depleted depths. Changes in water temperature and circulation left traces in the sediments showing how microbes methylated mercury during each warming period.
Different microbial species employed distinct mechanisms for mercury methylation over time, increasing the risk: the more species capable of methylation, the higher the likelihood of toxin formation whenever oxygen levels drop.
Today, major sources of methylmercury include industrial mercury emissions and nutrient runoff that cause eutrophication. Climate change amplifies these effects: warm water and low oxygen provide ideal conditions for microbial activity.
Methylmercury does not dissolve or settle; it moves up the food chain. Predatory fish like tuna and swordfish accumulate the toxin, which can then reach humans.