Less oxygen, more poison: Warming seas fuel mercury risks

Climate change doesn’t have to add more mercury to the ocean to make seafood riskier – it can do that simply by taking oxygen away.

A new study led by Umeå University found that thousands of years ago, natural episodes of deoxygenation in the Black Sea supercharged the microbes that convert ordinary mercury into methylmercury. This potent neurotoxin climbs the marine food chain and ends up on our plates.

Microbes made mercury deadly

Methylmercury is dangerous because it biomagnifies: plankton take it up, small fish eat the plankton, bigger fish eat the small fish, and the dose grows at every step. The compound forms under low oxygen conditions when specialized microbes “methylate” inorganic mercury.

As modern oceans warm and stratify, those oxygen-poor zones are expanding in many coastal seas and enclosed basins. The Black Sea offers a long, natural experiment in how that plays out.

Researchers analyzed ancient DNA preserved in Black Sea sediments spanning roughly the last 13,500 years. They focused on the hgcA gene – a reliable genetic marker for organisms that can make methylmercury.

DNA reveals ancient warning

The signal spiked during a warm, humid interval about 9,000 to 5,500 years ago, when oxygen levels in the water column dropped sharply. Thus, climate-driven oxygen loss alone was enough to prime the sea for methylmercury production.

The Black Sea is uniquely suited to this kind of detective work. It’s the world’s largest permanently stratified marine basin. A relatively thin, oxygenated surface layer sits on top of vast, anoxic depths

As climate and circulation shifted through the Holocene, that boundary moved, leaving a clear imprint that settled to the seafloor.

Layer by layer, the sediments recorded when methylating microbes were abundant. Those genetic peaks line up with independent reconstructions of warmer, wetter conditions and pronounced deoxygenation.

The DNA also shows that multiple microbial lineages carried the methylation toolkit across time. That diversity matters.

If many branches of the microbial tree can play the role when conditions favor them, then expanding low oxygen zones will reliably bring methylmercury with them.

Climate’s repeating pattern

Today, human mercury emissions and nutrient pollution shape where and how much methylmercury forms. While industrial mercury provides the raw material, eutrophication deepens oxygen deficits that help the microbes thrive.

Thousands of years ago, the drivers were much simpler. Warming and stratification slowed mixing, and productivity rose. Organic matter consumed oxygen at depth, and the stage was set for methylation.

The new study doesn’t claim that modern mercury controls are futile. Cutting emissions remains essential. Rather, it shows that climate alone can nudge ecosystems into a state where methylmercury becomes easier to make and harder to avoid. That’s a crucial nuance for risk management.

Toxins climb the food chain

The path from a gene in ancient mud to a fillet at dinner runs through physical oceanography and ecology. Stratified waters trap low oxygen layers below the sunlit surface.

When winds or currents occasionally mix the column, nutrients – and methylmercury – can be lofted upward. This movement fuels blooms and shuttles the toxin into plankton. Small fish graze those blooms; larger fish eat the small fish. The concentration ratchets up with every bite.

The Black Sea isn’t the only place where this recipe is coming together. Expanding oxygen-minimum zones have been documented in the eastern tropical Pacific, the Arabian Sea, and semi-enclosed basins like the Baltic Sea.

Warmer surface waters mix less, algal blooms flourish, and as that organic matter decomposes, deeper layers lose oxygen.

Cut mercury, restore ocean oxygen

The practical takeaway is two-pronged. First, keep pushing mercury emissions down, since fewer inputs mean less raw material to methylate.

Second, tackle the conditions that help methylmercury form and persist. That means curbing nutrient runoff that drives coastal eutrophication and restoring circulation where feasible.

Experts should also track oxygen levels and microbial communities in places known to tip into low-oxygen states.

Combining paleogenomics with modern observation systems can sharpen forecasts. Ancient DNA provides the long view of how ecosystems respond to warming and stratification, while today’s sensors and sampling reveal how quickly conditions can swing.

Lessons from a breathless sea

Together, these insights feed models that connect climate, oxygen loss, microbial dynamics, and toxin production – tools that can guide fisheries management and shape seafood advisories.

Not every environmental threat arrives in barrels or pipes. Some are baked into the way ecosystems respond to heat. The Black Sea’s ancient record shows that when the ocean loses breath, the microscopic machinery that makes methylmercury whirs to life.

If deoxygenation continues to spread in a warming century, we should expect that machinery to switch on more often. Acting now – on emissions, nutrients, and monitoring – won’t just protect fish. It will also protect the people who consume them.

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