Even the marine animals at the bottom of the Mariana Trench bear the signature of humanity, researchers report: The ecosystems of the deepest sea areas on earth are polluted by sinking fish carcasses with toxic mercury, which originally comes from human emissions into the atmosphere. This emerges from isotope analyzes of the heavy metal in inhabitants of the deep sea. The findings thus provide important information on how rising mercury emissions are affecting the creatures of the oceans, say the researchers.
Mercury also occurs naturally in the environment, but humans are driving the values into increasingly critical areas: More than 2000 tons of the toxic heavy metal are released into the atmosphere every year by coal-fired power stations, garbage fires or cement production. It is assumed that these emissions will continue to rise. The mercury can travel thousands of kilometers before settling on the land surface and in the ocean, where microorganisms convert some of it into methylmercury. This is a highly toxic organic form that can accumulate in quantities that are harmful to humans and animals via the food chain. Potential effects include damage to the nervous system, heart, and immune system.
On the trail of mercury
It has long been assumed that anthropogenic mercury is mainly confined to the upper 1000 meters of the oceans. However, as early as the summer of 2020, the researchers led by Joel Blum from the University of Michigan in Ann Arbor and a Chinese research group reported at a scientific meeting on mercury pollution of human origin in deep-sea trenches. But as Blum and his colleagues report, it was unclear how the heavy metal got there. It seemed possible that it had reached the depths through so-called detritus particles that trickled down like snow from higher layers. In their current publication, Blum and his colleagues contradict this declaration.
Their results are based on isotope analyzes of methylmercury from the tissue of deep-sea fish and crustaceans, which were collected by diving robots at depths of up to 11,000 meters in the Mariana Trench in the northwestern Pacific. Additional samples were obtained at depths of up to 10,000 meters in the Kermadec Trench in the southwest Pacific, northeast of New Zealand. “We examined these trench biotopes because they represent the deepest and most remote places on earth. One could have expected that the mercury there is almost exclusively of geological origin – from volcanic deep-sea sources, ”says Blum. But as it turned out, it wasn’t.
As the researchers explain, mercury has seven stable isotopes. The ratio of the versions to each other provides a unique chemical signature or fingerprint that can be used as a diagnostic tool to compare environmental samples from different locations. The researchers have now used this fingerprint technique to determine where the mercury in the deep-sea inhabitants originally came from.
Fish carcasses transport mercury
The analyzes showed: The heavy metal of the amphipods and fish from the two deep-sea trenches is of human origin and has a chemical signature that is similar to that of the mercury of fish species from the near-surface areas of the ocean. In contrast, the researchers found that the isotopic composition of the mercury in the sinking detritus particles does not match the chemical signature of the mercury in the trench organisms. They therefore come to the conclusion that most of the heavy metal pollution in the trench organisms was transported there in the carcasses of fish that ingested the anthropogenic mercury in the near-surface waters.
As the researchers explain, the results can now help to better understand the consequences of rising mercury emissions. Scientists rely on global models. According to Blum, their refinement requires the clearest possible understanding of the mercury cycles within the oceans and between the seas and the atmosphere. “We don’t eat fish from deep-sea trenches. However, we have to understand the mercury cycle through the entire ocean in order to be able to model future changes in the near-surface ocean, ”says Blum.
The result is also symbolic, as co-author Jeffrey Drazen from the University of Hawaii concludes: “Deep-sea trenches are seen as pristine ecosystems that have been spared from human activity. But earlier studies have already refuted this assumption. Traces of anthropogenic lead, carbon-14 from nuclear weapon tests and persistent organic pollutants were found in organisms that live even in the deepest part of the ocean, ”said the scientist. The latest findings on mercury confirm once again how human activities affect even the food webs in the most remote marine ecosystems on earth.
Source: University of Michigan, Article: PNAS, doi: 10.1073 / pnas.2012773117