Mercury finds a long-term home in marine ooze

Microscopic, single-celled algae may play a big role in sequestering mercury pollution, a new study finds. German geochemists have found that a type of photosynthetic algae called diatoms can trap the toxic metal in deep-sea sediments. These diatom-rich oozes may store up to 20% of the mercury released into the atmosphere by human activity over the past 150 years (Science 2018, DOI: 10.1126/science.aat2735).

As a volatile metal, mercury easily enters the atmosphere from both natural sources, like erupting volcanoes, and human sources, in particular coal burning. The metal can travel great distances around the globe through the atmosphere, ultimately ending up back on land or in water.

Scientists previously thought that the vast majority of mercury deposited in the ocean eventually re-entered the atmosphere, says Harald Biester, a geochemist at the Technical University of Braunschweig and senior author of the new study.

Biester’s team has now found that diatoms somehow adsorb or scavenge a significant portion of the dissolved mercury, pulling it down to the seafloor when the microorganisms die.

The researchers identified and sampled diatom oozes in sediment cores taken from the bottom of the Southern Ocean at three locations around Antarctica. By measuring mercury concentration at various depths throughout the cores, the scientists reconstructed a history of how much of the metal the tiny algae sequestered during the past 8,000 to 10,000 years.

The diatom ooze showed some of the highest mercury accumulation rates measured in the marine environment, surpassing other areas that exhibit high mercury accumulation, like estuaries. The scientists estimated that diatom oozes around the world might have accumulated between 22,000 and 84,000 metric tons of mercury since 1850, when the cores indicate that pollution from human activity began.

Scientists haven’t historically accounted for mercury trapped in ocean sediments, notes Robert P. Mason, a biogeochemist at the University of Connecticut. Given the mercury levels found in diatom ooze, he thinks scientists may be underestimating the amount of mercury humans have emitted into the atmosphere. “These results will lead to a re-evaluation of the response of not only the ocean but the entire biosphere to human inputs of mercury in the past and future,” Mason says.