Researchers at the Institut méditerranéen d'océanographie (MIO/PYTHÉAS, CNRS / Université de Toulon / IRD / AMU) have just provided an initial response to the paradox surrounding the production of methylmercury in the ocean. Using metagenomic and metatranscriptomic datasets from the TARA OCEANS expedition, they have highlighted the major role played by the microaerophilic Nitrospina bacterium in mercury methylation in all the world's oceans. Understanding how methylmercury is formed and transferred in the ocean is important because this compound is the most toxic form of mercury for humans and ecosystems.
Protecting humans from exposure to mercury is the main objective of the United Nations Minamata Convention, ratified in August 2017. However, while this convention focuses on the need to reduce anthropogenic emissions of inorganic mercury, it does not take into account the organic form of mercury, methylmercury, which is the most toxic and, above all, the only form capable of spreading along the trophic chain and thus endangering marine ecosystems and human health. Consumption of marine fish is the main route by which humans are exposed to mercury.
Methylmercury is produced in the ocean by microorganisms from inorganic mercury1. This methylation of marine inorganic mercury has been confirmed in all ocean basins, regardless of the oxygenation levels of their waters. However, until 2016, only anaerobic microorganisms2 were known to produce methylmercury. This was the paradox of the origin of methylmercury.
In 2016, the microaerophilic2 Nitrospina bacterium detected in the Antarctic ice pack was shown to possess mercury methylation genes. It has also been suggested that it could also be responsible for methylmercury production in other oxic (oxygen-rich) environments, although this has not been confirmed by the available ocean metagenomes. The question of "who is methylating mercury in the global ocean" therefore remained unresolved. Solving this enigma is of vital importance if we are to better understand the links between anthropogenic mercury emissions and the transfer of methylmercury along trophic chains.
Young MIO researchers have combined their expertise in bioinformatics, microbiology and marine biogeochemistry to tackle this problem. On the basis of exhaustive metagenomic3 and metatranscriptomic3 datasets from the TARA OCEANS project (www.oceans.taraexpeditions.org), they were able to identify mercury methylation genes in all the ocean basins covered by the expedition. These genes are not only present, but also transcribed4, demonstrating the active role of the microorganisms identified in mercury methylation.
The results identified the nitrite-oxidising microaerophilic bacterium Nitrospina as the predominant and ubiquitous producer of methylmercury in the oxic subsurface waters of the global ocean. On the other hand, sulphate-reducing bacteria, already well known and considered to be the main producers of methylmercury in other ecosystems such as sediments, are thought to represent a minor fraction of bacteria carrying the mercury methylation gene in the ocean.
While resolving the apparent paradox of the origin of marine methylmercury, this work is leading to a profound reconsideration of the role of Nitrospina in methylmercury production, a role that is proving to be major not only in the Antarctic ice pack but also in the Pacific, Atlantic, Indian and Southern Oceans. In so doing, they are making a major contribution to our understanding of the global mercury cycle, which will enable us to better estimate the effects of climate change on marine methylmercury production.
Source
Villar, E.; Cabrol, L.; Heimbürger-Boavida, L.E. Widespread Microbial Mercury Methylation Genes in the Global Ocean. Environ. Microbiol. Rep. 2020, 1758-2229.12829.
https://sfamjournals.onlinelibrary.wiley.com/doi/abs/10.1111/1758-2229.1…