Gionfriddo C. M., Tate M. T., Wick R.R., Schultz M. B., Zemla A., Thelen M. P., Schofield R., Krabbenhoft D. P., Holt K. E., and Moreau J. W. Microbial mercury methylation in Antarctic sea ice. Nature Microbiology. 29 June 2016. DOI: 10.1038/nmicrobiol.2016.127.
Mercury is an odd metal. Unlike other metals, it is a liquid in its elemental form. You may have encountered mercury in the capillary of a thermometer your mom or dad used to take your temperature when you were a kid. Mercury is released into the atmosphere naturally from mineral deposits, but about half of the mercury released to the environment is actually due to human activities, primarily coal-burning power plants. Mercury exists either in “organic” forms in complex with carbon or “inorganic” forms without carbon. While mercury is generally considered to be toxic, organic mercury in particular ranges from pretty toxic (methyl mercury) to one of the most potent neurotoxins known to humans (dimethyl mercury). In a tragic case in 1997, a scientist exposed to just one drop of methyl mercury at Dartmouth College fell ill and died less than a year later. The further trouble with organic mercury is that it is easily absorbed by the fatty tissues of living things, where it can linger and accumulate over time. This is why you can get a strong dose of organic mercury from eating salmon or other delicious fatty fish. Therefore, understanding the factors that influence the abundance of the various forms of mercury is important from both the perspective of conservation and human health.
Mercury is cycled through the environment with a little help from sun and biology (i.e. bacteria) in a process that scientists like to call the biogeochemical mercury cycle.
Bio. Geo. What?
Let’s break it down.
Inorganic mercury exists in two forms, namely, the neutral element Hg0 (where ‘Hg’ is the elemental symbol for mercury, and the zero denotes a neutral charge) or the neutral element less two electrons denoted Hg2+ (where the ‘two plus’ indicated a vacancy of two electrons). The “geo” part of the cycle begins with the escape of elemental mercury trapped in rocks and sediments into the atmosphere by natural and human activities, where, greeted by energetic cosmic rays, it is “ionized” to Hg2+. This ionic mercury falls back to earth when it rains. Once on earth, the “bio” part of the biogeochemical cycles comes into play when Hg2+ is encountered by bacteria that can either add one or two carbons (in the form of “methyl” groups), forming methyl- or dimethyl- mercury, or where it can be “reduced” back to the neutral element (that is, our old friend Hg0). Dear reader, this has been the biogeochemical mercury cycle.
Despite a longstanding knowledge of the role of bacteria in producing organic mercury, the question of exactly who and how (let alone why) it is produced has only recently begun to be answered. Indeed, 2013 marked a breakthrough year for the field of “mercury methylation” (that is, formation of organic mercury) thanks to the discovery of the genes that could explain mercury methylation by bacteria that reside in environments devoid of oxygen (a.k.a. “anaerobic” environments). However, this discovery was only the tip of the iceberg. It could not explain global concentrations of organic mercury in, for example, the Southern Ocean of Antarctica. Fast-forward to 2016, and a multi-national team of scientists from the USA and Australia think they’ve found the first evidence for mercury methylation in oxygen-tolerant bacterial dwelling in Antarctic sea ice. (What a time to be alive!) The team reports their results in a recent article published in the journal Nature Microbiology.
To get at the oxic paradox, the team lead by Caitlin Gionfriddo sequenced DNA collected from Antarctic sea ice and water. They searched the resulting “meta-genomes” (the complete genetic content captured by sequencing the soup of DNA collected from either environment) for genes known to be involved in mercury oxidation and methylation. The researchers found genetic sequences that looked an awful lot like methylation genes previously reported from anaerobic bacteria, although the order of the genes forming the genetic sentence (a.k.a. “operon”) was somewhat different. Taking a look at the genomic contexts of these genes, the researchers could determine that they came from a nitrogen-fixing (uses nitrogen to breathe) group of bacteria that inhabits a low-oxygen “microaerophilic” environment. Hence, the authors report the first example of a likely source of organic mercury from an oxygen-containing (a.k.a. “aerobic”) environment. The researchers also found genes involved in the conversion of Hg2+ to Hg0 (mercury reduction) by bacteria that breathe mercury. That is, in the sea ice researchers found the potential to both convert ionized mercury (Hg2+) to organic methyl mercury, and to take it back to elemental mercury (Hg0). In other words, bacteria occupying the same environment appear to be capable of performing competing processes with vastly different outcomes!
While the implications for mercury utilization in sea ice are not clear, the authors of the study speculate that sea ice might sequester both organic and elemental mercury. Measurements taken in this study suggest that the contribution of sea ice to methyl mercury concentrations in seawater is up to ten times that of those previously reported. In light of their findings, the authors of the study propose that tiny pockets of melted sea ice containing a soup of bacteria, Hg2+, and organic particulate matter as a source of carbon, could lead to formation of methyl mercury, which could eventually find its way into seawater. Further, the presence of bacteria that convert ionized mercury to neutral mercury could mean that sea ice, like rock and sediment, sequesters elemental mercury. Hence, accelerated sea ice melt caused by global warming could dramatically alter the mercury cycle through both the release of organic mercury into seawater and inorganic mercury into the atmosphere. The relative and overall contributions of these competing processes to global mercury cycling remains to be seen. With the recent spate of progress in the field, answers are well on their way.
For further reading on this topic:
A reflective blog post by Caitlin Gionfriddo who lead the study.
A summary of the article by Dr. Elsie Sunderland and Dr. Amina Schartup.
Abrahim is a PhD student at Scripps Institution of Oceanography in San Diego where he studies marine chemical biology.