“In the world everyone knows enough to pursue what he does not know, but no one knows enough to pursue what he already knows.” The Complete Works of Chuang Tzu

The work on mercury always held a particular fascination for us scientifically as we worked to understand the processes and food webs through which mercury accumulates in seafood. We are currently developing a mobile app for pregnant women to use when making decisions about which beneficial fish to consume and which fish to avoid.

We asked questions about seasonality, trophic level etc. even measuring mercury levels in Sargassum as well as smaller non-commercial species. Along the way in different traditional communities we encountered tantalizing evidence of the ways that some peoples may have deliberately sought to avoid mercury-laden fish during pregnancy.

The question of limiting mercury consumption has always been primary; we never suspected, however, that the fish may have been working on a solution themselves...

This story has its beginnings with an article from 1976 in the journal Pacific Science: The Distribution of Total and Organic Mercury in Seven Tissues of the Pacific Blue Marlin, Makaira nigricansIby Cythia D Shultz and David C Crear.

Dr. Eric Dewailly brought this article to our attention in 2013. On the surface it seems like a fairly regular article discussing measurements of mercury in Marlin flesh, something with which we are all familiar today.

The high mercury levels in Pacific Marlin, a large "top of the food chain" fish, in the 1970s, in hindsight, is not surprising, but what caught Eric’s attention was the article's specific attention to the distribution of organic mercury as a percentage of the total. In particular one finding stood out: that the majority of the body burden of mercury was in its inorganic form – 90% of it.

In simplified terms, mercury, as it comes to find itself in humans and animals is absorbed as organic and inorganic. The inorganic, demethylated form is toxic, as is all mercury, but is comparatively less bioavailable than organic or methylated mercury, meaning that it is not as readily absorbed deep into tissues and cells. This has a material impact on the perniciousness of its toxicity – the deeper it can be absorbed into tissues and cells the more persistent it is in the body and the more of an impact it can have on health.

For this reason inorganic mercury is removed from the body in a matter of weeks whereas organic mercury can be resident for many months. Its penetration deeper into tissues and cells means that it also has a greater impact on human physiology, the systems of the body, having a broad spectrum of impacts.

Eric had always wondered how these large predator animals such as Marlin managed to function with such elevated levels of mercury. How could their basic physiology, their neurology bypass what would be devastatingly toxic handicapping effects in other organisms? How was it possible that these large animals with such high body burdens of methyl mercury were not be neurologically impaired – how did they live with it?

The article from 1976 which highlighted this most curios data literally turned on its head the established truism of mercury ratios in fish being consistently 4 to 1 organic to inorganic.

The measurements from 1972 and ‘73 are the first data we have seen that suggest that this inversion exists in any fish.

From an evolutionary perspective it makes sense that an organism would evolve systems to diminish or manage the toxic elements of its regular diet. But why is this fish, amongst all other top predators that have been measured for mercury since 1972, the only one to have the ability to so drastically reduce its levels of organic mercury?

For Eric this was the ultimate ‘curious data’ and opened the door to answering the question of how Marlin manages to do it – what is the ‘magic’ that they use.

Sufficiently inspired in 2014, a small team from this Ocean Human Health Research Program from the Atlantis Mobile Labs in Bermuda and Emma Smith at the University of West Indies decided to test whether the solution to Marlin demethylation of mercury could be happening in the gut.

There are well known demethylating bacteria, and it is possible that Marlin somehow culture these so that food transiting the gut is demethylated.

A quick microbiology experiment was carried out isolating the gut bacteria of Atlantic Blue Marlin, growing it in the lab and exposing it to tuna flesh with a known quantity of mercury and measuring the flesh after exposure. The test was inconclusive for a number of reasons, but it was a long shot that may still prove to be correct. The ultimate purpose, however, of quickly trying to find the solution was to begin looking at this conundrum and for curious minds to become engaged.

To begin with we will be copying the 1976 Pacific Marlin study and sampling and analyzing Atlantic Marlin to see if the data we collect matches their findings.

If they match and Atlantic Marlin do invert mercury ratios, then we will design an Atlantic program that engages the small community of fish physiology and mercury specialists to begin testing hypotheses on the possible mechanisms of demethylation used by Marlin.

Our plan is to build an open-source testing think tank on this very specific topic and to help coordinate and fund, when needed, the lab work associated with, hopefully, developing an understanding of this question.

Philippe Max Rouja PhD