每周一次的 Nature Podcast ，本周由Noah Baker和 Nick Howe 带来的一周科学故事，讨论鱼体内的汞。
音频文本：
Interviewer: Nick Howe
Billions of people around the world rely on fish for nutrition and for good reason. They’re a great source of protein, vitamins and minerals. But fish are also the main route by which people are exposed to methylmercury, a toxin which is formed when mercury is released into the environment by various human processes. Ingestion of large amounts of methylmercury can cause heart problems and brain disorders. To prevent human exposure, an international treaty, the Minamata Convention, was introduced to prevent the release of mercury by humans into the environment. This effort has led to less methylmercury in the sea. You’d think then that there’d be less of it in fish too.
Interviewee: Amina Schartup
One thing we noticed was that depending on the fish species that people were talking about in their work, it was either the concentrations are going up or down or remaining flat, even though the atmospheric levels of mercury have been declining. So, the question was, why do we see all those different directions in terms of mercury levels in fish?
Interviewer: Nick Howe
This is Amina Schartup, a biochemist who looks at toxic chemicals in the environment. This week in Nature, she’s publishing a paper that’s trying to work out why people are seeing increases in methylmercury levels in some fish, even though the amount in the environment is declining, and how the concentrations in fish may change in the future. I gave her a call to find out more and started out by asking how she was answering these questions.
Interviewee: Amina Schartup
We wanted to construct a model that actually allows us to literally play around with different environmental parameters to see if seawater temperatures was part of activity levels in the ocean, how is that going to impact this particular fish mercury level versus another fish?
Interviewer: Nick Howe
So, what was this model based on?
Interviewee: Amina Schartup
The idea was just to build a fish from scratch, right. So, using mathematical equations, we will make a fish and have it grow in this mathematical universe we created for it and then it will eat other fish that are also robot math fish and then see how, depending on their diet and the conditions around that fish, how that is going to impact mercury levels.
Interviewer: Nick Howe
And so, what did you find by doing this?
Interviewee: Amina Schartup
As mercury levels in seawater have been declining and have recently plateaued due to regulatory efforts, we have noticed, at least in our model, that despite the decline in seawater concentration, we haven’t seen any mercury decline in tuna, in the bluefin tuna we’re working on. And actually, if we project the increases in temperature in the Gulf of Maine which is the region we’re working on, we see that the bluefin tuna mercury concentrations are going to increase despite a decline in mercury levels, which actually means that we also need to keep an eye on our carbon emissions and the implication of those emissions have on seawater temperature because even despite declining mercury levels, we may see an increase in mercury levels in the fish just driven by seawater temperature.
Interviewer: Nick Howe
And how exactly does an increase in temperature lead to an increase in mercury in the fish?
Interviewee: Amina Schartup
The reason we see an increase is when it’s warm, and these are for the most part cold-water animals, they are sensitive to temperature, so when the temperature of seawater increases a tiny bit, their activity also level increases, and so as their activity level increases, their consumption of food increases but they don’t grow fast enough to compensate for the fact that they’re consuming more.
Interviewer: Nick Howe
Right, okay, so when things get warmer, they eat more and they just end up accumulating more mercury in their system.
Interviewee: Amina Schartup
Right.
Interviewer: Nick Howe
Also in the paper, you talk a little bit about overfishing as well. What impact is that having?
Interviewee: Amina Schartup
Yes, so diet is quite important and depending on what a fish eats, it will have a different level of exposure to mercury. The issue with overfishing certain species is that if, for example, that particular species was low in mercury, like a herring for example, and you have another predatory fish that used to consume that herring but now you overfish the herring and so your other predatory fish decides it needs to eat something else. So, depending on what that predatory fish is going to switch to – is it going to be a higher mercury level fish or a lower mercury level fish – you will see either a decline or an increase in mercury levels in that predatory fish.
Interviewer: Nick Howe
So, in terms of thinking about sort of climate change action and things, is that what needs to be done then, in order to prevent such increases in the mercury concentrations in fish?
Interviewee: Amina Schartup
So, we often talk about climate change as this very abstract thing and it’s really hard to see what our daily implications can be for some people aside from extreme events. But this one was really trying to link the impact of climate change to all the other little things that we do in our daily lives, so it’s going to impact our plates and what we like to eat. It’s not just those physical things around us, it’s also in our food.
Interviewer: Nick Howe
That was Amina Schartup of the Harvard School of Engineering and Applied Sciences in the US. You can find her paper over at nature.com.
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