Source: BBC Radio 4: The Forum
URL: http://www.bbc.co.uk/programmes/b068lsp5
Date: 05/09/2015
Event: Anne Cohen on seashells and ocean acidification
Credit: BBC Radio 4
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Bridget Kendall: When was the last time you stopped to pick up a beautiful shell on a sea shore? Beautifully intricate, sometimes fragile, sometimes unbelievably strong, and when you press some shells to your ear, an eerie crashing sound like ocean waves... But these treasures of the ocean hold many other secrets. And today we'll be hearing about astonishing new research into their ancient role in human evolution, more recently what using shells as money does to a region's economy and the glimpse they give us of how rising acidity in oceans might change the world we live in. With me are the Dutch archaeologist Josephine Joordens, the British cultural historian Toby Green and, joining us from Boston in the United States, the South African marine scientist Anne Cohen. Welcome to all three of you, and I just want to start off by asking each of you briefly about one of your favourite shells. Josephine...
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Bridget Kendall: But let's hear more about your work now, Anne Cohen, you're a marine scientist from South Africa who's now based at Woods Hole Oceanographic Institution in Massachusetts in the US, from where you join us now, and your work largely focuses on climate change and its impact on life in the ocean, particularly how it affects seashells. And your most recent research has been looking at how well the shells of marine organisms - from crabs and lobsters to oysters and scallops, all the things we love to eat - are standing up to an increasingly acidic ocean. So, first of all, explain a bit more about how shells form in the first place.
Anne Cohen: Shells are made from calcium carbonate. So what these organisms are doing is they're combining calcium ions in sea water with carbonate ions to make calcium carbonate crystals. And that's very much like chalk - well, actually chalk is calcium carbonate and different from bone, obviously - but all these organisms - shellfish, lobsters, corals, crabs - are all using calcium carbonate ions to make their shells.
Bridget Kendall: And what happens when the ocean becomes more acidic, or less alkaline? How does this affect these shells?
Anne Cohen: So when the ocean takes up carbon dioxide from the atmosphere, what's happening is that the pH is declining, and that's what is referred to as the process of ocean acidification. Now at the same time, what's happening is that the concentration of carbonate ions, the ions that these organisms are using to make calcium carbonate shells, is decreasing. There's just fewer carbonate ions with which to make the shells, and makes it much harder for the organisms to build shells and skeletons.
Bridget Kendall: And the point is that the organisms are vulnerable when they're in the early stages of making the shell, before it hardens off, like the very hard shell that Josephine's brought in for us to look at.
Anne Cohen: So what's really interesting about shellfish, what makes them so fascinating to me is that they use different kinds of calcium carbonate to make the shells. So oysters, for example, are largely calcidic - they use a calcium carbonate called calcite. And then there's other shellfish that use a calcium carbonate called aragonite. And these are both made of calcium carbonate but they have very different properties. So for example, calcite is harder and more brittle and it is less soluble in seawater, whereas aragonite - which is also calcium carbonate, just a different form - is softer and it is more soluble in seawater. And what's important to realise is that even though the adult shell can be made of calcite, which is the harder, less soluble form, all the larvae, the shells of all those babies, all use aragonite, which is the more soluble form. And that's why they are more sensitive to the effects of ocean acidification.
Bridget Kendall: So they literally get corroded by the higher degrees of carbonic acid in the water, right?
Anne Cohen: Yes. So that can happen if the pH and the carbon ion concentration are very, very low. Then the shells can actually start to dissolve, and we see that in our experiments when we set up and we bubble different amounts of carbon dioxide into the aquarium seawater. And that gives us a range of experimental conditions that mimic what the ocean's going to be, in terms of its pH, over the next decade, over the next two decades, three decades, et cetera.
Bridget Kendall: What did you find, because you've put in different sorts of organisms, haven't you - blue crab, lobster, sea scallops, tropical urchins...
Anne Cohen: Yes.
Bridget Kendall: What sort of - what's the difference in the different types of damage that you see on - in these different experiments?
Anne Cohen: That's a really good question, because what we saw was a wide range of responses. The crabs and lobsters make a different kind of shell - carapace, which has a lot of organic material embedded in their high-magnesium calcite crystals. And they also shed their shells, they moult. And we saw, primarily amongst that group of organisms, that calcification rates actually increased when we lowered the pH. But amongst other organisms, and especially amongst the shellfish, the molluscs, we saw a decrease, consistently a decrease in the ability of these organisms to make their shells, as the pH decreased.
Bridget Kendall: And what's your prediction, then? Are you saying that in 100 years' time, people won't be able to eat sea scallops any more, because they just won't be there any more?
Anne Cohen: Well actually, Bridget, it might be sooner than 100 years from now, depending on where you live, because the rate of acidification of the coastal oceans is not homogenous around the world - in fact, we're finding that rates of acidification on the East Coast of the US are much higher, much faster than the global climate models project. A lot has to do with the decomposition of organic matter in the estuaries and along the coast, and that puts more carbon dioxide into the water. In fact, if you just look at a latitudinal gradient, you see that rates of acidification are higher in the colder regions, or faster in the colder regions than they are in the tropics. And there's a reason for that, because cold water takes up carbon dioxide from the atmosphere much more quickly than warm water, and so those areas are more vulnerable, and ocean acidification is occurring faster there.
Bridget Kendall: And, in a way, you're looking at the future, but Josephine, you look at the past, don't you, and after all, this isn't the first time that the world's oceans have been made more acidic, some scientists think that mass extinctions - how long ago? 250 million years ago.
Josephine Joordens: There are many, many extinctions but what I'm thinking when I hear your - Anne - what the role of evolution would be, because it reminds me very much about the conditions in the freshwater ecosystem, where you have this decomposition of organic matter, and you have naturally highly acid conditions, often, and the shellfish living in them protect themselves against it by forming a kind of protective layer on their shells. And, for instance in the case of the shells I've been working on now, for my study in Java -
Bridget Kendall: This is in Indonesia.
Josephine Joordens: Indonesia, yes. They have a brown protective layer, originally, and I was thinking maybe, if evolution is fast enough, marine shells could evolve such a layer as well. Would that be a possibility, or is this simply not possible?
Anne Cohen: That's a really good question, Josephine, and it's something that scientists in my community are looking at in a very focused way, the question of potential adaptation. All shells, even marine shells, have that organic layer, that organic covering called a periostracum, that protects the outer shell. What we find is that the significant impacts are at the larval stage.
Josephine Joordens: Okay.
Anne Cohen: So, for example, a baby oyster that's just been fertilised, it's developing into a embryo and within about 24 hours it's starting to deposit calcium carbonate, it's starting to build its little D-shell, which is only about 50 microns in diameter, it's tiny, you can't see it with the naked eye. What we see in our experiments is that it becomes much more difficult, and they take a lot longer to make that first shell. The shells are thin, they are often mutated, so they don't form normal shells - this is even before the animal can make the protective organic covering of its shell.
Bridget Kendall: It's very, very fascinating and it is a microcosm, which is actually giving us a glimpse of the future, possibly.
Josephine Joordens: Yeah.
Bridget Kendall: But let's go on now to have a glimpse at the past...