Plankton
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BBC Sounds, music, radio, podcasts. This is In Our Time from BBC Radio 4 and this is one of more than a thousand episodes you can find on BBC Sounds and on our website. If you scroll down the page for this edition, you'll find a reading list to go with it. I hope you enjoy the programme. Hello, whenever you breathe in, half the oxygen in your lungs came from plankton, the tiny drifting lifeforms in the ocean.

The plant-like ones among them are the great photosynthesizers and the diet of animal-like ones which the fish then eat and so the food chain continues to the billions of us who depend on the seas for food.

and at dawn the animal ones sink to the depths of the ocean to avoid predators. Only to rise again at night is the largest migration of life on the planet, a daily pulsation like Earth's heartbeat. We meet to discuss plankton, our Carl Robinson, Professor of Marine Sciences at the University of East Anglia,

Christopher Lowe, lecturer in marine biology at Swansea University, and Abigail McWatters-Gollop, associate professor of marine conservation at the University of Plymouth.

Abigail, can you give us an idea of the diversity of plankton in all their shapes and sizes? Yes, plankton are wonderfully diverse. So they come in many, many kinds of shapes and sizes. There's two sort of groups of plankton when we think about plankton. The phytoplankton, which are algae, so they're single cells, but they can form chains together. And then the zooplankton, which are tiny microscopic animals.

They have all kinds of morphologies, which means body shapes and constructions. So the phytoplankton can be long and thin or spherical or in a chain form. But zooplankton are really cool because they're little tiny animals. So they have all kinds of animal features like eyes and arms and legs and that kind of thing and tails. But what's really interesting about zooplankton is a lot of them

are the larval stage or juvenile stage of other animals in the ocean like crabs and lobsters. But when their larvae are in the plankton, they look completely different from the adult animals that we know in the marine environments. Like larval crabs have these really long powerful tails and this big spine on their noses and big spine on top of their head. And that keeps them, protects them from becoming prey for other plankton or for larval fish.

So if you look through a microscope at a plankton sample, you'll see all these different kinds of organisms together and you'll see them eating each other and fighting each other. Why are they grouped together and called plankton? Why are they grouped together, first of all? So they're called plankton because they cannot swim against the currents. So plankton comes from this Greek word that means to wander.

And the key kind of characteristic all planktons share is that they just go wherever the currents take them. So they can move up and down in the water column, some of them. They have adaptations to make sure they just don't kind of all sink to the bottom. But they can't fight against the currents.

How many are there? Have you any idea how many there are? Oh, wow. There's probably thousands of different species of plankton in the ocean. There's so many different species. But one that I can't... My favourite one is jellyfish, which is one of the few plankton that you can see without a microscope. So even though they're big and they can be over a metre long, they still can't swim against the currents and they still drift through the seas. Thank you very much, Chris Lowe.

When do we see them with the naked eye, these plankton? If we go down to the smallest plankton that we come across, something along the lines of a single-cell bacterium, something like a species called Prochlorococcus, which is probably the smallest plankton species that we get. It's a very small bacterium that we get offshore. These things are absolutely tiny. They're about one micron across, so that's about a thousandth of a millimetre. So we could get 100 of these lined up in the width of a human hair.

Now, obviously, the human brain kind of struggles to comprehend a bit that size. Let's increase the size of that pleurochlorococcus to about the size of a human, about 1.7 metres high. If we then took another plankton animal, so one of these jellyfish that Abiyu was talking about, let's say maybe a small one at five centimetres, if that single bacterium was the same size as me, that jellyfish would probably be about the size of Greater London. So there's this massive, massive, massive variation in the size of what we see.

So conceptually, then, we'd think the things that we would see and we'd interact with most are going to be these larger plankton. If you happen to be a scuba diver and at the right time of year you're going up and down in the water column, we'll see these sort of mucousy bits floating around or perhaps some crunchy stuff floating around as well. These are these zooplankton, some of the larger ones. But in reality, what we actually probably see more of or what is more evident in many ways are these really small phytoplankton, because what they do is they change the colour of the water.

The earliest record that I can find that I'm aware of of plankton being observed, which actually goes back to Gerald Wales in 1188, in his trip around Wales, he mentions the fact that the local population around Llins y Fad and Llangors Lake in mid-Wales

used to refer to the waters of the lake being red or green, and this representing basically being a portent of evil times. And it's almost certain that this is actually a report of changes as a result of the plankton growing in these lakes. Move forward now to sort of present day, and the way that oceanographers, so people who study this sort of thing, will often look at is the changes in what we can see in the sea, so the changes in colour.

And now we can look down from hundreds of miles above at the sea in front of us and take photographs and get those same colours. So we are able to visualise these tiny, tiny, tiny little organisms, which are thousands of a millimetre across, but they're in vast, vast numbers. And because they're in vast numbers, they change the colour of the sea and we can see them even from space and track their movements around the sea. Can you say something about the plant-like ones? No.

The way that plants developed or plant-like organisms developed in the sea is quite different to where we ended up on land. So essentially the way that primary producers, plants on land evolved, they basically only did once. So way back about a couple of billion years ago. But back in the sea, that happened a number of other times as well. So we have other cells which are more closely related to fungi, which are more closely related to things like seaweeds, which also did the same thing and took chloroplasts on board as well.

And that means that the systems used, the chemicals used for primary production for photosynthesis in these species is slightly different. And it means we've got a larger diversity of primary production in the sea than we do on land. What's the significance of the plant-like ones in terms of the history of life on Earth?

We go back to about 2.4 billion years ago when this all really started with the primary producers. That's when some of these individual bacteria which started to photosynthesise started to come about. Back then, the atmosphere of the planet basically didn't have any oxygen in it. So it was this early primary production which really oxygenated the atmospheres above the sea and allowed for life to appear as we know it.

Thank you. Carol, Carol Robinson, can you tell us about the biological carbon pump? Yes, of course. The biological carbon pump is a term that we use to describe a suite of physical and biological processes which together ultimately result in the drawdown of carbon dioxide from the atmosphere and the storage of that carbon in the deep sea or in the sediment. So it's called a pump because it's moving carbon from the surface to depth

And it sets up a gradient of carbon so that carbon dioxide is lower in surface waters than it is at depth. And in fact, modelers suggest that if there was no biological carbon pump, atmospheric concentrations of carbon dioxide would be about 30% higher than they are currently today.

So there are a number of mechanisms by which drawdown of carbon dioxide occurs. And we start with photosynthesis in the surface waters. So phytoplankton photosynthesize, take up inorganic carbon, produce carbon in terms of more phytosynthesis.

phytoplankton cells, which can then form the basis of the marine food web. So these tiny phytoplankton can be eaten by small zooplankton, eaten by larger zooplankton, eventually eaten by fish. So we've set up a food web of how phytoplankton are underpinning the feeding, the food security of billions of people on Earth. But then when the phytoplankton die and when the zooplankton defecate and produce faecal pellets,

These particles rain down through the ocean, so we call it marine snow because you can imagine it drifting slowly down through the depths to 4,000 metres. And as the marine snow falls, it is degraded and utilised by more zooplankton and by bacteria. And these organisms respire, so they take up the organic carbon from the particles and produce inorganic carbon, carbon dioxide.

So you have something like 100 billion tonnes of carbon being taken up by photosynthesis...

and something like 98 billion tonnes of carbon being produced by respiration. So two enormous rate processes that are very closely in balance... except for about 2 billion tonnes of carbon. And this 2 billion tonnes of carbon essentially relates... to the drawdown of carbon dioxide from the atmosphere... the storage of carbon in the sediments...

And also the production of oxygen, which, as Chris mentioned, is how the Earth's atmosphere became oxygenated. But we also have a flux that's mediated by zooplankton, as you mentioned, the greatest migration on Earth.

where zooplankton feed on phytoplankton in the surface waters at night, so they're avoiding their predators because it's dark, and then they swim down at sunrise, down to 300 or 500 metres, and at that depth they avoid their predators, they defecate, they excrete, they respire and they molt. So that's a very quick transport of carbon from surface waters to depths. This is daily? This is daily, yes.

So if we imagine our particles gently floating down through gravity, they are sinking at about 100 metres per day. Whereas if you have zooplankton that are feeding at the surface, taking that carbon down to depth to 500 metres, then your faecal pellets, your carbon, can get down to depth three times faster.

And this depth distribution is incredibly important because the amount of carbon dioxide that's drawn down from the atmosphere is dependent on the depth at which the faecal pellets and the other particles are respired back to carbon dioxide. If most of the respiration occurs in the surface waters, then the timescale over which that carbon dioxide can go back to the atmosphere is very short, whereas if most of the respiration occurs at depth,

then that carbon dioxide can stay trapped in the ocean for hundreds of years. Abigail, can you tell us how we started to know so much about plankton?

We monitor plankton in lots of different ways, using bottles or nets. But one of the most kind of extraordinary ways that we monitor plankton is with something called the Continuous Plankton Recorder Survey, or the CPR survey. So the CPR survey, it's run from Plymouth. How long, when did it start? The Marine Biological Association. The CPR survey started in 1931. Wow.

And one of the most amazing things about the survey is that the method and the machine has remained almost completely unchanged during that time period. So Sir Alistair Hardy is the one who developed the CPR. And this was before we had computers. So he used things that he had to hand to make this kind of device. It's about a meter long. It's metal. And it's got a spool of silk that travels or winds as the machine is pulled behind a boat.

So Sir Alistair Hardy used what he had around him to construct this. The thing that holds the silk down and keeps it flat was a bit of a pasta maker. And the things, the wheels that turn the propeller on the back of it were from a grandfather clock, like the cogs inside the clock, because they all fit together. And as the ship is pulling the CPR, the water turns this impeller and these cogs turn to turn the silk and the pasta maker bit keeps the silk flat.

So now CPRs are produced at a workshop in Plymouth. So what's it doing exactly? The plankton comes in through a hole in the front of the CPR...

And it's filtered onto the silk. So, you know, if you're driving through a country road and you get insects splattered on your windscreen, it's a little bit like that when you look at a CPR sample under a microscope. So the plankton are in the water, the water hits the silk and the plankton are like splattered against the silk. You can see they're a bit broken and squashed and that silk is put in some preservative.

And then when the CPR comes back to Plymouth, they unwind the silk and they know from where the ship has been, where these samples were taken. So they can match up where the plankton that they're looking at, where those were collected. An amazing thing about the CPR survey, not only has it not changed, so we have this very long, consistent time series where we can see things like climate change, but

But it's really spatially extensive, which is really unique. So the ships of opportunity, like so the Merchant Navy ferries, cargo ships, they tow these CPRs all over the ocean, right through the middle of the Atlantic, right across the Pacific. And that's quite rare in plankton sampling because usually we sample from shore or from a research vessel that is limited in where it goes.

Thank you. Chris, Chris Lowe, can you tell us more about mixotropes? When we were talking about there being these two main groups, the phytoplankton and the zooplankton, it's a little bit more arm-wavy than that because there's kind of a group of organisms that fit between the two of them. And these are the mixotropes. So we have phytoplankton, which are autotropes. They are things that take in light and convert that into chemical energy, into glucose. And then we have the zooplankton, which eat them.

Now, there's this group of organisms called the mixotropes, which do a bit of both. So they have the machinery involved inside them to be able to photosynthesise. And so while the sun's shining, they make hay and bring energy in that way. But when they end up in situations where they are struggling to get organic material or there's not sufficient light, they have the ability to switch over.

So as well as photosynthesising, they can go out hunting, basically. And what they're going to be going after is things of a similar size to them. So they tend to be relatively large single-cell organisms, but they have lots of little flagella, so little spindly things that stick out the side of them.

So they can go chasing after other individual cells and they'll go up and catch another one, harpoon it basically with one of these flagella, draw that in, scoop out the insides of their prey, or they may completely envelop the other cell they've caught, or they can actually create basically an external stomach which they surround the cell with and digest it that way.

So they're really opening up a sort of extra niches as a species to things they wouldn't be able to do otherwise. And a very, well, to be honest, an increasing proportion of certainly the phytoplankton species are increasingly being identified as being myxotrophs. That's particularly true of the dinoflagellates. So these are also the species that perhaps is another one that the people have seen out in the environment because the dinoflagellates are the species that tend to give us bioluminescence.

So this is this phenomenon where if you happen to go out into the coastal areas of the UK in late spring, early summer, on a nice warm day, and you're there at night when it's properly, really nice and dark, as the waves come in, you've seen flashes of light that are there in the waves. And these are these single cell organisms which are producing light, essentially to try and drive off predators.

Thank you. Carol, the fundamentals of the life here are the nutrients in the water. Where do they come from and what are they?

Just like ourselves and plants, plankton need nutrients, things like nitrate, phosphate, silicate. And there are a number of different sources and sinks of these nutrients. So, for example, they can come into the ocean through rivers, through runoff off the land, atmospheric deposition through rain and through dust. But nitrogen also has a biological source and a biological sink.

because they're a very specialised bacterioplankton which can use gaseous nitrogen from the atmosphere and transform that into ammonium, which is another nitrogen source that phytoplankton can use to grow.

And there's also a sink of nitrogen whereby specialised bacteria that live in low oxygen environments and sediments can transform nitrate back into nitrogen gas through a process of reactions that also include the production of nitrous oxide.

But the major movement of nitrogen through the water column of the ocean is intimately related to the biological carbon pump that we talked about earlier. Because just as the phytoplankton photosynthesize and take up carbon dioxide, they also take up these nutrients, ammonium, nitrate, phosphate, etc.,

And when the phytoplankton are degraded by bacteria, the bacteria use the nutrient components of the phytoplankton cells and respire them back to dissolved nutrients that phytoplankton can then use again. So it's a real cycle. The phytoplankton are taking up nutrients from the water, the bacteria are degrading the phytoplankton cells and putting the nutrients back into the water. The way you talked about what was happening in the ocean is like a non-stop laboratory. Yes.

Absolutely, yes. So what is so fascinating about this is all of these cycles have real global relevance and yet we cannot see the organisms with the naked eye. Abigail, what happens when the balance of nutrients is disturbed? What disturbs it?

Carol mentioned where some nutrients come from, and humans are a huge source of nutrients. So we put a lot of nutrients into the water. It happens a couple different ways. So through sewage is one way. Nutrients come into the water through sewage or through rivers, and those nutrients get into the river often through farming. So imagine if you've got cows in a field and there's manure and it pours rain and gets really swampy and all that water

water that runs off the field goes into a stream that goes into a river that goes into the sea. That manure is full of nutrients. And we call these anthropogenic nutrients or human nutrients. And the plankton, the phytoplankton, use those nutrients to live and to reproduce and

And they can do that really, really quickly when the conditions are right. So especially if the water is warm, tons of nutrients can cause the phytoplankton to bloom, we say. So reproduce really, really quickly. And this can have impacts on our marine ecosystem. So Chris mentioned earlier how the water can be different colors. So when we have these

blooms, the water can go from clear, being able to see when you're scuba diving, for example, see 10 meters, and then we might have a plankton bloom, and all of a sudden you can only see a couple of meters in front of you. But there's also lots of ecological impacts. So for example, if we have a huge bloom of phytoplankton, eventually they'll use all the nutrients, and then they'll die, and they'll sink down to the seabed.

And what can happen then is that they decay. So bacteria consume these dead phytoplankton, but that draws the oxygen out of the water, creating a dead zone. And these can be permanent or they can be temporary. But that means that any cell

seabed benthic organism that can't get away might die. And so this can happen. This has happened all over the world in different instances. And you see dead crabs or dead lobsters or dead shellfish because all the oxygen has disappeared from the benthic environment. And the Black Sea had a really bad problem with this, for example, in the 1980s. And every summer, the Black Sea would have all of these benthic organisms die off

because they've had these huge phytoplankton blooms caused by too many nutrients in the rivers going into the Black Sea. Chris, can you tell us more about how the plant-like ones react to light? What we've spoken about in the last few minutes has been largely talking about nutrients. The job of a phytoplankton is basically to produce more phytoplankton. That's how life works.

And there are two main controls to control how fast that can happen. The first one is the neutrons we've talked about. So that's kind of the building blocks that they need to build a baby phytoplankton. And the second thing is light. So you need light for photosynthesis, which is where the photo in photosynthesis comes from.

To do this, what phytoplankton will do is they use pigments. Now then, that's great. Chlorophyll is an amazing molecule, but it has a few disadvantages. One is it's very energetically expensive to create. It's also quite unstable, so once you've made it, it does tend to fall apart quite quickly as well. And also, it's fairly limited in the wavelengths that it's able to absorb.

And so what phytoplankton do is they have, all right, they do have this chlorophyll a, that's kind of an important part of the structure, but they also have these things called accessory pigments. And they have the advantage that they are less energetically expensive to make. They're more stable and they can absorb light at different wavelengths as well. So again, this is increasing the niches available for different species of phytoplankton by being able to absorb light at different wavelengths. It does also go the other way.

So phytoplankton, if they're in the very surface waters, are in light intensities which are sufficiently high that the energy coming in from above from the sun is sufficiently energetic that they're unable to process all of the light that comes into them. And that energy will break down and convert into free radicals, which are basically particles within the cell, which have this effect of essentially sterilising the cell. They break down the DNA in the cell and kill the phytoplankton.

So what these phytoplankton do instead of absorbing light and trying to put it through into the photosynthetic system, they create further pigments. And these pigments, these photoprotectant carotenoids, they absorb light and they dissipate it as heat. So basically it's acting as this factor 50 sunscreen for our phytoplankton, meaning that, again, there is an area of the sea where phytoplankton can live, which they wouldn't be able to otherwise simply because essentially they'd be sterilised.

Thank you very much. Carol Robinson, what happens if the balance of the sea temperature changes? Quite a lot, a range of different effects of increasing temperature on plankton. So there are direct effects. So the processes I was mentioning earlier, photosynthesis and respiration, are dependent on enzymes. And enzymes tend to work faster if we warm them up.

So with an increase in temperature, you would expect photosynthesis and respiration both to increase. However, they are not equally sensitive to temperature. There's a differential influence of increasing temperature. So for the same increase in temperature, respiration will increase more than photosynthesis.

So this means that the biological carbon pump would potentially be less efficient because more of your carbon is respired back to carbon dioxide into the atmosphere and less is stored in the ocean. And in fact, models have suggested that just due to this differential effect of temperature on photosynthesis and respiration, by 2100, the uptake of carbon dioxide by the biological carbon pump will have been reduced by 21%.

What would that mean? Well, therefore, you have more carbon dioxide remaining in the atmosphere and less stored in the ocean. And therefore, all of the problems of increased carbon dioxide in the atmosphere, so global warming, ocean acidification, would be enhanced.

There are also indirect effects of temperature on plankton. So if you think about it, temperature will change the mixing of the water and that will mean that the availability of nutrients and the availability of light will be different for plankton. And it will also change the habitat of those plankton that live in sea ice.

So some long-term data that's being collected from British Antarctic Survey scientists have shown a linkage between the decrease in the extent of sea ice and the decrease in the duration of winter sea ice, with less phytoplankton being able to live in the ice, being less of a food source for Antarctic krill. Less Antarctic krill is a food source for penguins and seals.

And in the following year, the success rate of hatching of penguin eggs and the survival rate of seal pups is decreased. So in years when there's less ice, there's less phytoplankton, less krill and less successful hatching and survival rate of seal pups. That's a sea temperature. Abigail, what about if the world temperature changes? Is that an extension of what's been said?

Yeah, as climate change progresses, the atmosphere gets warmer, and this actually warms the ocean as well. So we've seen lots of effects on plankton, so Carol's mentioned a few. But what I think is what I'm really interested in is how the distribution of which plankton live where, how that's changed with increasing temperature because of climate change.

So I mentioned earlier the Continuous Plankton Recorder Survey has collected plankton data from our world oceans for over 90 years. And because it's such a long time series, we've been able to map through space and through time the changes in the plankton community. And what we're seeing around the UK, for example, is something called tropicalization. So as our seas are warming...

Plankton that are better suited for a warm environment further south are now moving northward to be around UK waters, while plankton that previously were really abundant here that are quite cold temperate plankton are being squeezed poleward. So we're seeing this tropicalization, this change in the community composition of the plankton that's driven by temperature.

And so there's some food web implications for that. So, for example, big fish that we eat in the UK, like cod, their larval stages feed on plankton. And often these colder water types of plankton are better fish food for things like larval cod because they're bigger, they're fattier.

And the warmer water plankton that are moving into our northern waters are smaller in size. So they're not quite as good of a fish food. So we would expect these big fish like cod to also follow their prey and start to move northwards as well. So this change in the plankton that's being driven because of changes in temperature is having effects all throughout our food web.

Thank you. Can we take this on in a way, Chris? Are there ways in which humankind has had an impact on, not so much global warming, but other ways in which it's had an impact on plankton and the movement of plankton? The distribution of plankton around is largely going to be driven by these changes that we're seeing. But as humans, we've also had an effect on where plankton can be found in other ways.

And the big way that we've done this is through shipping, so the movement really of goods around the world. So there are a few ways this can have an effect. The first one is essentially ships obviously go from port to port. They go across ocean basins. And as they go along, you will have different species of organism will settle onto the hulls of these ships. And these things that settle onto ships generally are species that don't move very much. So they often don't move at all as adults.

And that means that to reproduce, to spread around, what they tend to do is have planktonic larvae. So sessile organisms, organisms that don't move very much, will spit out larvae. They're things called mirror plankton. They spend part of their life in the plankton, part of their life as things sitting on the seabed. So the bits that sit on the seabed don't usually move around very much and will often not be able to spread very far with the plankton.

If instead you settle down on a ship and that ship goes from Swansea to New York, all of a sudden you have a whole new area open to you. So these adults, which normally wouldn't be able to move, are able to produce larvae, which can be distributed and settled in different parts of the world that would do otherwise. So you have direct movement of adults from one place to the other and the opportunity to resettle whole areas as a result of that. A secondary but similar thing is going through ballast water.

So as ships move around the world, when they, for instance, empty an oil cargo, they change the weight of the ship and they have to take water on board to act as ballast to stabilise that boat, that ship. That ship's ballast water could contain plankton. And so when that ship moves to the other side of the planet and pushes that water back out, potentially there are planktonic species in there as well, which could be colonising areas they wouldn't otherwise have access to.

And then finally, we can look at massive infrastructure projects as well. So things like the Panama Canal, the Suez Canal. So these are things that we have built which directly link ocean basins, which otherwise wouldn't be connected. So go between the Pacific and the Atlantic and allows potentially movement of plankton species that otherwise you'd have land blocks between them, which wouldn't otherwise be able to move across. So species that were in one ocean basin connected.

can transfer themselves across these bridges that we've created by creating these canals and allow access to invasive species, which could potentially have quite devastating effects on different communities. So there have been a number of cases of this over the last few decades, things like the jellyfish nemiopsis ladyi, which, again, Abby was talking about the Black Sea having all sorts of problems. We had an influx of a particular species of jellyfish

which essentially completely changed the dynamics of the planktonic system as a result of one of these invasive movements. So although we have this movement of plankton due to changes in temperature, changes in light, changes in nutrients, there's also direct movement as well, which we can affect. Thank you. Carol Robinson, when the oceans become more acidic, what happens to plankton then?

The reason that the ocean becomes more acidic is because more carbon dioxide in the atmosphere dissolves into the seawater. It reduces the pH, which then also reduces the carbonate ion concentration, but it increases the carbon dioxide concentration. So there are a number of different effects. First of all, the increase in carbon dioxide can have a fertilising effect on phytoplankton growth.

So you get an increase in photosynthesis because there's more carbon dioxide in the water.

But again, we have a differential effect so that some phytoplankton are better able to grow in higher concentrations of carbon dioxide. So one experiment I can remember was done in the Antarctic where they grew phytoplankton in carbon dioxide concentrations that we have nowadays. And the dominant phytoplankton was a very small needle-shaped plant.

phytoplankton. And then when they grew the same sample of water in higher concentrations of carbon dioxide, the dominant phytoplankton became larger chain-forming phytoplankton. So the increase in carbon dioxide can shift the dominant phytoplankton organism.

There are also phytoplankton which produce a calcium carbonate or chalky shell around themselves. And one of these groups of phytoplankton are known as coccolithophores. And these look like small, tiny golf balls when you look at them under the microscope.

And there are also zooplankton, which produce a calcium carbonate shell. Things like tiny snails, which move in such a majestic way that they're called sea butterflies. They also have a translucent calcium carbonate shell. And actually the coccolithophores...

form blooms or cover areas of the North Atlantic, hundreds of thousands of square kilometres of the North Atlantic. So they are very important species within the phytoplankton.

But these organisms that produce this calcium carbonate shell, because of the low pH and the low carbonate ion concentration, have much more difficulty in producing this calcium carbonate shell. So you have these opposing effects of the fertilisation of more carbon dioxide means more and more cells, but the inhibition of producing calcium carbonate shells means that they are less calcified.

And this means they're less heavy and they can sink faster.

more slowly and this goes back again to the biological carbon pump so the depth at which these cells are degraded and respired back to carbon dioxide if you're a cell that has ballast of calcium carbonate you can sink very fast and you're degraded at depth and the respiration the production of carbon dioxide occurs at depth and that carbon dioxide stays trapped in the ocean for hundreds of years.

If you're a cell that has lost your calcium carbonate shell, then you'll sink much slower, you'll be degraded, and the respiration will produce carbon dioxide in the shallow water, which can very easily then evade back to the atmosphere. Thank you very much. Abigail, we've told a lot about the importance of rainforests and the importance for the future of the planet. What would happen if we lost plankton?

If we lost plankton, we wouldn't have the same planet that we have today. So we've heard a bit about how plankton produce, like you said earlier, 50% of oxygen. So one out of every two breaths you take has come from the sea. It's come from phytoplankton. So without phytoplankton, we wouldn't even have an atmosphere that we could breathe today.

And these phytoplankton support the whole marine food web. So small zooplankton eat the phytoplankton, big zooplankton eat the small ones, then larval fish eat the small zooplankton, then big fish eat the little fish and even whales. So even the blue whale, 100 feet long, largest animal to ever live on Earth, eats plankton. So they eat crustaceans, a type of zooplankton.

So if we just removed phytoplankton and removed zooplankton from the sea, the whole food web would collapse. So phytoplankton and zooplankton are really critical for supporting the marine food web. So from the fish that we eat...

to all those organisms that you see on the rocky shore that started out in the plankton, like Chris described, that were meroplankton, and then settled in their adult stages to the rocky shore. We wouldn't have any of those species if we removed the plankton. So the plankton really are critical to the functioning of our entire marine ecosystem.

What, if anything, can be done to support plankton? Plankton themselves are essentially going to survive more or less whatever we do. Why are you so sure? Because essentially when we talk about plankton, the subject is so big. When we're talking about these things, it is essentially the entire diaspora of life on Earth. If there are conditions there that life can survive in any way, then there'll be plankton species that can do it.

that has to be really quite extreme for that not to be the case. However, if we're looking at trying to maintain things in the way that they are now, that's a little bit different. Probably the best thing to do is for the entire human race to emigrate to Mars because it is going to be us driving change that are causing those problems. But if we want to maybe think in terms of supporting the plankton in such a way that perhaps they can help with the homeostasis of the planet, maintaining things as they are, there are a few potential things to think about.

One idea that was sort of floated in the middle of last century by John Martin was the idea that potentially there are other things out there other than sort of the main nutrients that we were talking about and light, which are restricting the growth of phytoplankton. His suggestion was that give me half a tanker of iron and I'll give you a second ice age.

And this comes from the concept that it's not just the macronutrients, the nitrates, the phosphates and the silicates which are causing problems and holding back our growth of plankton. There are large parts of the earth where we have high concentrations of these nutrients and we have plenty of light as well. So there should be lots of primary reduction going on.

But when we look at these places with satellites, they sort of come across as being this lovely deep blue area, which is indicating there's not much going on. And the suggestion was that actually the problem here isn't those major nutrients, it's something different, and that's iron. So iron is a part of a lot of biological molecules that are required for growth. But it's something that's not actually present in very high concentrations around the oceans. And that's because it's very much a land-based chemical to get.

So this tends to come in from the atmosphere, from dust coming in from deserts and then rain bringing it down, or indeed from meteorites coming down, which have a fairly high iron content as well. That tends to be accumulating mainly around the coast that we get the input from that. So we have these large areas which are probably very depleted in iron, and so what we can do potentially is go out and sieve these areas with iron. So back in the 90s and the early 2000s,

We had a couple of cruises where we went out and basically pushed iron out of the back of these boats.

And as a result of that, we saw very high increases in primary production from our satellite images. So we're seeing increases in populations of these phytoplankton and potentially, as a result, increasing the drawdown of carbon dioxide, which could help to reduce climate change rate. However, this is probably a relatively short-term thing and may not have that great of an effect. We're towards the end now. Carl, how much confidence do you have in somehow engineering the oceans to support plankton?

We're not at the stage, I think, where we can quantify our certainty or our confidence. We will unlikely remain below the 1.5 degree Celsius warming that we want to remain below by 2100. And that probably the way to maintain confidence

Our global warming, less than 1.5 degrees, is to also undertake so-called carbon dioxide removal approaches. And these can range from restoring, conserving coastal environments that we know are good stores of carbon, things like seagrass beds, mangroves, salt marshes.

to adding alkaline powdered rocks to the sea, just as we add lime to our gardens, to also enhance supply of nutrients, as Chris mentioned, with iron fertilisation. However, it's incredibly difficult to know how these would play out in the real world. We know that it works on small-scale experiments, but remember we're trying to draw down billions of tonnes of carbon dioxide.

We don't know whether or not these processes would work at that scale.

And we also have the challenge of our methods of measuring and determining the biological carbon pump. We don't know whether we can determine the variability in the biological carbon pump well enough to be able to measure the additional carbon dioxide that we would hope one of these approaches would cause. And we don't know that we can attribute any additional drawdown of carbon dioxide to a process.

And of course, we're very worried. We've been talking about the wonder and the beauty of the plankton, our marine environment, and we don't want to do anything that would harm that. We need to have an efficient biological carbon pump because, as we've said, it supports food production, it mediates climate, it keeps the atmosphere oxygenated.

We have to be very careful. Thank you very much indeed, Kyle Robinson, Abigail McWatters-Gollop and Chris Lowe and our studio engineer, Steve Greenwood. Next week, we're in 1787 with the Federalist Papers, Hamilton, Madison and Jay's influential arguments for a new United States Constitution. Thank you very much for listening. And the In Our Time podcast gets some extra time now with a few minutes of bonus material from Melvin and his guests.

Thank you. Thank you. Now, just a second. I'm afraid there's more work for you to do. What would you like to have said that you didn't have time to say, starting with you?

So one aspect of the plankton I'm really interested in is their ability to inform decision making. So I work with policymakers in Westminster and Brussels and in other countries too, to take all the science that my colleagues that we're doing on plankton and all this understanding that we've developed about how plankton are changing in response to human pressures, to climate change.

and put that into evidence that policymakers can use to make decisions about how to manage our environment in a way that's sustainable. So, for example, if we're managing our fisheries, yes, we are catching fish, but fish, larval fish, eat plankton. So we also have to understand that

how changes in the plankton are affecting the diet of those larval fish so that we can manage the amount of fish that we're fishing in a way that doesn't overfish our fisheries. And there's loads of different examples where we can use plankton information to make more informed decisions about how we manage the marine ecosystem. So seabirds is another great example. So puffins, for example, eat these little tiny fish called sand eels.

that start out in the plankton and that eat plankton. And if we're seeing changes in sand eels, which we've seen in the North Atlantic, it tells us that, okay, that's a warning sign. We should look and see if there's changes in seabirds. And when we do look, we do see, yes, there's changes in puffins because there's been changes to their diet. So in this way, all this...

science that's happening around plankton can be used to kind of support and inform decision making from policymakers about how to manage our marine environment. And I think it's remarkable that these organisms that we can mostly only see with the microscope can provide this kind of information to help us manage our marine environment in a better way.

I think that fundamentally the concept that I have with this is this is such a massive subject area. When we talk about plankton, it's essentially, like I said, all of life in that everything that is on Earth, apart from mammals, birds and reptiles, are represented in the plankton. So anything that's having massive scale effects on these organisms is something that we really need to take notice of.

And the fact that we're able to see these changes that we're having on these things from space, and we're able to track this over a period of decades, and we are seeing these changes from these satellite images over the last 25, 30 years, is something that really needs to be a wake-up call to people, and very much tying with what Avi was just saying, in that if we are seeing these changes over only a period of a few decades, we've been seeing massive changes in the seasonality of when plankton are appearing in the Antarctic oceans.

It's obvious that these changes are happening. One thing we haven't really had a chance to discuss here was how this ties in with the physics and chemistry of the oceans as well. And we're making records of how that's changing. So when we bring all these things together, it really does bring everything into sharp focus as to how big a change we are having on the planet and how we can see that even from space, the effect that it's having and the potential disastrous effect of that on the population of the planet.

Yes, picking up on that. So we're talking about global effects and therefore this means as a scientific community we need to work in an international way. And this depends on international funders that are prepared to bring together scientists from around the world to form networks and working groups to share data and to share best practices.

And it also means that we have to have a very close relationship between technologists and engineers, as well as natural scientists, because the way that we will continue to monitor and understand the world, the marine environment, is to have better sensors, a network of floats and underwater gliders, and to have a better understanding of the world.

that can give us the information of what is occurring, where the plankton are occurring and how they are reacting to the changing environment. We talked about an increase in temperature and we talked about an increase in carbon dioxide and a reduction in pH. But remember there are lots of other things happening at the same time. The oxygen concentrations are decreasing, we have changes in nutrients.

And the complexity of the biological carbon pump is such that we need to understand much better the feedbacks that could occur within the marine ecosystem. I think this is a very exciting time for marine science and for society, really, to understand and appreciate value and, as Abby says, manage sustainably the marine environment.

Everything Carol said, I just, you know, bringing together scientists is not just bringing together the data. But as scientists, we're always struggling to find funding to do this kind of work and even to meet together as groups of scientists where we can be advance our knowledge. And that is really key to this. There's loads of great science happening. But if we can't figure out how to work together and if that's not funded or supported, you know, all these questions.

changes that we need to be making will just happen more slowly. So these networks of scientists and their data are really critical to figuring out how to manage our oceans. I think Simon Tillerson is about to offer you a delicate cup of tea. Does anyone want tea or coffee? No, thanks. Melvin, tea? I've had enough tea for a lifetime. I'll go for coffee, please. Coffee, coffee. Tea, please. One coffee, one tea. Nobody else? I'll have a bit more tea, please. Two teas, one coffee. Thank you very much. Thank you.

In Our Time with Melvyn Bragg is produced by Simon Tillotson. Have you ever wondered who you really are? It clicked in my mind suddenly. I was like, why have I never done this? I'm Jenny Kleeman, a writer and journalist. In my new series, The Gift, from BBC Radio 4, I've been uncovering extraordinary truths that emerge when people take at-home DNA tests.

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And for other people to not be hurt, for things to be different in the future. To bring it into the light and almost alchemise some of that evil stuff that went on and take back the power. World of Secrets, Season 6, The Bad Guru. Listen wherever you get your podcasts.