Plant Science, Composting and Mosquito Repellents

15 January 2006
Presented by Chris Smith, Kat Arney



This week we go green as Alison Smith discusses how algae get their vitamin B12, Tim Upson reveals the science behind composting, Heather Gorringe and Richard Fishbourne dish the dirt on what worms get up to in your compost heap, and John Pickett talks about natural mosquito repellents. Sticking with the insect theme, Nigel Franks describes how the first example of animal teaching has been found in ants, while in Kitchen Science, Derek is itching to reveal how ants lay chemical trails to their food.

In this episode

Stardust Mission Makes it Safely Back To Earth

Space missions are fraught with potential disasters. In 2004, the Genesis mission crashed back down to earth, raising fears that its cargo of precious particles captured from the solar wind had been lost. And we all remember what happened to the Beagle mission to Mars. So hundreds of space scientists were understandably nervous as their latest baby, the comet-chasing Stardust capsule, came hurtling back to earth at 29,000 miles per hour on Sunday morning. Since it left earth in 1999, the US$212 million capsule has been on an amazing mission to brush past the comet Wild 2, which orbits out past Neptune in an area of the solar system called the Kuiper belt. Comets are rather like the clump of ice and frozen peas that you find at the back of the freezer. They are full of material that has been around since the origin of the solar system. This kind of material can tell us much about how our solar system was formed, almost like a time capsule dating back four and a half thousand million years. After a nail-biting finish, Stardust safely touched down in the Utah desert at 10:12 GMT on Sunday. Stardust has travelled an incredible three billion miles on its epic voyage, and was sent to within 149 miles of the comet's core. Using special tennis-racket shaped collected filled with aerogel - an incredibly light substance - Stardust picked up tiny particles shaken off by the comet as it hurtled through space. The capsule has been transported to NASA's Housten Space Centre for opening, but around 150 scientists all over the world be be able to study the spacedust inside. This includes Professor Monica Grady at the Open University (link to Monica's show) and researchers at Imperial College, London. They will asking many questions, such as whether comets could have deposited the water essential for life onto the earth's surface.

Ant-i Biotics

Scientists at the University of Wisconsin have discovered how a species of leaf cutter ant produces its own home-made antibiotics to keep fungal infections at bay in the nest. Leaf cutters are nature's gardeners. They crop green plants and carry the pieces back to their nests where they infect them with a fungus which breaks down the plant matter and is then, in turn, itself eaten by the ants. But just as weeds grow in even the most carefully tended gardens, the ants nests are threatened with invasion by a poisonous fungus, called Escovopsis, which the ants have to keep at bay. They do this by coating their body surface with a species of filamentous bacterium, called pseudonocardia, which provides the ants with an antibiotic chemical weapon with which to combact the invading fungus. Until now it wasn't known how the ants carried or nurtured their bacterial charges, but writing in this week's edition of the journal Science, Cameron Currie and his colleagues at the university of Wisconsin have discovered that the ants have evolved a special system of pouches, called crypts, each of which is fed by the ant-equivalent of a sweat gland. These provide the ideal home for the bacteria which take up residence and turn themselves into portable pharmacies, producing a cocktail of antibiotics which the ants can dispense around their nests. So it looks as though Alexander Fleming may have to surrender the crown for discovering antibiotics - these ants must have been using them for millions of years!

- Nigel

The Naked Scientists spoke to What we've been able to show for the first time is a really good example of teaching in animals. In fact teaching hasn't been...

with What we've been able to show for the first time is a really good example of teaching in animals. In fact teaching hasn't been shown in any non-human animal ever before. The remarkable thing is that we haven't been a

Chris - And this is one ant showing another ant how to go somewhere to find food or something.

Nigel - That's exactly it. But what's really interesting about it is that it's a very slow process. What we see is that the follower stops periodically, and to all intents and purposes we're sure that's it's looking round for landmarks to learn. Only when it's learnt a landmark does it signal to the leader that the lesson can continue.

Chris - How do they actually tell each other that they're doing it right? How is the information passed between the teacher and the pupil?

Nigel - It's a very interesting thing, and what happens is an absolutely gorgeous bit of behaviour. The follower taps on the hind legs and abdomen of the leader, and only when the leader is very frequently tapped by the antennae of the follower, does the leader proceed. What you can do is take the follower away and use a hair to tap on the leader. If you tap very frequently, she'll keep running. In other words, it's a signal from the pupil that says that they've learnt this part of the path and now they can proceed.

Chris - Now in medicine we have this motto where you see one, do one, teach one, and you might apply that to an operation for example. Does the same thing happen in ants?

Nigel - I think it does, because once the follower has got to the food and knows where it is, will actually return to the nest and very frequently begin to teach somebody else. So even though tandem running is rather slow and ponderous, the beauty of it is that the pupil can, in turn, become a teacher and so the information can flow through the society.

- Natural Mosquito Repellents

The Naked Scientists spoke to Prof. John Pickett, Rothamsted Research, Hertfordshire

Natural Mosquito Repellents
with Prof. John Pickett, Rothamsted Research, Hertfordshire

Chris - Tell us about your research. How did you actually get into this and what have you found?

John - Well there are a lot of stories about people being differently attractive to mosquitoes and other nuisance flies. The perceived wisdom is that the people who aren't attacked lack some attractant material. In fact what we've found is quite the opposite. People who aren't attacked produce some extra chemistry and put off the mosquitoes and other flies. The way we found this was to look first of all at the emanations that come out of people's skin. We chose a number of volunteers and looked at the way the mosquitoes would fly towards their arms in a device called a wind tunnel.

Chris - Presumably you had to pay people quite a lot to take part in this research!

John - No, they volunteered, and we didn't actually allow them to be attacked by the insects. We stopped them before they reached the arms so we could count them as they approached. What we did then was to take the volatile chemicals coming out of the skin of the volunteers. We did this by putting the volunteers into a kind of bag, which we kept a positive pressure into, so we could draw air out of it and take out the chemicals that the volunteers were putting into the air. By that means we could show that the same people that were not attacked by the mosquitoes also produced a range of chemicals that had the same effect.

Chris - How did you actually identify what the chemicals were?

John - Well that's even more clever. We use a lot of chemical analysis in this kind of work, but we can also use the insect itself. We can put small microelectrodes into the antennae of the mosquito and find out which chemicals in the complicated mixture given off by the skin were actually repelling the mosquitoes.

Chris - So essentially you attach a wire onto a nerve fibre coming from the antennae and record from that when you present different chemicals to the mosquito and see what the response is electrically.

John - Yes. We can either record from individual neurons or we can record from the whole antenna. In fact, we used a preparation that used the whole antenna in this work.

Chris - And if a mosquito is particularly put off by a smell, does you still get a positive response from the antennae or does it switch off?

John - The antennae is just a recording device. It's the central nervous system in the animal that decides whether an animal is going to approach the source of the material, or whether it goes away. What we've done is looked at the chemicals that are produced by those people who are attractive, and then we've tried to find the extra ones that the mosquitoes respond to from people who are not attractive. These have then been tried out in this wind tunnel device to see if they will put mosquitoes off from going to the attractive people's hands. That works.

Chris - So you can confer unattractiveness on individuals by decorating them with this smell.

John - Yes, and in the field. With help from our colleagues at the University of Aberdeen where they have the Scottish biting midge, we've shown that you can reduce almost to nothing the normal attacks that you get from the Scottish biting midge.

Chris - Why do some people have these chemicals, but not others? You would have thought that it would be a huge advantage if you think of places like Africa where they have malaria. If you have gene that means that you have components in your sweat that put mosquitoes off, it would be so advantageous that everyone should have it.

John - Yes that's right. The close relationship between some animals and these biting flies have meant that the mosquitoes are always keeping up with how we try to evolve away from them. But the kind of chemistry we're seeing sees to relate to stress. Of course, mosquitoes have to put in quite an investment to get a blood meal from somebody. They have to inject I chemicals that dilate the blood vessels and stop the blood coagulating. That's quite an expense for the mosquito, so they're very keen not to go to somebody who's stressed. We think that might be the reason for it, but we're not certain that this is true at the moment.

Kat - Are these pheromones going to be commercially available?

John - Well it's creating a lot of interest at the moment. We've been able to use one of the main commercial repellents, deet, in our comparative work in Scotland and we can certainly beat that in the short term. So there's a lot of interest. What we're doing at the moment is writing up the work and patenting it. That will of course facilitate commercial development.

- How Algae Get Their Vitamins

The Naked Scientists spoke to Dr Alison Smith, Department of Plant Sciences, University of Cambridge

How Algae Get Their Vitamins
with Dr Alison Smith, Department of Plant Sciences, University of Cambridge

Chris - Tell us about your algae.

Alison - My algae constitute the largest group of organisms that fix carbon dioxide from the atmosphere, which means they take the carbon dioxide and turn it into sugars. They use the energy of the sun to do that. Algae are responsible for about 505 of the world's carbon dioxide fixation, so they're very important in terms of global carbon cycles. What we've been interested in is the fact that algae aren't completely able to live on their own. This is because they require a source of vitamins. In that respect, they are quite like animals. There is one vitamin, vitamin B12, that plants on the land can't make . So if you're a very strict vegetarian, you won't get it from your food unless you don't wash your vegetables very well.

Chris - So what do algae do?

Alison - These algae appear to take it up from their environment, but they can't get it from sea water because the levels of vitamin B12 in the sea water are just to low to support their growth. Instead what they appear to do is get it from bacteria.

Chris - So these algae have bacterial clusters hanging around with them that are feeding them vitamin B12?

Alison - Well it appears to be like that. It's hard to show it happening in the seas, but what you can do in the laboratory is grow the algae in a flask. You can then see that around the outside of the algae there's a glue-like mucilage and the bacteria stick to that.

Chris - So in return for giving the algae B12, do the bacteria get some protection then?

Alison - It's not so much protection as they actually get some of the sugars that the algae make.

Chris - So it's a symbiotic relationship.

Alison - Exactly.

Chris - Now, I want you just to comment briefly on this recent methane story. Frank Keppler from the Max Planck Institute said that they were just messing around in the lab and found that plants produce large amounts of methane. They don't know why they do it, and even the leaves they drop my produce bits and pieces of methane. But why should they do that and what do you think the implications are? What will this be doing to plant scientists around the world now?

Alison - I think it's something that's completely unexpected, like you say. It really does require a considerable effort on behalf of plant scientists to work out why plants are doing this, what the benefit is and what's the chemistry behind it. These are all very interesting questions. In fact plants give out all sorts of volatiles and I think John knows about one of these.

John - Yes. Methane is potentially very useful but it's not valuable enough to capture from plants. We're working on an idea at the moment to try and capture isoprene, which is a really very valuable chemical. It's the type of chemical you can make rubber out of, and if we can capture that from plants we'd stop it becoming a greenhouse gas problem and get something really very useful for industry.

- Composting And Wiggly Wigglers

The Naked Scientists spoke to Heather Gorringe and Richard Fishbourne

Composting And Wiggly Wigglers
with Heather Gorringe and Richard Fishbourne

Chris - So tell us, what actually is Wiggly Wigglers?

Heather - We're a natural gardening company, but we started our business worm composting, so supplying kits to individuals so that they can turn their kitchen waste into lovely worm casts. I think you had one in Australia!

Chris - Yes, when I was in Australia I did have one on my balcony because we lived in a flat and we had a roof garden. We found this thing could consume kitchen waste faster than you could put the kitchen waste in there. The worms turned it into this wonderful juice.

Heather - Yes, but not only juice, but they also make a cast as well. They like dog hair and bananas too!

Chris - You've structured your company around trying to get people to use science to get their garden to perform a bit better. How long have you been going?

Heather - We've been going for sixteen years now. When we started it was just myself , and now there's fourteen of us - thirteen women and one man!

Kat - And that's the man we've got here! I've got a quick question for you. My dad is really into composting, and he likes to pee on the compost heap because he heard it was good for it. Why is that?

Richard - In conventional composting it's probably quite a good idea because of the urea involved. If you're looking to compost with worms, it's probably worth avoiding because generally, worms don't like acidic conditions. Some worms have a calciferous gland for excreting and producing calcium. This is why you don't tend to find worms in places where you find acidic soil and rhododendrons for instance.

Chris - So you encourage people to add a dose of worms to their garden to improve the quality of the soil. But does it actually work?

Richard - It does. Worms are the most amazing improvers of soil. When we set up a worm kit, what you're doing is harnessing a natural phenomenon really. You're introducing organic material and the worms are consuming it. The casts that they produce are something like five times higher in nitrogen, seven times higher in phosphorus and a thousand times higher in the number of bacteria in the casts than the original soil.

Chris - So if you've just moved into a house on a housing estate that's just been built and you have that wonderful clay that they give you for your garden, if you sling some of these worms on there, does it make a difference to the soil?

Richard - Absolutely. Worms are obviously going to improve to soil by increasing the aeration and allow oxygen to diffuse into the soil. They also increase the soil permeability. Interestingly enough, they also increase soil water retention, especially around plant roots.

Kat - Where do worms go when they die? Do they get eaten by other worms?

Richard - Probably! Indirectly at least! There are species of flatworms that exclusively feed on our indigenous earthworms. Generally if a worm dies, there'll probably be eaten by nematode worms.

Kat - So if I was to set up a wormery, what sort of thing should I do? Is it hard to look after worms?

Heather - They're not too fussy actually. We sell all different sorts of wormeries and you can just compost with worms on their own. The easiest way to do it is to buy a ready-made thing. Thatw ay you can get the compost out so much easier.

Chris - they're also not huge are they Heather? They're not too big and in the way. It doesn't smell and is quite compact.

Heather - They're about 50 centimetres wide and about 75 centimetres tall, so they can fit in most places.

Chris - How much does it cost to set up?

Heather - You can start with worms and bedding from about £13 or you can have the whole thing for around £89. So there's a full range of prices.

Kat - Is there anything you shouldn't give to your worms?

Heather - You should go easy on citrus peel, and if you do put a lot of it on, you should add extra egg shells or calcified seaweed or something. Other than that, they're pretty cool. They like things like cardboard, cooked waste and raw vegetables. Potato peelings take a little bit longer to break down, but it's really one of those things where you use the natural resource to its best ability. If it goes really cold, they slow down a little bit.

Chris - Are they going to last forever? They would hopefully breed in your wormery, or would they need replacing from time to time.

Heather - It's bad news if they need replacing. I had a customer once who insisted on feeding his son porridge every day but his son didn't like it. It ended up on the compost heap and it did cause a bit of a film over the top! However, it was easily fixed with a bit of cardboard and a bit of variation in their diet. If you do that, the worms will stick anything!

- The Science Behind Composting

The Naked Scientists spoke to Dr Tim Upson, Superintendent of Cambridge University Botanic Gardens

The Science Behind Composting
with Dr Tim Upson, Superintendent of Cambridge University Botanic Gardens

Tim - Composting is perfectly natural process. It's the process by which any organic matter decays and basically composting in gardens, for example, creates a very useful substance which we usually use to enrich our soil.

Chris - What actually does the composting process? Presumably it's some sort of thin gin the microbiological world.

Tim - That's correct. It's things like bacteria that are the primary decomposers. Then as the compost matures, other organisms such as fungi and particularly worms, woodlice and even slugs in very mature heaps come in a really refine it. If you get those organisms coming in you produce this really fine substance that's really nice to handle and to use in the garden. Of course, plants love it and it promotes plant growth and produces very healthy soil. This is all necessary whether you're growing ornamental plants or vegetables in your garden.

Chris - What are the best things to sling on a compost heap, and are there any definite no-nos?

Tim - Yes, I suppose it's a little bit like cooking to a degree. What you want to do to create an ideal mixture is to include greens. These are things like lawn clippings, annual weeds and anything which is green and breaks down easily. This needs to be mixed in with something called the browns. These would be things like twiggy branches, all the way through to household products like toilet roll holders. That adds bulk, keeps the compost open, and allows the air to penetrate. All of the bacteria and worms can keep functioning properly because they have a good air supply.

Chris - Dot he worms actually make much of a contribution, because presumably if you do get worms flocking to your compost heap, they mix things up and down. Do they do anything else?

Tim - Yes, worms are the gardener's best friend in many ways and of course they naturally occur in the soil. If you've got your compost heap directly on the soil then they will penetrate into it.

Chris - What about adding worms? If you buy in worms, is it useful?

Tim - Yes, you can do that. In fact there are now systems available which take advantage of the fact that worms eat organic matter. It passes through their gut and comes out the other end as worm poo and that produces something called worm compost. It's actually very rich, and usually it's diluted down by mixing with a normal potting compost from a garden centre or some other compost from heap. If you use it on its own, it's too rich, but it's a wonderful substance and if you use it in the right way, it can really promote growth. In the botanic garden we have a display showing a whole range of different composting options. Some require lots of space but I always feel that one of these little worm bins, even if you live in a flat that maybe has a balcony, you can put it out there and keep it going with kitchen waste rather than putting it into the usual landfill stream.

- Why is a gunshot wound so deadly?

It's always intrigued me that someone can have an accident, crash into a fence that pierces their body, and survive without any problems,...

Why is a gunshot wound so deadly?

It's all down to the momentum of the thing concerned. The Stardust mission comet Kat spoke about earlier was actually travelling thousands and thousands of miles and hour faster than the probe was, and the tiny particles it was throwing off were capable of destroying that probe. And this is all down to momentum. The momentum of something is the mass of something multiplied by how fast it's going. That's how hard something slams into you. So it doesn't actually matter how heavy or big something is because the faster it hits you, the harder it hits you. If you drive into a fence at 60 miles per hour, the slamming effect will be the weight of your body multiplied by the speed that you're going. If you're hit by a bullet, although the mass of the bullet is much less than you hitting the fence post, it's going a lot faster. The average bullet coming out of a pistol is going at around 700 miles per hour, which is a lot faster than someone in a racing car. As a result, the momentum of a bullet is much higher. The reason bullets cause such a problem is because although they go in and make a very small entry wound, the shock wave of the bullet hitting your tissues rips you apart internally. That's why if you look at someone who's been shot from the front, they don't look too bad until you look at them from the back where there's often a very big exit wound. This is because as the bullet goes through, it compresses all the tissues it passes through and pulls them to pieces. It's like being hit with a fence post but much much harder.

- Why do mosquitoes bite some people more than others?

I'm curious about why some people get bitten more than others. In summer I can have some quite good bites on me. Is there any way that I ...

Why do mosquitoes bite some people more than others?

That's a very good question. I think that it's not just attraction that can give you bites. It can depend on whether you show a good response when the fly or mosquito gets there, such as if you notice it or not and swat it away. But in terms of attraction, you can take certain things in your diet. If you have very strong plant smelling chemicals in your diet that come through the skin, such as with garlic, then these can put off some flies. They're obviously carnivorous, so they don't want to come to somebody smelling very strongly of plants. However, it's not very reliable. Although quite a lot of the mosquito repellents we use commercially are based on that kind of effect, they're really not that good and the mosquitoes can break through because they can see the attractant chemical through the ones we try to mask ourselves with. That's why it's been very important to look at individual human beings to find out how it is that some of us are actually repellent to mosquitoes and other flies.

- Do humans release smells that attract flies?

For the last week there have been flies at work and they're more annoying than ever. By this I mean that they continually fly around your...

Do humans release smells that attract flies?

Many flies, which includes the mosquito and the biting midge, use carbon dioxide, heat and various other chemicals that are quite commonly produced. But they will also use specific compounds to see human beings. For example, the malaria mosquito can find a human being in the middle of a very large group of cows, which are oozing carbon dioxide and heat. It uses specific chemicals to find that human subject. It will be the same for the flies that are annoying the friend in Melbourne. These insects are able to perceive what kind of potential host they've got, even if it's not blood they're after, just the mucus around our eyes and mouths.

Where do flies go at night time?

Flies normally fly during the day, which is why they have very good eye sight. They roost in nooks and crannies and so on ready to get a meal from you. The carrot fly, which is a pest of carrots, actually roosts in hedgerows and we devised a means of passing a disease to them in the hedgerow. Unfortunately, unlike the mosquito repellents, that's a very long way from commercial development.

- How do Venus fly traps sense their prey without nerve cells and how do they contract without muscles?

How do Venus fly traps sense their prey without nerve cells and how do they contract without muscles?

How do Venus fly traps sense their prey without nerve cells and how do they contract without muscles?

That's a very good question, and in fact some work has been done on that quite recently. It turns out that the Venus fly trap has two disc-shaped leaves that invert themselves when an insect touches them. This happens very quickly, and they discovered this by doing time-lapse photography. The scientists who discovered it painted ultraviolet dots on the Venus fly trap so they would show up when they shone UV light on to the plant. They took 400 photographs per second while triggering a Venus fly trap and could map what happened. The analogy they used is that if you take a tennis ball, cut it in half and then turn it inside out, it will sit there quite happily. However, if you come along and just touch it very lightly, it pings back again.

- Why does bicarbonate of soda alter the colour of cabbage?

Why does the addition of bicarbonate of soda to the cooking of cabbage make it much softer and the colour stay much brighter?

Why does bicarbonate of soda alter the colour of cabbage?

The reason that cabbage goes a nasty brown colour when you cook it for too long is because it changes the chemical nature of the green stuff in the leaves, or the chlorophyll. If you put bicarb in there, it changes the conditions by making it more alkaline. This means that the magnesium ion that would pop out of the leaves in normal water, stays in there.

- What can comets reveal about the origin of the Universe?

I want to whether the analysis of the Stardust comet dust can tell us about the origins of the universe and the theory of the Big Bang.

What can comets reveal about the origin of the Universe?

The answer is probably no, because the Big Bang was 14.9 billion years ago, and scientists mainly study that by looking at left over radio waves. In contrast, the Stardust comet mission is going to tell us about the formation of the solar system, which is about 4 and a half thousand million years ago. So it probably won't tell us anything about things at the beginning of the universe.


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