Some enzymes have bluer blood than others and this antioxidant is more electric than most.
Robin: Welcome to you, Copper, in our interview series here at Exploring the Bio-edge. From time to time we feature trace elements of metabolic importance and you’re certainly a shining example of a micronutrient.
Copper: Thank you, Robin and Honey Badger, I appreciate being met as more than a mere metal.
The Honey Badger: We’d better first remind listeners that you’re going to be speaking mainly in your biological – as opposed to industrial or financial – voice.
Copper: Yes, a lot of humans don’t realise that metals exist in a kind of parallel reality inside the bodies of organisms, invisibly helping to run a non-metallic, living world (see Figure 1).
The Honey Badger: We’re not implying that DNA contains any metals, are we?
Copper: No. But it may come as a surprise that living cells contain metallomes as well as genomes.
Robin: Because many biologists may not be used to thinking of metals, could you offer us an outline of your basic nature? Perhaps an abstract identity on which to hang the facts? I’m thinking of something beyond the common knowledge that you are one of the best conductors of electricity.
Copper: Well, my electrical talent will actually help biologists to understand my organic roles. But let me divulge two equally basic aspects of my character.
Robin: What are those?
Copper: One is that Silver is my sibling but I’m married to Zinc. If you look at the Periodic Table of Elements, you’ll see that’s how I’m placed. Now, Silver and Zinc differ quite radically because one is basically an antibiotic whereas the other is about the most nutritious of trace metals. So although the electronic configuration I’ve inherited is a potentially toxic one, my wholesome marriage has made me a kind of natural medicine for many organisms.
The Honey Badger: Are you hinting that trace metals form some kind of biological alloy within organisms?
Copper: ‘Alloy’ is a bit too metallurgical. But living cells do contain interactive combinations of metallic elements, and so maybe a similar principle is involved. Anyway, just a suggestion to help listeners appreciate my purer self by first relating me to the significant other metals in my life.
Figure 1. A small paper clip of standard size, if composed of copper, would contain the equivalent of the total copper content of ten adult human bodies. The arithmetic is simple: the paper clip weighs about 1 gram whereas each human body contains a total of only 0.1 grams of copper. This limited requirement makes it remarkable that the body does not store and recycle a fixed capital of copper but instead absorbs and defecates a little of the metal each day.
The Honey Badger: And the second of your two basic faces, what would that be?
Copper: Well, of all the trace metals I’m the one that binds most strongly to organic matter. That’s just a consequence of my atomic structure.
Robin: Let’s ground ourselves in your metallic relationships first. If you’re a sibling to Silver but a spouse to Zinc, your alloys with those elements are respectively billon and brass, right?
Copper: That’s right. The alloy of Copper and Silver is billon, pronounced as in billionaire. And billon was in fact used by the Romans for their original coinage, although these days metallic cash is mainly Copper and modern coins actually contain no Silver at all. The alloy of Copper and Zinc is brass, which even today is still the material chosen to make, for example, some of the boldest musical instruments.
Robin: So a way for biologists to imagine your nature is that on the one hand you have a mercenary materialism, while on the other hand you’re as lively as a brass band?
Copper: (chuckling) You could put it that way.
The Honey Badger: Well, how about first giving us an example of your cold, hard, anti-biological side?
Copper: Well, I’m widely used as a biocide. Just spraying copper sulfate or copper nitrate on crops kills disease-causing fungi while actually nourishing the plants, because small organisms with large surface areas are over-sensitive to me. A drenching of Copper is lethal to fungal blights and rusts, a case of far too much of a good thing for them. This toxicity shows my kinship with Silver, which is still one of the most effective antiseptics ever discovered. Silver dropped from favour in medicine when penicillin was discovered, but it’s returning by necessity today with so many bacteria developing resistance to antibiotics.
Robin: I take it microbes and fungi don’t use you as a nutrient as plants do?
Copper: That’s not true; the real reason they die is just a matter of excessive quantity. I’m actually a nutrient to most small organisms as I am to large organisms. I can offer you a few examples.
Robin: Please do.
Copper: Well, both tea and cocoa only appeal because humans process them by means of Copper-dependent bacterial fermentations that get rid of bad-tasting compounds in the leaves or seeds. A second example is that fungi especially need Copper to produce their spores (see Figure 2). But if a little Copper is good for microbes, a little more can be bad for them. At increased concentrations (yet still little enough to be harmless to humans) I can also be used to kill microscopic algae in swimming pools.
Figure 2. Mosaic puffball (Handkea utriformis), a pasture fungus which produces large numbers of copper-rich spores. This puffball fungus can reach 20 centimetres diameter and contains more than 200 parts per million copper in its dry matter, making it more than tenfold richer in copper than most plants [photo by Luridiformis under the Creative Commons Attribution 3.0 Unported licence].
The Honey Badger: But why are microbes and fungi more vulnerable than large organisms to Copper toxicity? I thought bacteria could metabolise and neutralise all sorts of toxins?
Copper: Because small organisms have a big surface area relative to their body volume and I stick to organic matter like glue. They can’t avoid absorbing me in excess and then they find it hard to shift me once I latch on to organelles within the cell. Those physical and chemical principles are hard to overcome.
Robin: Is this particular susceptibility of microbes the reason why humans use Copper alloys – as opposed to wood or plastic – to manufacture hygienic handles and handrails?
Copper: Yes, the bacteria and viruses from grubby hands die on any coppery surface before the next hand comes along to pick them up. Likewise, water pipes were made of Copper before chlorine was introduced to sanitise the water supply.
Robin: Speaking of sanitary water, does copper sulfate keep a swimming pool blue by dissolving its blue crystals into the water?
Copper: The blue colours are a coincidence. The swimming pool stays blue because it is clean enough to reflect the colour of the sky, not because of the copper sulfate, which is far too dilute to add colour. What the copper sulfate does is to kill the microscopic algae that would otherwise turn the pool green.
The Honey Badger: Thinking laterally, did humans ever use copper containers to preserve food from bacteria, before refrigeration?
Copper: Indeed they did, and this succeeded so well that, in at least one case, wine and soup buried ceremonially in Copper-based containers have lasted up to 2400 years in the ancient Chinese capital of Xi’an.
The Honey Badger: Wow. Are you talking about the resting place of the First Emperor of China, with its famous Terracotta Army?
Copper: Yes, that’s the site. Archaeologists were surprised in 2010 when they dug up still-recognisable wine and soup.
Robin: That’s certainly an impressive example of antibiotic use of a metal. Hasn’t there also been something in the news about termites harnessing your toxicity for defensive purposes?
Copper: Yes, that’s a particularly graphic example of my nastier side. The workers of the South American termite you mention accumulate copper in a special pouch on their bodies. When defending the colony, they commit suicide by suddenly rupturing this and another pouch to shower enemies, such as ants, with a combination of copper and sticky substances. You could describe this as a ballistic chemical cocktail, with the concentrated copper, released into solution in this process, helping to kill the enemy insects to which it adheres.
Robin: But you’re less likely to be toxic to bigger organisms because their size buffers them?
Copper: Well, that’s how it works in general. But a strange fact is that several types of microbes are remarkably impervious to my toxicity.
Robin: Which ones are those?
Copper: Well, for example certain primitive prokaryotes, which have similar cells to bacteria but grow so slowly that I cannot harm them much. And then there’s the biomining bacterium, a rock-dweller which can live in iron and Copper ores, actually oxidising the metals and sulphides as an energy source.
Robin: This bacterium ‘eats’ Copper?
Copper: That’s right. It powers its rapid metabolism partly by oxidising Copper in the ores, in some places creating flows of copper sulphate which can be scooped up by miners on a scale that is commercially viable.
Robin: How does the biomining bacterium manage to break a rule as basic as your toxicity to metabolism?
Copper: Nobody knows for sure, but don’t forget that it oxidises iron and sulphide at the same time. I suspect that its trick may be to neutralise the electronic effects of Copper by means of the electronic effects of iron. A bit like fighting fire with fire. The ‘bioalloy’ concept we mentioned earlier is about this playing off of one metal against another.
The Honey Badger: What about an example of your vivacious side, your joie de vivre?
Copper: Well, apart from boosting immunity, as Silver and Zinc also do by the way, I act as a catalyst for sundry biological processes. Maybe because many of my roles haven’t yet been discovered by biochemists, nobody has yet been able to figure out a common denominator to all the chemical reactions I orchestrate.
The Honey Badger: It sounds like biochemists still see your nutritional values as a bit unpredictable. Could you elaborate on some of the benefits that have been documented?
Copper: Well, certain enzymes I activate are part of photosynthesis and the formation of haemoglobin. Iron is co-involved in both cases – yet another hint of a ‘bioalloy’. I’m an oxygen-carrying metal in the blood of invertebrates, in the place of the iron that the two of you use for the same purpose in your haemoglobin. My various enzymes also help to shape skeletons and connective tissues in both plants and animals. For example, I help to make bone, collagen, elastin, and lignin. But one of my most visible effects is that I catalyse the production of melanin pigments which give colour to a wide range of organisms from bacterial cells and fungal spores to mammalian fur and human skin. And, by being part of enzymes for making phospholipids and the myelin sheaths of nerve cells, I help to build and maintain brains and spinal cords.
The Honey Badger: That’s an impressive versatility.
Copper: Thank you.
Robin: Didn’t you say earlier that you act as a kind of medicine too?
Copper: What I meant there was that perhaps my most important service is as an antioxidant – which overlaps with some of the functions I’ve just listed, such as production of melanin.
Robin: Antioxidant, now there’s a word that every biologist will know even without studying metals or catalysts. Could you please remind us as to what exactly antioxidants do?
Copper: Any substance that protects life by keeping oxidation under control is an antioxidant. Organisms metabolise rather than rusting or combusting but there’s a fine line between these processes, chemically speaking. So every living thing needs to keep redox reactions on a short leash as it were, to bring out the good side of oxygen radicals and to prevent their reactions from flaring out of control and corrupting the tissue of life.
The Honey Badger: It’s easy to understand that guard dogs are useful animals as long as they are disciplined, but how can a metal like you keep a ‘leash’ on oxidation?
Copper: You could say it’s a matter of electronic juggling. When you respire or something combusts, what basically happens is an unravelling of the electronic fabric of the fuel used. In respiration there’s a nice orderly release of energy, but lethal chaos is never far away at a subatomic level. My special nature is that my ion, the Copper ion, is versatile enough both to receive and to deliver electrons in an orderly manner such that the potential chaos – when glucose or other fuels are ‘burnt’ – is controlled.
The Honey Badger: I see.
Copper: So I have the ability to appease an oxidant radical that would otherwise home-wreck by stealing an electron from a living molecule, leading to a kind of electronic contagion, a destructive chain-reaction that consumes the cells themselves. I can effortlessly juggle my spare electron back and forth as a dampener of oxidant radicals in an indefinite series of nano-appeasements. And so, even though my quantities are invisibly small, I can help to ensure that the only organic molecules to unravel are fuels, such as the sugars in cells, and that they do so at the right pace.
The Honey Badger: So your presence in certain enzymes ensures that respiration doesn’t get out of hand and become a self-consuming smoulder. And you can do this in both bacteria and elephants?
Robin: Ah, is that the link with electricity? Many listeners probably realise that electric currents in metals come from electrons vibrating on the spot rather than actually travelling down a wire. Are you saying that whether you’re conducting electricity or making sure that metabolism doesn’t get out of control, it’s because each of your ions can juggle electrons at lightning speed?
Copper: That’s a bit oversimplified, but you’re more right than wrong. In a Copper wire I can play fast and loose with electrons but as an antioxidant I concentrate on juggling one electron at a time, which means alternating between my cuprous and cupric ions. These descriptions are a bit of an abstraction but may give listeners some idea as to why metals like me can be nutritious to living things, even though organisms are electrical devices only to a lateral thinker.
Robin: So when you’re in a wire you can conduct an electrocution, but when you’re in an enzyme you prefer to conduct the jazz of life?
Copper: (smiling) You may be getting a bit too metaphorical now, but it’s true that the same properties that make me an antioxidant in small quantities also make me toxic in larger quantities.
The Honey Badger: Can you name a few actual chemical compounds in which you act as a vital antioxidant?
Copper: Well, the enzymes known as superoxide dismutases are important antioxidants in nearly all cells exposed to oxygen. Sorry if that’s chemispeak to some listeners; but it is worth making the point because they are so ubiquitous. Only a few types of bacteria use a different mechanism.
Robin: And how do your near-and-dear, Silver and Zinc, compare as antioxidants?
Copper: Well, Silver is an even better electrical conductor than I – too good in fact, for it usually encourages oxidant radicals rather than appeases them and so tends to exacerbate oxidative hindrance in living cells. Zinc, however, leans the other way. As a metal with limited valence it doesn’t stoop to what we call redox chemistry, but instead has even more virtues than I as a catalyst. So Silver can be medicinal against infections but is seldom a nutrient, while Zinc is very much a nutrient, and of the three of us I’m the one that can best be described as an antioxidant element.
The Honey Badger: So, you have convinced me of your importance, but does an animal like me risk any deficiency of Copper?
Copper: It’s not likely in your case because you eat mainly animals and their internal organs are a good source of me. The plants in which I’m most likely to be deficient are the ones growing on alkaline peats, because of the binding of Copper to soil organic matter that I mentioned earlier. If a deficiency in Copper is passed from such soils up the food chain, herbivorous or omnivorous mammals can suffer from, for example, heart attacks or scurvy-like symptoms.
The Honey Badger: Oh, I get the link – vitamin C is another antioxidant, isn’t it?
Copper: That’s right. But neither Copper-deficiency nor Copper-toxicity is easy to diagnose in most organisms. That’s partly because I’m only one of a whole team of antioxidant and catalytic substances, some of which are metal-free vitamins, and it’s the balance of various players that keeps the metabolic system resilient. What’s important to know is that there is one group of mammals that is so susceptible to deficiency that, for them, I’m more like a food than a medicine.
The Honey Badger: Which mammals are those?
Copper: The commonest forms of livestock: ruminants. Only in cud-chewers, such as cattle and sheep, do clear cases of Copper-deficiency crop up on a wide range of soils, not just alkaline peats.
Robin: Why ruminants particularly?
Copper: It’s their special digestive system. Humans keep ruminants as the main producers of meat and milk because these animals grow so fast. This productivity is the consequence of their complicated gut design that allows green foods to be digested wonderfully efficiently. The way the animals do it is by culturing hordes of bacteria in their stomach chambers as a kind of digestive outsourcing – or should that be insourcing? But everything has a cost and the extremely successful nutritional system of ruminants has its Achilles’ Hock.
Robin: How so?
Copper: All large animals depend on microbial enzymes for the fermentation of fibrous foods and the digestion of polymers like cellulose, because no animal larger than a snail can produce the necessary enzymes itself. It’s a win-win relationship because the ruminant profits and so does the bacteria, which get living quarters and food on tap. But one of the downsides of this dependence on symbiosis can be the binding of Copper to the organic substances and other metals pooled as a complex soup in the stomach vat of the ruminant.
Robin: Ah, a link between Copper-deficiency for crops on alkaline peaty soils and Copper-deficiency in the alkaline foregut of ruminants?
Copper: Yes, when organically bound in the fermentation of a gut, I can be hard to absorb and I tend to be wasted in faeces instead. This makes ruminants more sensitive to Copper-deficiency than other mammals and it explains why ruminants are prone to spinal abnormalities or bleached-looking fur even though they store an unusual amount of Copper in their livers in compensation.
Robin: And the remedy for Copper-deficiency is eating more Copper?
Copper: That’s the general idea. Ruminants do supplement other metals, particularly cobalt, by licking cobalt-enriched earths (see Figure 3). But for some reason it doesn’t seem to work that way in my case; geophagy seldom seems to supply extra Copper to animals. Even commercial lick-blocks don’t seem to be the best way to give extra Copper to livestock.
Figure 3. Beisa oryx (Oryx beisa beisa) (top, featuring an adult at the border between Samburu and Buffalo Springs National Reserves, Kenya) and Rocky Mountain bighorn sheep (Ovis Canadensis Canadensis) (bottom, featuring two juveniles in Jasper National Park, Alberta, Canada), eating inorganic matter directly from the surface of earth licks that are frequently visited by large wild herbivores. For reasons that remain poorly understood, copper is seldom among the micronutrients supplemented at geophagic sites such as these [photos by Gareth M. Jones and Alison Brown, respectively].
Robin: So what’s a poor farmer to do?
Copper: The supplemental Copper usually comes from industrial mining, and can just be injected straight into the veins of the ailing ruminant. In some cases it also helps if the molybdenum and sulfur in the diet are reduced, because these elements also tend to prevent the absorption of Copper. But even farmers who understand the chemistry can find themselves between the ore and a metal plate in supplementing Copper.
The Honey Badger: What’s the hard choice?
Copper: They have to administer just the right amount, not too little and not too much. Too little, and the adult cattle and sheep have normal fur colour but their newborns can still suffer from swayback because a subclinical deficiency of Copper has remained in the adults and the foetuses are most sensitive to it. Too much, and the adults develop the same bleached fur from excess Copper as they would have from a clinical deficiency in Copper. Strangely enough, that’s a common pattern in micronutrients: too much and too little tend to produce the same symptoms.
Robin: And the same pastures produce no signs of Copper-deficiency in non-ruminants, such as the horse?
Copper: That’s true.
Robin: I understand that horses and their relatives have simpler guts that can absorb Copper more easily. And presumably they just keep recycling the Copper within their own bodies?
Copper: Well, one of the mysteries of nutrition is that, actually, animals don’t manage to recycle me or other metals very much, and instead have to keep eating a little metal every day while a corresponding amount is excreted. So you wouldn’t know the difference between horse and ruminants from their faeces, which contain similar amounts of Copper. After the absorbed Copper has done its job in the blood and body of the horse, it is excreted via bile produced by the liver, through the bile ducts and into the intestines. The spent Copper then of course leaves the body in the faeces. So both the ruminant and the horse usually lose me in their faeces but in one case I’ve done my job in the body and in the other I may not even have entered the bloodstream.
The Honey Badger: Horse and pig have similar guts, but I seem to recall that pig farmers use Copper supplements too. The pig isn’t a ruminant, is it?
Copper: No, it isn’t, but unlike the horse it can grow as fast as ruminants. Up to ten weeks of age, the juvenile pig grows fastest if it receives tenfold the required concentration of Copper.
The Honey Badger: What is the function of the Copper if it is not absorbed by the juvenile pig?
Copper: It acts as a growth stimulant, perhaps by controlling gut bacteria that would compete with the pig for food. Because this is a medicinal rather than nutritional role of Copper, the extra Copper ends up in the pig faeces rather than the pork. The resulting slurry from commercial pig pens is so rich in Copper that it is close to being classified as toxic waste. You won’t find Copper on this scale when cleaning even the plushest stable for thoroughbreds.
The Honey Badger: I suppose that the wild ancestor of the pig might even have been able to self-medicate with Copper swallowed in the form of earth, because it naturally forages by rootling for soiled foods?
Copper: Trust a professional digger to notice that. It hadn’t even occurred to me.
Robin: So the next time a human sits down to a meal of cheese, beef, or bacon, you’ve been working behind the scenes to put that food on their plate in the first place, as supplementary Copper fed to ruminants and medicinal Copper fed to the pig?
Copper: That’s right, and chances are the Copper used was a product of mining rather than farming.
Robin: Why do you claim to be particularly important in feeding humans if there is a complex metallome and the antioxidant role is spread among several elements? Don’t you see the other micronutrients as being equally important?
Copper: Well, what many don’t realise is that, when fuel is used, most of the energy is actually released at the final stage with the formation of water. It may sound strange to be focussing on the wet stuff in combustion, but whether we’re talking about respiration in a living cell or even a bonfire or jet-fuel engine, most of the power is released by the chemical production of water – the ultimate counterpoint to the fact that, in photosynthesis, water must be split by the green cell to produce carbohydrate from carbon dioxide.
Robin: So making water molecules may sound like the simplest thing but actually turns out to be rocket science?
Copper: (chuckling) Indeed. My crucial importance is that I’m up to the task of controlling the electrons at that ultimate point in the cycle of life, when a molecule of water is remade from hydrogen and oxygen and a burst of energy is released. And cytochrome oxidase is the Copper-containing enzyme, or – if you like – antioxidant, that accomplishes that whether we’re talking about a bacterial cell or a human cell.
Robin: I see.
Copper: Although I see myself as one of the most important elements in the equation of producing food and releasing controlled energy within cells, I do acknowledge that there are some strong competitors for the supreme title. Elements like manganese may even beat my versatility in managing electrons, but I won’t go into those virtues because I understand that my rival is in line for an interview in your series.
The Honey Badger: You mentioned the trace metal molybdenum as antagonising Copper in the guts of livestock. Do any plants try to manipulate herbivory by taking up that element in addition to their own metabolic requirements?
Copper: Yes, the balance between Copper and molybdenum in leaves does seem to make a difference. For example, one widespread African tree(see Figure 4) is ignored by ruminants such as the giraffe but is eaten by the elephant and is a preferred food for large caterpillars. The plant still loses a lot to herbivory, as you can see from the fact that the local Africans collect these edible larvae in large numbers for their pantries. But caterpillars seem tolerant of ratios of molybdenum to Copper that deter ruminants. Meanwhile there are many trees and shrubs in other habitats that can sustain large ruminants even as winter-bare twigs, maybe partly because twigs retain an order of magnitude more Copper than molybdenum in the dormant cambium.
The Honey Badger: And do any plants actually lace themselves with Copper to favour herbivory? You know, plants like lawn grasses that do what it takes to attract grazers because they can compete best with other plants when under heavy herbivory?
Figure 4. A widespread African tree (Burkea africana) typical of nutrient-poor sands, which is unpalatable to ruminant herbivores partly because of its richness in molybdenum relative to copper.
Copper: That’s something that ecologists might want to investigate in future. But lawn grasses don’t seem to contain more Copper than most other grasses, and in general plants are pretty strict in keeping only the necessary traces of Copper in their leaves. It seems that any benefits in attracting herbivores would be outweighed by the risk of toxicity. Even plant species used by prospectors as indicators of Copper ores (see Figure 5) tend not to hyperaccumulate Copper in their foliage. They prefer to prevent any extra Copper from travelling up their stems even if they can’t avoid it entering their roots.
The Honey Badger: Can you tell us any other juicy miscellania about your role in animal bodies?
Copper: Well, I don’t know if you could call them juicy, but certain toads – you know, the terrestrial amphibians – accumulate me (see Figure 6). I suspect this is, at least partly, because they eat a lot of crustaceans such as isopods and molluscs such as snails, which may themselves be richer in Copper than the average animal. I’m not sure whether the toads actually use Copper for anything special, like their own defensive toxins, but I don’t seem to do the toads any harm. Something for zoologists to investigate further.
Figure 5. An everlasting daisy (Helichrysum candolleanum) widespread in southern Africa, which occurs only where copper ores are so close to the surface that other plants cannot grow [photos by N. Dreber and D. Wesuls, respectively (Photo Guide to Plants of Southern Africa). www.southernafricanplants.net]
The Honey Badger: Why are some of the invertebrates eaten by toads rich in Copper?
Copper: You remember, I mentioned that the blood of some invertebrates use a Copper- instead of iron-based pigment to carry oxygen safely through the body? We’re talking about faintly blue blood instead of bright red blood. Certain crustaceans also put Copper in their shells for some reason.
Figure 6. Southern toad (Anaxyrus terrestris) of North America, which is known to be able to accumulate copper in its body for unknown reasons.
Robin: We feel new appreciation for the unseen Copper in our bodies. So is the oddly blue-green Statue of Liberty in New York a case of art imitating life?
Copper: What do you mean?
Robin: Well, I’m told the whole surface of that 46-metre statue is Copper, made of 27 tonnes of Copper in 2.4 millimetre-thick sheeting. Haven’t I heard that it too is protected from the weather by a greenish compound of Copper?
Copper: Now that you mention that, it is another funny coincidence. The verdigris patina on the Statue of Liberty (see Figure 7) does protect it from oxidation but in a different way from crustacean blood. The metal plates self-seal with copper carbonate, which doesn’t occur in blood or anywhere else in organisms. Copper starts to rust as iron does, but unlike iron the oxide is soon converted to carbonate which stops further rusting. The blueish blood of some animals and the greenish patina on the Statue of Liberty are both coppery, and in both cases the coloured substance is an antioxidant in the loosest sense of the word, but that’s where the similarity between haemocyanin and copper carbonate ends.
Figure 7. The Statue of Liberty in New York, showing the natural colour of the copper surface, which has not been painted. This verdigris patina has resulted spontaneously from the exposure of the copper to air.
Robin: And unfortunately with this freedom from the tyranny of oxygen we now have to close because of the tyranny of time.
The Honey Badger: If there have been any dull moments in this interview, they were at least pleasantly pastel-coloured.
Robin: Copper, thanks for sharing your true mettle and your true colours with us.
Copper: Thanks, and I trust that listeners will remember me the next time the text says “C u later”!
Our sincere thanks go to:
- the Department of Special Collections, University of Notre Dame Libraries, for permission to use the photo of the 1988 Bermuda cent on the front cover; and
- Juergens, N.; Rügheimer, S. et al; Erb, E.; Wesuls, D.; Schmiedel, U.; Schrenk, J.; Dreber, N.; Mayer, C.; Oncken, I.; Schulz, A.; Kwembeya, E.; Strohbach, B.; Ihlenfeldt, H.-D.; Niesler, I.; Reddig, C. 2012: Photo Guide to Plants of Southern African plants. BioCentre Klein Flottbek, Hamburg, Germany, www.southernafricanplants.net, for permission to use the photos of Helichrysum candolleanum in Figure 5.
- Figures without attribution are in the public domain.
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