Oxygen
Page 15
Oxygen and the Rise of the Giants • 101
ous.7 It seems that the Bolsover dragonfly really was only able to fly, and so hunt its prey and survive, in an oxygen-rich atmosphere.
Dragonflies were not the only giants of the Carboniferous — many other creatures attained sizes never matched again. Some mayflies had wingspans of nearly half a metre [19 inches], millipedes stretched for over a metre [39 inches], and the Megarachne, a spider-like arachnid with a leg-span of nearly half a metre [18 inches], would have chilled the marrow of Indiana Jones. Even more terrifyingly, scorpions reached lengths of a metre, dwarfing their modern cousins, the largest of which barely manages a fifth of that length. Among the terrestrial vertebrates, amphibians grew from newt-like proportions to reach body lengths of 5 metres [16 feet].
They left some of the oldest footprints in England, at Howick in Northumberland — 18 centimetres [7 inches] long and 14 centimetres
[5½ inches] across. In the plant world, ferns turned into trees, while the giant lycopods reached heights of nearly 50 metres [164 feet]. Their only survivors today are the diminutive herbaceous club-mosses, such as the ground pine ( Lycopodium obscurum), which rarely grow higher than 30
centimetres [12 inches].
Was all this rampant gigantism related to oxygen? It is certainly possible. Like the dragonfly, each of these organisms depends on the passive diffusion of gases in one way or another. The size of amphibians, for instance, is restricted by their capacity to absorb oxygen by diffusion across the skin, whereas the height of plants depends on the thickness of their structural support, which in turn is limited by the need for gases to reach internal tissues. But while it is plausible that high oxygen levels might enable greater size, it is hard to back the claim with direct evolutionary evidence. There is one tantalizing suggestion from modern ecosystems, however, that this is indeed the case.
Tucked away in the ‘Scientific Correspondence’ section of Nature in May 1999 was a short paper on the size of crustaceans — the class that includes shrimps, crabs and lobsters — in polar regions. This paper solved a long-standing riddle rather neatly: the relationship between gigantism and oxygen availability. The authors, Gauthier Chapelle of the Royal 7 Because the extra oxygen was added to the existing atmospheric gases, the overall density of the air would have increased. Higher density aids lift (by increasing the Reynold’s number) and would have encouraged the evolution of flight. In a series of fascinating papers, Robert Dudley has connected the origins of flight in insects, birds and bats with changes in atmospheric density.
102 • THE BOLSOVER DRAGONFLY
Institute of Natural Sciences in Belgium, and Lloyd Peck of the British Antarctic Survey, examined length data for nearly 2000 species of crustaceans from polar to tropical latitudes and from marine to freshwater environments. They focused on a single group, known as amphipods, which are cold-blooded, shrimp-like creatures, ranging in length from a couple of millimetres [1/25 inch] to about 9 centimetres [3½ inches]. The amphipods are not exclusively marine, and are best known to most of us as sand-hoppers, or the shiny brown animals that leap about when pot plants are moved in the garden.
The thousands of marine species of amphipod are a cornerstone of polar food chains, being the staple diet of juvenile cod, which are in turn preyed on by seals, and the seals by polar bears. In some bottom sediments, amphipods are found at an extraordinary density of 40 000 per square metre [4000 per square foot]. These tiny creatures offer even more of a square meal in polar waters: the largest Antarctic species are some five times larger than their tropical cousins — true giants by amphipod standards. In this respect, amphipods are not alone. For the past hundred years or so, scientists have catalogued numerous giant species in polar seas. Although polar gigantism is usually ascribed to the low temperatures and the reduced metabolic rates of cold-blooded animals, the relationship is not straightforward. Surprisingly, polar gigantism had never been satis-factorily explained. The trouble is that the inverse correlation between size and temperature is curved rather than linear, and has a number of puzzling exceptions. In particular, many species achieve far greater sizes in freshwater environments than they ought to on the basis of temperature alone. Freshwater amphipods from Lake Baikal in Russia, for example, are twice as large as those in the sea at the same temperature.
Then Chapelle and Peck had a clever idea and applied it to their amphipod data. What if the true correlation was not with water temperature at all, but with the dissolved oxygen concentration? Oxygen dissolves better in colder water and is nearly twice as soluble in polar seas than in tropical waters. The salt content also affects the solubility of oxygen, which dissolves 25 per cent better in fresh water than in saline.
The highest oxygen saturation is therefore in large freshwater lakes verg-ing on the Arctic tundra, such as Lake Baikal — and this is where the largest crustaceans are to be found. When Chapelle and Peck re-plotted their length data against the oxygen saturation of the water, they got a nearly perfect fit (Figure 6). While it is true that a correlation says nothing about mechanism, it seems likely that inadequate oxygen availability
Oxygen and the Rise of the Giants • 103
(a)
60
Lake Baikal
50
40
(mm)
30
95/5
Caspian Sea
TS
Black Sea
20
10
030
25
20
15
10
5
0
–5
Mean annual surface temperature (°C)
(b)
60
Lake Baikal
50
40
(mm)
30
95/5
TS
Caspian Sea
20
Black Sea
10
0150
200
250
300
350
400
450
Water oxygen content (μmol/kg)
Figure 6: Correlation between body length of amphipods in millimetres (given as an index of average length, TS95/5) and (a) temperature and (b) oxygen concentration. Lake Baikal, the Caspian Sea and the Black Sea are outliers from the temperature curve because they are brackish or freshwater, rather than saline. Oxygen dissolves more readily in freshwater, so the correlation is restored by plotting the length of amphipods against the dissolved oxygen concentration. Reproduced with permission from Chapelle & Peck, and Nature.
104 • THE BOLSOVER DRAGONFLY
limits size in many species, or conversely, that high oxygen raises the barrier to gigantism.
Of course the dependence of giants on high oxygen means that they are perilously susceptible to falling oxygen levels. In a stark closing line, Chapelle and Peck predict that giant amphipods will be among the first species to disappear if global temperatures rise, or if oxygen levels decline.
We can hardly begin to imagine what effect this might have on the rest of the food chain.
The case for changeable oxygen levels in the atmosphere is not easy to dismiss. This conclusion stands in opposition to Lovelock’s Gaia theory, which argues that the living biosphere has regulated the levels of oxygen in the atmosphere over the past 500 million years. While this may be true for much of the time, there have surely been periods when the biosphere lost control over oxygen levels. If anything, the possibility that Gaia cannot always maintain the physiological balance that Lovelock ascribes to her only strengthens his concerns about our impact on the planet.
Given the unequivocal evidence of global glaciation punctuating the Earth’s history, it is plain that Gaia does not have a tight control over temperature. Something similar seems to be true of oxygen. We have a tenuous grasp of the factors that control oxygen or carbon dioxide levels, but the fact that there have been times when they h
ave tipped out of balance means that it can happen again, perhaps with our help. The feedback mechanisms postulated by Lovelock and others have the power to resist change for a period. If we are to judge from the case of oxygen, they do not have indefinite flexibility and cannot resist catastrophic change.
We should beware.
With the exception of fire, there is remarkably little evidence that high oxygen levels are in any way detrimental to life. On the contrary, high oxygen may have opened evolutionary doors that are closed to us today. Falling oxygen closes these doors, and the species left outside are unlikely to survive. Most of the giants of the Carboniferous, for example, failed to survive until the end of the Permian period, when Robert Berner’s calculations suggest that oxygen levels plummeted to 15 per cent as the climate became cooler and drier.
We must conclude that high oxygen is good, low oxygen is bad. But we saw in Chapter 1 that high levels of oxygen are toxic, causing lung
Oxygen and the Rise of the Giants • 105
damage, convulsions, coma and death, and we are told that oxygen free radicals are at the root of ageing and disease. What is going on: is oxygen toxic or not? This paradox did not escape the notice of Barry Halliwell and John Gutteridge, authors of Free Radicals in Biology and Medicine, the standard text on the subject, who remarked laconically that “the plants and animals existing in the Carboniferous times must presumably have had enhanced antioxidant defences, which would be fascinating to study if these species could ever be resurrected.” Yes indeed. How did they overcome oxygen toxicity? Might there be some way that we can imitate them to protect ourselves against free radicals in disease? It is time to look in a little more detail at the strange spectre of oxygen toxicity, and what life does about it.
C H A P T E R S I X
Treachery in the Air
Oxygen Poisoning and X-Irradiation:
A Mechanism in Common
In 1891 a shy 24-year old polish girl named Marya Salomee Skl/odowska, arrived in Paris with the dream of becoming a scientist. In the chauvinist academic culture of France at the time, such a dream stood little chance of fulfilment, but Marya possessed a brilliant mind and stubborn determination. Nor was she a stranger to hardship. The Poland of her youth was occupied by the Russians, and her mother had died when she was just four. The youngest of five children, she had been raised in poverty by an idealistic father, and educated in the Flying University, which met each week in different locations to avoid detection by the Russians — the Poles resisted oppression through education, and Polish culture flowered in underground centres of learning. Not surprisingly, the passion for learning that swept her homeland left an indelible mark on Marya.
When she was 18, Marya made a pact with her sister Bronya. Bronya would pursue her own dream of studying medicine in Paris, and Marya would support her by working as a private tutor in Warsaw; then Bronya would return the favour. Marya duly worked as a governess for six years, while continuing her underground studies in chemistry and mathematics and suffering an unhappy love affair. In the meantime, Bronya completed medical school and married a fellow medical student. So it was that Marya arrived as a mature student in Paris, changed her name to the French
Oxygen Poisoning and X-Irradiation: A Mechanism in Common • 107
version, Marie, and enrolled at the Sorbonne. She passed a master’s degree in physics with flying colours in 1893, and a second in mathematics in 1894. Then, while seeking extra laboratory space to conduct more elaborate experiments, she was introduced to an equally brilliant, intro-verted and free-thinking Frenchman, who had already made a reputation for his work on crystallography and magnetism. They quickly fell in love, and he wrote to her saying how nice it would be “to spend life side by side, in the sway of our dreams: your patriotic dream, our humanitarian dream and our scientific dream.” Marie and Pierre married in 1895, going on a cycle tour around France for their honeymoon, and when Marie found fame as a scientist it was under her new name, Marie Curie.
In the next two years, Pierre gained a teaching position at a science college while Marie studied for a teaching certificate. In 1897 their first child, Irène, was born, and that same year Marie began work on her doctoral studies — another pioneering step for a woman at the time; she was to become the first woman in Europe to receive a doctorate in science.
Although both Marie and Pierre had been interested primarily in magnetism until then (and the field still pays homage to their name in the ‘Curie point’, the temperature at which materials lose their magnetism), the Curies had become close friends with another brilliant young French scientist, Henri Becquerel. Inheriting the large phosphorescent mineral collection of his scientist father, Becquerel had just discovered that if crystals of uranium sulphate were exposed to sunlight, then placed on photographic plates and wrapped in paper, an image of the crystals would form when the plates were developed. At first he assumed that the rays emitted by the crystals were a type of fluorescence derived from the sunlight, but this theory was confounded by the overcast skies of Paris that February. Becquerel returned his equipment to a drawer and waited for better weather, but after a few gloomy days he decided to develop his plates anyway, anticipating no more than faint images. To his surprise, the images turned out to be clear and strong, and Becquerel realized that the crystals must have emitted rays even without an external source of energy such as sunlight. He soon showed that the rays came from the small amounts of uranium in the crystals and that all substances containing uranium gave off similar rays. He even found that uranium causes the air around it to conduct electricity. His excitement transmitted to the Curies, and Marie decided to study the strange phenomenon, which she later termed radioactivity, for her doctorate.
108 • TREACHERY IN THE AIR
Marie set to work on a uranium ore known as pitchblende. She and Pierre had realized that radioactivity could be measured by the strength of the electric field that it generates in the surrounding air, and Pierre invented an instrument that could detect the electric charge around mineral samples. Using this instrument, Marie discovered that the radioactivity of pitchblende was three times greater than that of uranium, and concluded that there must have been at least one unknown substance in pitchblende, with a much higher activity than uranium. By chemically separating the elements from uranium ore and measuring their radioactivity, the Curies discovered a new element that was 400 times more radioactive than uranium, which they named polonium after her native Poland. Later, Marie discovered tiny quantities of another element, this time a million times more radioactive than uranium, and called it radium.
Pierre tested a tiny piece of radium on his skin, and found it caused a burn, which developed into a wound. The Curies recognised its potential as an anti-cancer treatment. Radium was first used for this purpose by S. W. Goldberg in St Petersburg, as early as 1903. Radium needles are still inserted into tumours as a cancer therapy today.
To study the properties of radium in detail, the Curies needed to isolate more, and to do this entailed working with tonnes of pitchblende to isolate just a few milligrams of radium. Radium is present in such small quantities that, even today, world production amounts to only a few hundred grams. The Curies worked in what must have been appalling conditions. Their lab was described by a contemporary chemist as looking more like a stable or a potato cellar. Refusing to patent radium, for humanitarian reasons, the Curies continued to struggle on. Despite their financial hardship and poor conditions, they took great pleasure in their work, especially at night, when they could see all around them “the luminous silhouettes of the beakers and capsules that contained our products.”
For their work on natural radioactivity, the Curies and Becquerel received the Nobel Prize for physics in 1903. The year after that, Marie and Pierre had a second daughter, Eve. It must have been the best time of their lives. In 1906, Pierre, weakened by radiation, was killed in a road accident, his head crushed beneath the wheel of a horse-drawn cart.
Traumatized, Ma
rie began writing to him in a diary, which she kept for many years, but her scientific resolve did not falter, and she determined to complete alone the work they had undertaken together. She struggled against the French establishment for recognition, finally taking up her husband’s old position at the Sorbonne in 1908 — the first woman in its
Oxygen Poisoning and X-Irradiation: A Mechanism in Common • 109
650-year history to be appointed professor there. In 1911 she received a second Nobel Prize, for the isolation of pure radium, and in 1914 she founded the Radium Institute, now renamed the Curie Institute, with its humanitarian goal of easing human suffering. Throughout the First World War she trained nurses to detect shrapnel and bullets lodged in wounds, using mobile x-ray vans, and after the war, with her daughter Irène alongside her, she pioneered the use of radium to treat cancer patients. Irène herself, with her husband Frédéric Joliot, went on to receive a Nobel Prize for the discovery of artificial radioactivity in 1935.
Marie did not live to see her daughter’s Nobel laurels. She died of leukaemia on 4 July 1934 at the age of 67, exhausted and almost blinded by cataracts, her fingers burnt and stigmatized by her beloved radium. She had not been the first to die of radiation poisoning, nor was she the last.
During the 1920s, several workers at the Radium Institute had died of a cancer that other doctors attributed to radioactivity. Not believing the truth, Marie put it down to a lack of fresh air. Later, her daughter Irène also died of leukaemia.
Today, with the experience of Hiroshima and Chernobyl behind us, radiation is not seen in quite the same humanitarian light. High doses of radiation kill cancer cells, but also kill normal cells. Within weeks of the discovery of x-rays there had been reports of tissue damage among researchers who worked for many hours a day with x-ray-producing discharge tubes. Many of them lost their hair and developed skin irritations, which sometimes festered into severe burns. Lower doses of radiation were found to increase the risk of cancer. The signs were there even in Marie Curie’s time. Forty per cent of the early researchers in radioactivity died of cancer. They were joined by others who worked with radioactive materials. In 1929, doctors in Germany and Czechoslovakia noticed that 50 per cent of the miners working in Europe’s only uranium mine, in Bohemia in northern Czechoslovakia, had lung cancer, which was attributed to their inhalation of radon gas, a radioactive decay product of uranium (via radium), escaping from the ore. The incidence of lung cancer among uranium miners in the United States was also much higher than normal.