Poisoned by Oxygen

There was a good reason for the scientists’ interest in oxygen poisoning. The problem was how to facilitate the work of divers. A man can survive in an atmosphere of pure oxygen for about twenty-four hours. If he breathes oxygen for longer than that, pneumonia ensues and, strange as it may seem, death due to asphyxia, which is a shortage of oxygen in the most important organs and tissues. A man can endure a pressure of two to three atmospheres not longer than one and a half to two hours. Then he becomes intoxicated with oxygen, loses coordination of movement, and suffers from mental distraction and loss of memory. If the oxygen pressure exceeds three atmospheres, convulsions will soon follow causing death.

Oxygen proves even more poisonous for animals which live where there is a critical lack of oxygen. This is how ascarides living in human intestines are combated. Oxygen is fed into the intestines, causing no danger to the man himself, but surely killing the parasites.

An excess of oxygen is not only detrimental to animals, but also to plants. It is interesting that, although plants saturate the atmosphere of our planet with oxygen, the Earth’s atmosphere is not good for them. They are rather short of carbon dioxide and, strange as it may seem, there is too much oxygen for them. According to recent investi­gations not only the usual concentration of oxygen but even as little as two per cent, that is one-tenth of what is to be found in the atmosphere, considerably retards photosynthesis. This means that plants have created an atmosphere quite unsuitable for themselves. Had there been less oxygen they would have grown and developed more rapidly.

Combatting Oxygen Shortages

Animal kingdom emerged on our planet when the atmosphere was still very poor in oxygen. It is no wonder that living organisms had to adapt themselves to an environment where oxygen was in short supply. However, we usually fail to notice another much more puzzling phenom­enon, namely, that animals living in the presence of exces­sive oxygen have managed to restrain the intensity of the oxidation processes taking place in their bodies as if they were always ready to extinguish a constantly threatening fire.

The amount of environmental oxygen is constant, and, if it does alter, it decreases. This explains why animals have different means of combatting oxygen shortages, but no means of protection against excess oxygen.

Paul Bert was the first to discover that breathing pure oxygen can be poisonous around a hundred years ago. This was such an unexpected discovery that scientists did not believe him and a suspicion arose that the oxygen used by Bert contained various poisonous admixtures. The experiments were repeated many times, but no matter how thoroughly the oxygen was purified, the animals which breathed it for prolonged periods inevitably perished.

Oxygen, Energy and Wastefulness

The question arises why living organisms use atmospheric oxygen if energy can be obtained by mere fermentation. There are many important reasons for this. Fermentation never results in the complete oxidation of a substance and, therefore, little energy is released. If one gram-molecule of glucose is completely oxidized to carbon dioxide and water, 673 large calories will be obtained. But with fermentation, which results in the formation of ethyl alcohol and carbon dioxide, only as little as 25 large calories will be released, i. e. almost 27 times less. This means that anaerobes have to use 27 times as much glucose as aerobes to obtain the same amount of energy. The difference is, of course, appre­ciable and nature cannot tolerate such wastefulness.

Another important reason is that substances such as ethyl and butyl alcohol, lactic and butyric acid, acetone, etc., which are bad for the organism, are formed as a result of fermentation. It is not easy to dispose of these harmful substances.

Respiration frequently produces combustible gases. Micro­organisms often release hydrogen. This is how microbes living in the intestine of termites breathe. Of the multicellular creatures, the larvae of some flies, in particular, release a great deal of hydrogen. Some organisms liberate not only hydrogen, but also methane and other gases, some of which are still not known, including spontaneously inflammable gases. It is a particularly beautiful sight when the gases, which have collected in the silt at the bottom of a pool, rise to the surface of the water and burn with a mysterious bluish flame.

How then have animals managed to change their way of breathing to such an extent and adapt themselves to an absence of oxygen? This did not prove difficult. At the dawn of life on the Earth there was little free oxygen and the earliest living creatures had to become anaerobes. It was not until the atmosphere became rich in oxygen that animals learned to burn energy-forming products completely. At the same time, the anaerobic method of breathing did not disappear but was passed on and finally came down to us.

As has been mentioned at the beginning, in all animals without exception the first stages of energy release proceed without oxygen. When aerobic animals felt like returning to the places where no oxygen could be obtained, they again had to restrict themselves to partial utilization of the energy contained in nutrient substances. To do this they had to remember how to render partially oxidized products harmless.

Methods of Oxidation

Oxidation by abstraction of hydrogen is termed fermen­tation; it results in the splitting of organic substances to form oxidized and reduced products and the liberation of the energy required by the organism.

The best known form of fermentation found in unicel­lular organisms is the breakdown of a glucose molecule into two molecules of ethyl alcohol (the reduced substance) and two molecules of carbon dioxide (the oxidized substance).

In multicellular organisms, the most common form of fermentation is lactic fermentation which involves the decom­position of carbohydrates, as, for instance, when a sugar molecule breaks down into two molecules of lactic acid which have less energy than the initial substance. The breakdown of carbohydrates is a gradual process consisting of a series of reactions. As a result, the oxygen in the molecule of sugar near to the inner carbon atom is trans­ferred to the external carbon atom. Energy is thereby liberated.

There is also another method of oxidation, that of electron loss, but whether it can be used by living organisms has not been adequately studied.

How Anaerobes Breathe

Still more primitive animals, primarily bacteria, have no haemoglobin and are therefore unable to actively extract oxygen from their surroundings. However, they are often doomed to environments where there is little or no oxygen at all. Nevertheless, these creatures are quite happy to reconcile themselves to an absence of oxygen. This led to their being named anaerobes, which means ‘one who lives without air’.

How do anaerobes manage to live without air? Not so long ago this has seemed to be a puzzle that could not be solved. Now we know that they do need oxygen all the same. Instead of extracting oxygen from the atmosphere the anaerobes simply take it from organic substances. Some bacteria even extract oxygen from inorganic substances, using nitrites and sulphites for the purpose.

Anaerobes breathe by oxidizing the products of metabolism without using additional oxygen and are quite content with the amount already present in the substance being oxidized. For, when a substance is oxidized, it makes no difference whatsoever whether oxygen is added to, or hydrogen removed from it.

Two Types of Hemoglobin

As yet we do not know how the red organs function, but it is clear that they play a major part in supplying the larva of the botfly with oxygen. This is proved by the presence of a large amount of hemoglobin which accounts for the red colour of the cells and whose affinity to oxygen, i. e. the ability to combine with oxygen even when small amounts of the gas are present, is hundreds of times higher than in mammals.

Ascarides are intestine dwellers often found in mammals. Even quite recently it has been maintained that they could manage without oxygen. However, scientists were astonished to find two kinds of hemoglobin in the body of the pig ascarid. This haemoglobin was concentrated at two points, in the wall of the body and in the parenteral liquid which fills the cavity of the body. The outer hemoglobin retains the oxygen 2500 times longer, and the inner hemoglobin 10000 times longer than the pig’s own haemoglobin.

Now why does the ascarid need haemoglobin if it can manage without oxygen? Theoretical calculations show that a system of two hemoglobin with a growing thirst for oxygen may serve as the ideal carrier, especially where there is a considerable oxygen deficiency.

Lepidosiren and Green Plants

With the onset of the breeding season the Lepidosiren male dons his wedding outfit: extremely long thread-like shoots grow from his abdominal fins. The male in fancy dress is an interesting sight courting the female or guarding the nest, his fins lowered completely onto the spawn. This wedding outfit serves not only to attract the female; the fins serve as hoses to supply oxygen to the spawn. The temporary shoots of the Lepidosiren males are filled with tiny blood vessels and this enables the oxygen to pass from their blood into the surrounding water.

Given a good spot — a small hole or a burrow in a shallow place, completely cut oft” from the main pool — it is simple to obtain oxygen supply for the spawn. In such conditions the male can readily take oxygen from the surface and, while remaining in position over the spawn, thus enrich his own blood, passing the oxygen at a greater rate into the surrounding water. This is easily accomplished in the stagnant water of the pool provided that the hole or burrow used for the nest is small.

Pools have yet another source of oxygen — green plants. If there are few green plants and the oxygen they liberate is not sufficient to saturate the water, the only thing to do, as large numbers of insects do, is to settle on the plants themselves as the concentration of oxygen will be greater there.

Methods of Adaptation of Eels and Lepidosiren

We have spoken about ecological niches and struggle for breathing, particularly about the electric eel. But what is the reason for the eels’ attraction? Do they occupy the best places in the pool? Not at all. It is simply because these terrible fish enrich the water around them with oxygen. An electric discharge of 600 volts can break down water into its constituents, oxygen and hydrogen, and this life-giving stream attracts the oxygen-starved fish from all directions.

With the electric discharge the water in the eel’s body also decomposes. The oxygen so formed is immediately transported by the blood all over the body, but the hydrogen has to be expelled. It is eliminated through the gills and rises to the surface in long jets of tiny bubbles. The bubbles show the Indian hunters where this dangerous fish is and they lose no time in killing it so that they may have fish for their own table.

Besides the eels, another rather interesting fish, the Lepidosiren, also lives in the swamps of South America. It can survive even in completely dried-up swamps where there is very little oxygen even in the rainy seasons. The adult fish manage with very little oxygen because their swim bladder has become a paired respiratory organ. They breathe air, but the problem is how to preserve the spawn in such water. The Lepidosiren has developed a unique way of caring for its offspring, a method of supplying the spawn with oxygen. The male is responsible for this. As soon as the rainy season comes, he either finds a small, but sufficiently deep, hole on the bottom or else a burrow and takes his female there. When the spawn is laid and fertilized, the female swims quietly away leaving the spawn to the father’s care.

Struggle for Breathing in Ecological Niches

There are many places on the Earth which have little or no oxygen. In most cases, living creatures themselves are responsible for this, bacteria being especially heavy consumers of oxygen. One milligram of bacteria is able to consume 200 cubic millimetres of oxygen per hour. It should be pointed out that a working muscle of comparable weight will, during the same period, use only 20 cubic millimetres of oxygen and only two and a half cubic millimetres when relaxed. Owing to the activities of bacteria and the larger microorganisms many nooks on our planet are becoming quite unsuitable for life, and so animals have to be more inventive to settle in such ecological niches.

One such niche is successfully inhabited by electric eels. These large fish live in the swamps and small rivers of South America. During the rainy season the rivers become turbulent, and the swamps are flooded with streams of muddy water. These streams are rich in oxygen and the dwellers of the underwater kingdom can breathe easily. But, during the drought which follows the rainy season, the rivers quickly become shallow, forming small lakes with narrow stretches of water between them, and the marshes begin to dry out. In the shallow pools heated by the tropical sun the plants rot and the microorganisms multiply rapidly, consuming oxygen at a greater rate than it diffuses from the air. Thus, breathing becomes more and more difficult for all the water-dwellers, dyspnoea develops.

But the electric eel feels tine and does not seem to sutler from the lack of oxygen. What is more, food is plentiful. All the inhabitants of the disappearing pools are attracted to the place where the eels have settled. We shall have something to say about animal power stations later, but now we shall only point out that the electric eels do not hunt for their prey. Liquid mud is brown like coffee dregs in which you cannot even see the tip of your nose. It is obvious that one cannot catch anything there, except quite by chance. The eels kill their prey with powerful electric shocks, without even looking or trying to see what kind of creature it is.

Striving For a Breath

Tiny oxygen bubbles can often be observed on plants. The macroplea, beetles pick up these bubbles with their tiny legs and carry them to their antennae. After some time, the bubble disappears which makes us think that the beetles breathe with their antennae. If there are no gas bubbles of oxygen, the beetles cut the plant and wait for air to escape from its air channels. The same method is used by water weevils.

The larvae of macroplea and donicia beetles make incisions in plants and attach their spiracles to them. Other insects stick their stylets into the plants and suck oxygen out from the intercellular space. These oxygen-rich intercellular spaces are places favoured for pupation.

However, the caterpillars of the Brazilian paraponyx are even more ingenious. They build themselves a house from bits of green plants and, when these wither away, replace them. Consequently, during the hours of daylight, there is always plenty of oxygen in their nests, but at night, so as not to be choked by the carbon dioxide liberated by the plants, the caterpillars have to climb outside.

The amount of oxygen found in the stomachs and intestine of vertebrates is negligible. But certain living organisms which could find no place under the sun learned how to obtain oxygen. Not the least among them is the bot (the larva of the botfly) which lives in the alimentary tract of horses. Like all other insects, the bot has a tracheal system for respiration which is stronger and more ramified than that of larva living in the open. It also has red organs which are a conjugate formation consisting of many large red cells. A tracheal stem enters each cell and then branches out into numerous tracheoles in its protoplasm.