Bacteria As Agents In Nature's Food Cycle
( Originally Published 1897 )
But the matter by no means ends here. When we come to think of it, it is a matter of consider-able surprise that the surface of the earth has been able to continue producing animals and plants for the many millions of years during which life has been in existence. Plants and animals both require food, animals depending wholly upon plants therefor. Plants, however, equally with animals, require food, and although they obtain a considerable portion of their food from the air, yet no inconsiderable part of it is obtained from the soil. The question is forced upon us, therefore, as to why the soil has not long since become exhausted of food. How could the soil continue to support plants year after year for millions of years, and yet remain as fertile as ever?
The explanation of this phenomenon is in the simple fact that the processes of Nature are such that the same food is used over and over again, first by the plant, then by the animal, and then again by the plant, and there is no necessity for any end of the process so long as the sun furnishes energy to keep the circulation continuous. One phase of this transference of food from animal to plant and from plant to animal is familiar to nearly every one. It is a well-known fact that animals in their respiration consume oxygen, but exhale it again in combination with carbon as carbonic dioxide. On the other hand, plants in their life consume the carbonic dioxide and exhale the oxygen again as free oxygen. Thus each of these kingdoms makes use of the excreted product of the other, and this process can go on indefinitely, the animals furnishing our atmosphere with plenty of carbonic: acid for plant life, and the plants excreting into the atmosphere at the same time an abundant sufficiency of oxygen for animal life. The oxygen thus passes in an endless round from animal to plant and from plant to animal.
A similar cycle is true of all the other foods of animal and plant life, though in regard to the others the operation is more complex and more members are required to complete the chain. The transference of matter through a series of changes by which it is brought from a condition in which it is proper food for plants back again into a condition when it is once more a proper food for plants, is one of the interesting discoveries of modern science, and one in which, as we shall see, bacteria play a most important part. This food cycle is illustrated roughly by the accompanying diagram; but in order to under-stand it, an explanation of the various steps in this cycle is necessary.
It will be noticed that at the bottom of the circle represented in Fig. 25, at A, are given various ingredients which are found in the soil and which form plant foods. Plant foods, as may be seen there, are obtained partly from the air as carbonic dioxide and water; but another portion comes from the soil. Among the soil ingredients the most prominent are nitrates, which are the forms of nitrogen compounds most easily made use of by plants as a source of this important element. It should be stated also that there are other compounds in the soil which furnish plants with part of their food—compounds containing potassium, phosphorus, and some other elements. For simplicity's sake, however, these will be left out of consideration. Beginning at the bottom of the cycle (Fig. 25 A), plant life seizes the gases from the air and these foods from the soil, and by means of the energy furnished it by the sun's rays builds these simple chemical compounds into more complex ones. This gives us the second step, as shown in Fig. 25 B, the products of plant life. These products of plant life consist of such materials as sugar, starches, fats, and proteids, all of which have been manufactured by the plant from the ingredients furnished it from the soil and air, and through the agency of the sun's rays. These products of plant life now form foods for the animal kingdom. Starches, fats, and proteids are animal foods, and upon such complex bodies alone can the animal kingdom be fed. Animal life, standing high up in the circle, is not capable of extracting its nutriment from the soil, but must take the more complex foods which have been manufactured by plant life. These complex foods enter now into the animal and take their place in the animal body. By the animal activities, some of the foods are at once decomposed into carbonic acid and water, which, being dissipated into the air, are brought back at once into the condition in which they can serve again as plant food. This part of the food is thus brought back again to the bottom of the circle (Fig. 25, dotted lines). But while it is true that animals do thus reduce some of their foods to the simple condition of carbonic acid and water, this is not true of most of the foods which contain nitrogen. The nitrogenous foods are as necessary for the life as the carbon foods, and animals do not reduce their nitrogenous foods to the condition in which plants can prey upon them. While plants furnish them with nitrogenous food, they can not give it back to the plants. Part of the nitrogenous foods animals build into new albumins (Fig. 25 C); but a part of them they reduce at once into a somewhat simpler condition known as urea. Urea is the form in which the nitrogen is commonly excreted from the animal body. But urea is not a plant food; for ordinary plants are entirely unable to make use of it Part of the nitrogen eaten by the animal is stored up in its body, and thus the body of the animal, after it has died, contains these nitrogen compounds of high complexity. But plants are not able to use these compounds. A plant can not be fed upon muscle tissue, nor upon fats, nor bones, for these are compounds so complex that the simple plant is unable to use them at all. So far, then, in the food cycle the compounds taken from the soil have been built up into compounds of greater and greater complexity ; they have reached the top of this circle, and no part of them, except part of the carbon and oxygen, has become reduced again to plant food. In order that this material should again become capable of entering into the life of plants so as to go over the circle again, it is necessary for it to be once more reduced from its highly complex condition into à simpler one.
Now come into play these decomposition agencies which we have been studying under the head of scavengers. It will be noticed that the next step in the food cycle is taken by the de-composition bacteria. These organisms, existing, as we have already seen, in the air, in the soil, in the water, and always ready to seize hold of any organic substance that may furnish them with food, feed upon the products of animal life, whether they are such products as muscle tissue, or fat, or sugar, or whether they are the excreted products of animal life, such as urea, and produce therein the chemical decomposition changes al-ready noticed. As a result of this chemical decomposition, the complex bodies are broken into simpler and simpler compounds, and the final result is a very thorough destruction of the animal body or the material excreted by animal life, and its reduction into forms simple enough for plants to use again as foods. Thus the bacteria come in as a necessary link to connect the animal body, or the excretion from the animal body, with the soil again, and therefore with that part of the circle in which the material can once more serve as plant food.
But in the decomposition that thus occurs through the agency of the putrefactive bacteria it very commonly happens that some of the food material is broken down into compounds too simple for use as plant food. As will be seen by a glance at the diagram (Fig. 25 D), a portion of the cleavage products resulting from the destruction of these animal foods takes the form of carbonic-acid gas and water. These ingredients are at once in condition for plant life, as shown by the dotted lines. They pass off into the air, and the green leaves of vegetation everywhere again seize them, assimilate them, and use them as food. Thus it is that the carbon and the oxygen have completed the cycle, and have come back again to the position in the circle where they started. In regard to the nitrogen portion of the food, however, it very commonly happens that the products which arise as the result of the decomposition processes are not yet in proper condition for plant food. They are reduced into a condition actually too simple for the use of plants. As a result of these putrefactive changes, the nitrogen products of animal life are broken frequently into compounds as simple as ammonia (NH3), or into compounds which the chemists speak of as nitrites (Fig. 25 at D). Now these compounds are not ordinarily within the reach of plant life. The luxuriant vegetation of the globe extracts its nitrogen from the soil in a form more complex than either of the compounds here mentioned ; for, as we have seen, it is nitrates chiefly that furnish plants with their nitrogen food factor. But nitrates contain considerable oxygen. Ammonia, which is one of the products of putrefactive de-composition, contains no oxygen, and nitrites, an-other factor, contains less oxygen than nitrates. These bodies are thus too simple for plants to make use of as a source of nitrogen. The chemical destruction of the food material which results from the action of the putrefactive bacteria is too thorough, and the nitrogen foods are not yet in condition to be used by plants.
Now comes in the agency of still another class of micro-organisms, the existence of which has been demonstrated to us during the last few years. In the soil everywhere, especially in fertile soil, is a class of bacteria which has received the name of nitrifying bacteria (Fig. 26). These organisms grow in the soil and feed upon the soil ingredients. In the course of their life they have somewhat the same action upon the simple nitrogen cleavage products just mentioned as we have already noticed the vinegar-producing species have upon alcohol, viz., the bringing about a union with oxygen. There are apparently several different kinds of nitrifying bacteria with different powers. Some of them cause an oxidation of the nitrogen products by means of which the ammonia is united with oxygen and built up into a series of products finally resulting in nitrates (Fig. 26). By the action of other species still higher nitrogen compounds, including the nitrites, are further oxidized and built up into the form of nitrates. Thus these nitrifying organisms form the last link in the chain that binds the animal kingdom to the vegetable kingdom (Fig. 25 at 4). For after the nitrifying organisms have oxidized nitrogen cleavage products, the results of the oxidation in the form of nitrates or nitric acid are left in the soil, and may now be seized upon by the roots of plants, and begin once more their journey around the food cycle. In this way it will be seen that while plants, by building up compounds, form the connecting link between the soil and animal life, bacteria in the other half of the cycle, by reducing them again, give us the connecting link between animal life and the soil. The food cycle would be as incomplete without the agency of bacterial life as it would be with-out the agency of plant life.
But even yet the food cycle is not complete. Some of the processes of decomposition appear to cause a portion of the nitrogen to fly out of the circle at a tangent. In the process of de-composition which is going on through the agency of micro-organisms, a considerable part of the nitrogen is dissipated into the air in the form of free nitrogen. When a bit of meat decays, part of the meat is, indeed, converted into ammonia or other nitrogen compounds, but if the putrefaction is allowed to go on, in the end a considerable portion of it will be broken into still simpler forms, and the nitrogen will finally be dissipated into the air in the form of free nitrogen. This dissipation of free nitrogen into the air is going on in the world wherever putrefaction takes place. Wherever decomposition of nitrogen products occurs some free nitrogen is eliminated. Now, this part of the nitrogen has passed beyond the reach of plants, for plants can not extract free nitrogen from the air. In the diagram this is represented as a portion of the material which, through the agency of the decomposition bacteria, has been thrown out of the cycle at a tan-gent (Fig. 25 E). It will, of course, be plain from this that the store of nitrogen food must be constantly diminishing. The soil may have been originally supplied with a given quantity of nitrogen compound, but if the decomposition products are causing considerable quantities of this nitrogen to be dissipated in the air, it plainly follows that the total amount of nitrogen food upon which the animal and vegetable kingdoms can depend is becoming constantly reduced by such dissipation.
There are still other methods by which nitrogen is being lost from the food cycle. First, we may notice that the ordinary processes of vegetation result in a gradual draining of the soil and a throwing of its nitrogen into the ocean. The body of any animal or any plant that chances to fall into a brook or river is eventually carried to the sea, and the products of its decomposition pass into the ocean and are, of course, lost to the soil. Now, while this gradual extraction of nitrogen from the soil by drainage is a slow one, it is nevertheless a sure one. It is far more rapid in these years of civilized life than in former times, since the products of the soil are given to the city, and then are thrown into its sewage.
Our cities, then, with our present system of disposing of sewage, are draining from the soil the nitrogen compounds and throwing them away.
In yet another direction must it be noticed that our nitrogen compounds are being lost to plant life—viz., by the use of various nitrogen compounds to form explosives. Gunpowder, nitro-glycerine, dynamite, in fact, nearly all the explosives that are used the world over for all sorts of purposes, are nitrogen compounds. When they are exploded the nitrogen of the compound is dissipated into the air in the form of gas, much of it in the form of free nitrogen. The basis from which explosive compounds are made contains nitrogen in the form in which it can be used by plants. Saltpetre, for example, is equally good as a fertilizer and as a basis for gunpowder. The products of the explosion are gases no longer capable of use by plants, and thus every explosion of nitrogen compounds aids in this gradual dissipation of nitrogen products, taking them from the store of plant foods and throwing them away.
All of these agencies contribute to reduce the amount of material circulating in the food cycle of Nature, and thus seem to tend inevitably in the end toward a termination of the processes of life; for as soon as the soil becomes exhausted of its nitrogen compounds, so soon will plant life cease from lack of nutrition, and the disappearance of animal life will follow rapidly. It is this loss of nitrogen in large measure that is forcing our agriculturists to purchase fertilizers. The last fifteen years have shown us, however, that here again we may look upon our friends, the bacteria, as agents for counteracting this disciparing tendency in the general processes of Nature. Bacterial life in at least two different ways appears to have the function of reclaiming from the atmosphere more or less of this dissipated free nitrogen.
In the first place, it has been found in the last few years that soil entirely free from all common plants, but containing certain kinds of bacteria, if allowed to stand in contact with the air, will slowly but surely gain in the amount of nitrogen compounds that it contains. These nitrogen compounds are plainly manufactured by the bacteria in the soil ; for unless the bacteria are present they do not accumulate, and they do ac-cumulate inevitably if the bacteria are present in the proper quantity and the proper species. It appears that, as a rule, this fixation of nitrogen is not performed by any one species of micro-organisms, but by two or three of them acting together. Certain combinations of bacteria have been found which, when inoculated in the soil, will bring about this fixation of nitrogen, but no one of the species is capable of producing this result alone. We do not know to what extent these organisms are distributed in the soil, nor how widely this nitrogen fixation through bacterial life is going on. It is only within a short time that it has been demonstrated to exist, but we must look upon bacteria in the soil as one of the factors in reclaiming from the atmosphere the dissipated free nitrogen.
The second method by which bacteria aid in the reclaiming of this lost nitrogen is by a combined action of certain species of bacteria and some of the higher plants. Ordinary green plants, as already noted, are unable to make use of the free nitrogen of the atmosphere. It was found, however, some fifteen years ago that some species of plants, chiefly the great family of legumes, which contains the pea plant, the bean, the clover, etc., are able, when growing in soil that is poor in nitrogen, to obtain nitrogen from some source other than the soil in which they grow. A pea plant in soil that contains no nitrogen products and watered with water that contains no nitrogen, will, after sprouting and growing for a length of time, be found to have accumulated a considerable quantity of fixed nitrogen in its tissues. The only source of this nitrogen has been evidently from the air which bathes the leaves of the plant or permeates the soil and bathes its roots. This fact was at first disputed, but subsequently demonstrated to be true, and was found later to be associated with the combined action of these legumes and certain soil bacteria. When a legume thus gains nitrogen from the air, it develops upon its roots little bunches known as root nodules or root tubercles. The nodules are sometimes the size of the head of a pin, and sometimes much larger than this, occasionally reaching the size of a large pea, or even larger. Upon microscopic examination they are found to be little nests of bacteria. In some way the soil organisms (Fig. 27) make their way into the roots of the sprouting plant, and finding there congenial environment, develop in considerable quantities and produce root tubercles in the root. Now, by some entirely unknown process, the legume and the bacteria growing together succeed in extracting the nitrogen from the atmosphere which permeates the soil, and fixing this nitrogen in the tubercles and the roots in the form of nitrogen compounds. The result is that, after a proper period of growth, the amount of fixed nitrogen in the plant. is found to have very decidedly increased.
This, of course, furnishes a starting point for the reclaiming of the lost atmospheric nitrogen. The legume continues to live its usual life, perhaps increasing the store of nitrogen in its roots and stems and leaves during the whole of its normal growth. Subsequently, after having finished its ordinary life, the plant will die, and then the roots and stems and leaves, falling upon the ground and becoming buried, will be seized upon by the decomposition bacteria already mentioned. The nitrogen which has thus become fixed in their tissues will undergo the destructive changes already described. This will result eventually in the production of nitrates. Thus some of the lost nitrogen is restored again to the soil in the form of nitrates, and may now start on its route once more around the cycle of food.
It will be seen, then, that the food cycle is a complete one. Beginning with the mineral ingredients in the soil, the food matter may start on its circulation from the soil to the plant, from the plant to the animal, from the animal to the bacterium, and from the bacterium through a series of other bacteria back again to the soil in the condition in which it started. If, perchance, in this progress around the circle some of the nitrogen is thrown off at a tangent, this, too, is brought back again to the circle through the agency of bacterial life. And so the food material of animals and plants continues in this never-ceasing circulation. It is the sunlight that furnishes the energy for the motion. It is the sunlight that forces the food around the circle and keeps up the endless change; and so long as the sun continues to shine upon the earth there seems to be no reason why the process should ever cease. It is this repeated circulation that has made the continuation of life possible for the millions and millions of years of the earth's history. It is this continued circulation that makes life possible still, and it is only this fact that the food is thus capable of ever circulating from animal to plant and from plant to animal that makes it possible for the living world to continue its existence. But, as we have seen, one half of this great circle of food change is dependent upon bacterial life. Without the bacterial life the animal body and the animal excretion could never be brought back again within the reach of the plant; and thus, were it not for the action of these micro-organisms the food cycle would be incomplete and life could not continue indefinitely upon the surface of the earth. At the very foundation, the continuation of the present condition of Nature and the existence of life during the past history of the world has been fundamentally based upon the ubiquitous presence of bacteria and upon their continual action in connection with both destructive and constructive processes.