II. The Soil as a Suitable Medium for Algal Growth.
Were it not for the recent advances that have been made in our knowledge of the mode of nutrition of many of the lower algæ, it would be very difficult to account for the widespread occurrence of algæ in the soil, for it is undoubtedly true of some of the more highly evolved algæ that their mode of nutrition is entirely typical of that of green plants in general. The application of bacteriological technique to the algæ, however, by Beijerinck, by Artari, and by Chodat and his pupils, and the introduction of pure-culture methods have led to a study of the physiology of some of the lower algæ, in the hope of getting to understand some of the fundamental problems underlying the nutrition of organisms containing chlorophyll. It is impossible here to do more than mention the names of a few of the more important of those who have worked along these lines, such as Chodat, Artari, Grintzesco, Pringsheim, Kufferath, Nakano, Boresch, Magnus and Schindler, and to condense into a few sentences some of their more important conclusions.
It is now established that although in the light the algæ are able to build up their substance from CO2 and water containing dilute mineral salts, yet in such conditions growth is sometimes very slow, and with some species at any rate it is greatly accelerated by the addition of a small quantity of certain organic compounds. The ability of the lower algæ to use organic food materials varies specifically, quite closely related forms often reacting very differently to the same substance, but there have been shown to be a considerable number of forms which can make use of organic compounds to such an extent that they can grow entirely independently of light. In such cases the nutrition of the organism becomes wholly saprophytic, and the chlorophyll may be completely lost; it has frequently been observed, however, that on suitable nutrient media, even in complete darkness, certain algæ continue to grow and retain their green colour, provided that a sufficient supply of a suitable nitrogenous compound is present.
Chlorella vulgaris, an alga frequently found in soil, has been shown to be extremely plastic in its relations to food substances. Given only a dilute mineral-salts solution as food source, it absorbs CO2 from the air, and grows in sunlight with moderate rapidity. The addition of glucose to the medium in the light greatly increases the rate and amount of growth and the size of the cells, while in the dark the colonies not only remain green but have been shown to develop more vigorously than in full daylight. The organism is also able to use peptone as a source of nitrogen in place of nitrates.
Stichococcus bacillaris and Scenedesmus spp., also occurring in soils, have been shown to be almost equally adaptable, though in these cases the organisms grow more slowly in the dark than on the corresponding medium in the light. Liquefaction of gelatine by the secretion of proteolytic enzymes has been shown to be a further property of certain species, resulting in the formation of amino acids such as glycocoll, phenylalanine, dipeptides, etc. This property is, however, possessed by only a limited number of species and in varying degree.
Up to the present very little work of this kind has been done upon algæ actually taken from the soil, and our knowledge is therefore very scanty. Of the species so far examined all show considerable increase in growth on the addition to the medium of glucose and other sugars, and tend to be partially saprophytic; a few have been shown to liquefy gelatine to some extent.
Servettaz, Von Ubisch, and Robbins have also demonstrated that the protonema of some mosses can make use of certain organic substances, especially the sugars, and grow vigorously in the dark. It has been shown, however, that light is essential for the development of the moss plant.
It was thought at Rothamsted that some light might be thrown upon the activities of the soil-algæ by making counts of the numbers present in samples of soil taken periodically within a circumscribed area. A dilution method similar to that in use in the protozoological laboratory was adopted and applied to samples of arable soil taken from the surface, and at depths of 2, 4, 6 and 12 inches vertically beneath. A considerable number of samples were examined in this way from two plots on Broadbalk wheat-field, viz.: the unmanured plot and that receiving a heavy annual dressing of farmyard manure. The numbers in the unmanured soil were observed to fall far short of those in that containing a large amount of organic matter, while in both plots the numbers varied considerably at different times of the year. The chief species in both plots were identical, and their vertical distribution was fairly uniform, but it was observed that the numbers of individuals varied according to the depth of the sample. The 6th and 12th inch samples contained very few individuals of comparatively few species, but the 4th inch samples yielded numbers that were not significantly less than those in the top inch. The 2nd inch sample was usually much poorer in individuals than either the top or the 4th inch.
It is unfortunate that this method of counting is not really satisfactory for the algæ, chiefly because it takes no account of the blue-green forms. The gelatinous envelope which encloses the filaments of these algæ prevents their breaking up into measurable units. Assuming, as appears to be the case for the two plots investigated, that the blue-green algæ are at least as numerous as the green forms, the total numbers should probably be at least twice as great as those calculated. Taking 100,000 as a rough estimate of the number of algæ per gram of manured soil in a given sample, and assuming the cells to be spherical and of average diameter 10µ, it has been calculated that the volume of algal protoplasm present was at least 3 times that of the bacteria though only one-third of that of the protozoa. This is probably only a minimum figure for this sample.
A soil population of this magnitude can not be without effect on the fertility of the soil. When growing on the surface of the ground exposed to sunlight the algæ must, by photosynthesis, add considerably to the organic matter of the soil, but when they live within the soil itself their nutrition must be wholly saprophytic, and they can be adding nothing either to the energy or to the food-content of the soil. How these organisms fit into the general scheme of life in the soil is at present undetermined, and there is a wide field for research in this direction.