Studies on the biology and taxonomy of soil algae

Abstract

1. In the introduction it is atternpted to give to the term "soil algae" a more precise delimitation, so as to make it comprise the species that normally occur on and in the soil. 2. A brief account is given of the development of our knowledge concerning soil algae through the ages. 3. The discovery , of the subterranean alga flora at the beginning of the 20th century is passed in review. 4. It is shown through the writer's investigations that in Danish soils, too , a subterranean alga flora is met with, on cultivated as well as on uncultivated land. The investigations were chiefly made in the spring, and it then turned out that the number of algal germs was generally much higher at the surface than in the lower layers. The algal species of the deeper layers are the same as are met with on the surface. This has been found, too, by previous investigators. 5. The investigation of a single locality in East Greenland shows that subterranean algae are present also in the absolutely virgin soil there. 6. A series of investigations, in part published before, show that soil algae do not multiply appreciably in soil samples in the dark; this bas been found for Diatoms, Cyanophyceae, and Chlorophyceae. In similar soil samples there was, on the other hand, a vigorous growth of the algae in the light. 7. The writer, therefore, could not but suppose (as surmised by some of the previous authors too) that the subterranean algae are derived from the surface of the soil and thence carried down into the lower layers. 8. After passing in review the literature on the responses of the motile phases of the algae, the writer arrives at the result that an active descent of the algae is not probable. The writer's own experiments gave negative results in all cases. 9. On the other hand, it was probable at the outset that seeping water, especially during heavy showers after a prolonged drought, might carry a large number of germs down into the soil. Experiments showed that water dripped on to the surface of a column of earth with an intensity as during a heavy shower is capable of carrying algal germs through the column of earth. Algae of ali the three systematic main groups were thus passed through a column of earth 12-15 cm deep. 10. It turned out that the algae passed more readily through soil which contained earthworms; whereas none of the writer's experiments showed that earthworms alone were able to convey algae in to the soil. 11. BRISTOL (1919 II) has found living algal germs in soil samples up to 73 years old. These contained 10 - 3 p.c. of water. The writer carried out experiments in a somewhat diflerent way, letting soil samples dry up in the course of about a month until they contained only about 1.5 p.c. of water. The experiment would seem to show that during the first time a not inconsiderable number of algal cells perish; but thereupon a state supervenes in which the number of living algal germs and the number of species remain constant for several years. A desiccation so intense and prolonged as that of the experiment will hardly occur under natural conditions in this country. 12. The data from the literature would seem to show that the effect of frosts or heat also will not be able to kill all the algae at the surface of the soil, though a smaller or larger number of them will perish. 13. It is pointed out that we know very little as to the relation which the acidity of the soil bears to the algae. 14. Other conditions being equal, the content of nutrient substances in the soil seems to determine the luxuriance of the algal vegetation. According to the data, manured soils possess a much richer algal flora than unmanured land. 15. A review of the literature on the capacity of algae to fix nitrogen gives as a result that such a capacity has not been shown to be present in any Chlorophyeeae or Diatoms. Among the Cyanophyceae species of Anabaena and Nostoc seem to be capable of binding free N in the light, but not in darkness. For Chlorophyceae as well as for Cyanophyceae there is much evidence to show that they can stimulate the activity of nitrogen-fixing bacteria. All the evidence would also seem to indicate that the algae can only exercise this influence in the light. The writer therefore arrives at the conclusion that only the epiterranean algae bear any relation to the fixation of nitrogen, while the subterranean algae are without importance in this respect. 16. In respect of the importance of the soil algae for the soil a sharp distinction must, according to BRISTOL ROACH (1927 I), be drawn between the epiterranean and the subterranean algae, since they must exercise contrary influences. Epiterranean algae are producers, subterranean algae consumers. The number of algae at the surface of the soil may under favourable circumstances be so high that they must be assumed to be capable of exerting their influence side by side with the bacteria. The number of the subterranean algae is always, as far as is kno\vn, so low that their importance in relation to the bacteria will be minimal. The significance of the epiterranean algae for the carbon cycle in the soil must be found in the fact that they produce organic substances by photosynthetic assimilation of carbon dioxide (preparation of "new soil"). In the nitrogen cycle most of them appear to play a part by stimulating the activity of nitrogen-fixing bacteria. Only some few genera of Cyanophyceae seem to be capable of assimilating free N in the light. According to the data at hand, the subterranean algae are exclusively consumers and bear no essential relation either to the carbon or the nitrogen cycle. 17. A list is given of the Danish soil alga flora as also of the algae met with in East Greenland, with systematic and biological notes on each species.