THE ROLE OF ALGAE IN THE NITROGEN CYCLE OF THE SOIL

Abstract

In the presence of actively growing cultures of 9 strains of green algae, Azotobacter chroococcum failed to grow under conditions where the sole limiting factor was a supply of available C and energy. Evidently the algae supplied Azotobacter with little or no available organic matter. The algae liberated small amte. of organic compounds into the surrounding medium. In most cases, this amounted to less than 10% of the total C assimilated by the algae. This accounts partly for the lack of marked development of Azotobacter in algal cultures; however, the most important probable factor in connection with the limited growth of the bacterium is the unavailability of even this part of the organic matter. Some multiplication of Azotobacter occurred when this organism was inoculated into filtrates of algal cultures, but this was slow and slight. Somewhat greater multiplication took place in the auto-claved algal suspensions, but this too was slow when compared with the rapid development of other bacteria that occasionally found their way, as contaminants, into the suspensions of algae. Qualitatively, the conception of a beneficial symbiosis between algae and Azotobacter may be true, since the 2 organisms do not appear to be antagonistic and since small increases in numbers of Azotobacter occurred in some cases; however, because of the extreme sensitivity of the biological method used, these increases must be interpreted with caution. Quantitatively, the degree of symbiosis appears to be very slight. The indirect role of the majority of soil algae in the fixation of atmospheric N either is nonexistent or functions to only a very small extent, for despite favorable environmental conditions and the presence of large numbers of algae, Azotobacter could not make appreciable growth with the amount of energy supplied by the algae. These optimum conditions rarely occur in nature where Azotobacter has to compete with numerous microbial forms such as fungi, actinomycetes, and other bacteria for the available organic matter. Blue-green algae must be considered as direct participants in the fixation of N in the soil. Their practical importance, however, still remains to be detd.