Symbiosis between Chlorella Sp. and Azotobacter Chroococcum and Nitrogen Fixation

Alfred Koch has given considerable attention to the subject of nitrogen fixation by bacteria as resulting from their symbiosis with alga~. Many others have also investigated various features of this problem. In spite of all this there are numerous questions which remain unsolved. The use of soil cultures for such studies is objectionable on account of the soil's complexity, its variety of flora and fauna, and the impossibility of observing the organisms while they are at work; and yet the soil culture method has been the one most used by investigators heretofore. A fortunate accident threw into our hands a species of Chlorella which we are using in some physiological experiments. The facile growth of this organism under ordinary laboratory conditions in tap water, in distilled water, and in weak culture solutions offered a ready means for experimentation on the question of a possible symbiosis with Azotobacter. This strain of Chlorella develops in solution cultures a beautiful green granular growth and with proper light conditions covers the walls of the culture flask completely. It is a single celled organism, somewhat elliptical in shape, and not much more than twice as large as the large cells of Azotobacter chroococcum. Its cell wall and chloroplasts are very distinct and the structure of the cell unusually clear under the microscope. The organism probably comes from the tap water of the city of Berkeley, but it is undoubtedly a common inhabitant of surface soils. Some unusually striking physiological peculiarities of this strain of Chlorella will be described in.a later publication from this laboratory. The Azotobacter culture used in this investigation was a pure culture of Azotobacter chroococcum (Madera strain) which was isolated from a California soil. 5O9