SOIL PROTOZOA 339 



found 100 in pure cultures of Azotobacter; this nitrogen would be pro- 

 duced as a result of autolysis. However, in impure cultures of Azoto- 

 bacter, considerable quantities of soluble nitrogen were found; this phe- 

 nomenon was ascribed to the action of amoebae. 



While the results of Cutler and associates seem to demonstrate defi- 

 nitely that protozoa are capable of reducing the number of bacteria in 

 the soil, due to their phagocytic action (as determined by the plate 

 method), there is very little information available concerning their 

 influence upon the activities of microorganisms in the soil and upon 

 soil processes in general. The activities of bacteria decomposing or- 

 ganic nitrogenous compounds may not be influenced injuriously by the 

 presence of protozoa and may even be favored, as indicated by an in- 

 crease in the amount of ammonia liberated (Hill, Waksman) or nitro- 

 gen fixed by Azotobacter (Nasir). Excessive development of bacteria 

 may become harmful to the growth of protozoa in artificial culture media, 

 although it remains to be seen to what extent this may take place in 

 the soil. 



The fact that the protozoa destroy some soil bacteria need not indi- 

 cate that they exert an injurious influence, but may result in a decided 

 benefit to soil biological processes. Decomposition of organic matter 

 as well as other biological activities are resultants of the multiplication 

 and growth of the bacterial cells. By destroying the excess of bacteria, 

 the protozoa may stimulate further bacterial development and, there- 

 fore, further biological transformations in the soil. The protozoa 

 themselves may become later a source of energy for bacteria. The 

 phenomena observed by Russell and Hutchinson as a result of partial 

 sterilization of soil may not be due at all to the destruction of protozoa, 

 but to other factors, as shown elsewhere (p. 757). 



The protozoa may also take a part in some definite soil processes, 

 such as the decomposition of certain organic substances. Cleveland 102 

 found that protozoa living in the intestinal tract of termites feed on wood 

 cellulose. When the protozoa are killed, the termites die on a wood 



100 Moler, T. Ein Beitrag zur Kenntnis der Entbindung des durch Azotobacter 

 fixierten Stickstoffes. Bot. Notiser. 1915, 163-175. (Centrbl. Bact. II, 47: 

 635-636. 1917.) 



101 Hill, T. L. The relation of protozoa to certain groups of soil bacteria. 

 Jour. Bact., 1: 423-433. 1916. 



102 Cleveland, L. R. The physiological and symbiotic relationships between 

 the intestinal protozoa of termites and their host, with special reference to 

 Reticulitermes flavipes Kollar. Biol. Bull., 46: 177-225. 1924. 



