316 PRINCIPLES OF SOIL MICROBIOLOGY 



lytic and other enzymes. 13 In some, if not in the majority of protozoa, 

 there seems to exist a certain selection for the food, some species of bac- 

 teria being preferred to others. This may be due to the fact that some 

 bacteria are digested more readily than others by the specific enzymes of 

 the protozoan or to the formation by the particular bacteria of substances 

 which are injurious to the protozoan. 14 Some protozoa, especially cer- 

 tain amoebae and flagellates, are capable of digesting complex proteins, 

 starches and in some cases even celluloses. 15 



The autotrophic protozoa have no apparent relation to the bacteria 

 in the soil; the heterotrophic forms may play an important part in the 

 soil by using the bacteria as food. The latter utilize the soluble inor- 

 ganic and organic compounds and serve, in their turn, as food for many 

 rhizopods and ciliates. Many of the latter are specifically adapted to 

 the feeding on bacteria. Some protozoa also feed on fungi, algae and 

 smaller protozoa. Doflein divides the protozoa, which require complex 

 organic substances as food, into 4 groups: those that feed on (1) bac- 

 teria, (2) on waste products, (3) on plants (diatoms and other algae), 

 and (4) on small animals. Most forms take in mixed food, feeding on 

 bacteria and waste products, but the fact that the different protozoa 

 differ from one another in their feeding habits and that some species 

 have considerable power of selecting their food, makes a more detailed 

 knowledge of their food requirements a necessity for the satisfactory 

 elucidation of their role in the soil economy. 



When an infusion of hay, straw, or moss is prepared and allowed to 

 incubate, bacterial development takes place immediately. This is soon 

 followed by abundant growth of various species of protozoa, including 

 the flagellates and amoebae, later followed by the ciliates; these feed on 

 the bacteria in the infusion. The protozoa come into the infusions 

 from the air, where they are present in the form of cysts; as many as 13 



13 Mouton, H. Recherches sur la digestion chez les amibes et sur leur diastase 

 intracellulaire. These. Paris, 1902, Charaire; Compt. Rend. Acad. Sci., 133: 

 244. 1901. Ann. Inst. Past,, 16: 457-509. 1902. 



14 Hargitt, G. T., and Fray, W. W. The growth of Paramecium in pure cul- 

 tures of bacteria. Jour. Exp. Zool., 22: 421-455. 1917; Phillips, R. L. The 

 growth of Paramecium in infusions of known bacterial content. Ibid., 36: 135- 

 183. 1922. 



15 Stole, A. Beobachtungen und Versuche liber die Verdauung und Bildung 

 der Kohlenhydrate bei einem amobenartigen organismus — Pelomyxa palustris 

 Greeff. Ztschr. wiss. Zool., 68: 625-668. 1900; Clevelend, L. R. The method 

 by which Trichonympha campanula, a protozoon in the intestine of termites, in- 

 gests solid particles of wood for food. Biol. Bui., 48: 282-287. 1925. 



