ECOLOGY 21 



um were less adaptive to salt water, rarely living in 60 per cent sea 

 water. 



Hydrogen-ion concentration. Closely related to the chemical 

 composition is the hydrogen-ion concentration (pH) of the water 

 which influences the distribution of Protozoa. The hydrogen-ion 

 concentration of freshwater bodies vary a great deal between 

 highly acid bog waters in which various testaceans may frequent- 

 ly be present, to highly alkaline water in which such forms as 

 Acanthocystis, Hyalobryon, etc., occur. In standing deep fresh 

 water, the bottom region is often acid because of the decomposing 

 organic matter, while the surface water is less acid or slightly 

 alkaline due to the photosynthesis of green plants which utilize 

 carbon dioxide. Several investigators have recently made obser- 

 vations on the pH range of the water or medium in which certain 

 protozoans live, grow, and multiply, which data are collected in 

 a table on page 22. 



Seemingly various Protozoa require a definite pH value in order 

 to carry on maximum metabolic activities. As a matter of fact, 

 Pringsheim, Hall, Loefer, Johnson, and others, found that sodium 

 acetate may increase or decrease the growth rate of various 

 Phytomastigina subject to the hydrogen-ion concentration of the 

 culture media. 



Food. The kind and amount of food available in a given body 

 of water also controls the distribution of Protozoa. The food is 

 ordinarily one of the deciding factors of the number of Protozoa 

 in a natural habitat. Species of Paramecium and many other 

 holozoic protozoans cannot live in waters in which bacteria or mi- 

 nute protozoans do not occur. If other conditions are favorable, 

 then the greater the number of food bacteria, the greater the 

 number of these protozoans. Didinium nasutum feeds almost ex- 

 clusively on Paramecium, hence it cannot live in the absence of 

 the latter ciliate. Euryphagous protozoans are widely distributed 

 and stenophagous forms are limited in their distribution. 



Some protozoans inhabit soil of various types and localities. 

 Under ordinary circumstances, they occur near the surface, their 

 maximum abundance being found at a depth of about 10-12 cm. 

 (Sandon, 1927). It is said that a very few protozoans occur in the 

 subsoil. Here also one notices a very wide geographical distribu- 

 tion of apparently one and the same species. For example, San- 

 don found Amoeba proteus in samples of soil collected from Green- 



