Chapter 2 

 Ecology 



WITH regard to their habitats, the Protozoa may be divided 

 into free-living forms and those living on or in other organisms. 

 Mastigophora, Sarcodina, Ciliata, and Suctoria include both free- 

 living and parasitic Protozoa, but Sporozoa are exclusively parasi- 

 tic. 



Free-living Protozoa 



The vegetative or trophic stages of free-living Protozoa have been 

 found in every type of fresh and salt water, soil and decaying or- 

 ganic matter. Even in the circumpolar regions or at extremely high 

 altitudes, certain protozoa occur at times in fairly large numbers. 

 The factors, which influence their distribution in a given body of wa- 

 ter, are temperature, light, chemical composition, acidity, kind and 

 amount of food, and degree of adaptability of the individual proto- 

 zoans to various environmental changes. Their early appearance as 

 living organisms, their adaptability to various habitats, and their ca- 

 pacity to remain viable in the encysted condition, probably account 

 for the wide distribution of the Protozoa throughout the world. The 

 common free-living amoebae, numerous testaceans and others, to 

 mention a few, of fresh waters, have been observed in innumerable 

 places of the world. 



Temperature. The majority of Protozoa are able to live only 

 within a small range of temperature variation, although in the en- 

 cysted state they can withstand a far greater temperature fluctua- 

 tion. The lower limit of the temperature is marked by the freezing of 

 the protoplasm, and the upper limit by the destructive chemical 

 change within the body protoplasm. The temperature toleration 

 seems to vary among different species of Protozoa; and even in the 

 same species under different conditions. For example, Chalkley 

 (1930) placed Paramecium caudatum in 4 culture media (balanced 

 saline, saline with potassium excess, saline with calcium excess, and 

 saline with sodium excess), all with pH from 5.8 or 6 to 8.4 or 8.6, at 

 40°C. for 2-16 minutes and found that (1) the resistance varies with 

 the hydrogen-ion concentration, maxima appearing in the alkaline 

 and acid ranges, and a minimum at or near about 7.0; (2) in a bal- 

 anced saline, and in saline with an excess of sodium or potassium, the 

 alkaline maximum is the higher, while in saline with an excess of 

 calcium, the acid maximum is the higher; (3) in general, acidity de- 

 creases and alkalinity increases resistance; and (4) between pH 6.6 



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