10 MICROORGANISMS IN SOILS AND WATER BASINS 



tensive mycelium in the soil, binding the particles together and pre- 

 venting their falling apart. 



Various bacteriolytic agents, including specific phages, have also been 

 demonstrated in the soil. The phage of root-nodule bacteria is of par- 

 ticular interest. It is readily adsorbed by the soil, but its presence can 

 easily be established. The repression of spore-forming bacteria and the 

 abundance of Pseudomonas fluorescens (139, 140) may be due to the 

 antagonistic action of the latter. 



SOILS AND WATER BASINS AS CULTURE 

 MEDIA 



Microorganisms require for their growth and respiration certain 

 energy sources and certain nutrients, as well as certain conditions favor- 

 able for their development. Different organisms show considerable 

 variation in this respect. The mineral elements required for growth and 

 multiplication are almost invariably present in the soil and to a large 

 extent also in many water basins. The available energy supply may be 

 limited, however, and thus usually becomes the most important factor 

 regulating the abundance and activities of microorganisms in natural 

 substrates. The autotrophic bacteria depend on the supply of oxidizable 

 minerals such as ammonium salts, nitrite, sulfur, iron, and manganese, 

 the oxidation of which makes energy available for their growth. The 

 heterotrophic organisms are dependent on the carbon compounds 

 brought into the soil in the form of plant and animal residues as well 

 as the bodies of many insects, earthworms, and other small animals. 

 The roots of plants also supply an abundance of easily available sub- 

 stances for microbial nutrition. 



Every organic compound produced in nature finds its way, sooner or 

 later, into the soil or into lakes and rivers, where it serves as a source 

 of energy for microorganisms. This energy becomes available to some 

 of the organisms through anaerobic or fermentative transformation and 

 to others through aerobic or oxidative processes. The net change in the 

 energy produced by any one organism or group of organisms is accom- 

 panied by a loss of free energy by the system to which the culture is 



