12 MICROORGANISMS IN SOILS AND WATER BASINS 



metabolism of these microbes is also highly complicated. Most of the in- 

 formation on their nutrition is based upon their growth on artificial cul- 

 ture media. In nature, however, these organisms live in associations and 

 vary considerably in the degree of their interdependence. As yet no 

 laboratory method has been developed that duplicates these conditions. 

 Microorganisms vary considerably in their nutrition and energy 

 utilization, as well as in the breakdown and transformation of the avail- 

 able nutrients. Certain elements or compounds are required for cell 

 synthesis. In some cases, certain trace elements as well as varying con- 

 centrations of growth-promoting substances are also essential. Among 

 the nutrient elements, nitrogen occupies a prominent place. Consider- 

 able variation exists in the ability of microorganisms to utilize different 

 types of nitrogen compounds: some can obtain their nitrogen from a 

 wide variety of substances j others are restricted to the use of a single 

 group of compounds such as proteins, amino acids, urea, ammonia, or 

 nitrate J a few are able to use atmospheric nitrogen. The variety of or- 

 ganic nitrogenous bodies supplied to microorganisms in soils and in 

 water basins is limited only by the number of such compounds synthe- 

 sized by plants and animals. The complex forms of nitrogen are broken 

 down to simpler compounds j these may be assimilated by organisms 

 and again built up into complex forms, or they may be utilized only by 

 other organisms. Microbial activity thus regulates the state of the nitro- 

 gen in natural substrates and is responsible for the continuous stream 

 of ammonia and nitrate forming the available sources of nitrogen that 

 make possible the growth of higher plants. 



THE GROWTH OF THE MICROBIAL CELL IN PURE 

 CULTURE AND IN MIXED POPULATIONS 



When nutrients are available in sufficient concentration and when the 

 environmental conditions are favorable for the development of the 

 microbial cell in pure culture, growth follows a definite sigmoid-shaped 

 curve. Slow multiplication is followed by rapid development, until a 

 certain maximum number of cells within a given volume of medium is 

 reached} the rate of growth then diminishes. The maximum population 

 of Aerohacter aero genes grown in a medium containing lactose and 



