Other Modes of Nutrition; Conservation of Food Elements - M7 



results from the nutrition of other sapro- 

 phytes (e.g., yeasts and molds). 



Decoy and Putrefaction. The saprophytic 

 bacteria are all spore formers (p. 56), and 

 bacterial spores are abundant everywhere ex- 

 cept in the purest atmospheres. Consequently 

 whenever a mass of organic material, such as 

 a dead body or other organic remnant, re- 

 mains exposed, bacterial spores fall upon it 

 and begin to grow. In their nutrition, the 

 saprophytic bacteria utilize the various or- 

 ganic substances present in the original ma- 

 terial, decomposing these compounds eventu- 

 ally into simple inorganic end products. This 

 process, as a whole, is designated as decay. 



Many different bacteria participate in de- 

 cay, and each species is very specific as to the 

 organic substances it utilizes as food. Some 

 grow on carbohydrates; for example, the 

 cellulose bacteria, which use the enzyme 

 cellulase to digest woody materials and ob- 

 tain glucose; the milk-souring bacteria, 

 which absorb lactose and convert it (anaero- 

 bically) into lactic acid (CH 3 CHOH- 

 COOH); and the vinegar bacteria, which 

 oxidize alcohol (present in hard ciders or 

 wines) into acetic acid (CH 3 COOH). Other 

 bacteria possess different enzymes, which en- 

 able them to grow on fatty substances. Thus, 

 the smell of rancid butter is due mainly to 

 butyric acid (C 3 H 7 -COOH), produced by 

 the action of certain bacteria upon butter 

 fat. 



The total decay of protein material is 

 designated by the special name of putrefac- 

 tion. Putrefaction involves the growth of a 

 variety of saprophytic bacteria, which act 

 simultaneously and successively upon the 

 protein, until nothing but inorganic end 

 products remain. During the intervening 

 stages, however, a number of foul-smelling 

 and toxic substances (for example, various 

 organic amines) are formed in the metab- 

 olism of some of the species, but these sub- 

 stances are finally absorbed and utilized by 

 other species. 



Return of Inorganic Nitrogen to the Soil 

 and Air. The liberation of the inorganic 



compounds of nitrogen, which results from 

 the bacterial decomposition of proteins and 

 other organic nitrogen compounds, is very 

 important in relation to soil fertility. Some 

 saprophytic bacteria utilize urea [CO(NH 2 ) L >], 

 hydrolyzing this compound into ammonia 

 (NH 3 ) and carbon dioxide (CO.,), which pass 

 forth into the environment. The nitrogen of 

 proteins and other complex nitrogenous 

 compounds is likewise liberated by the 

 putrefactive bacteria chiefly in the form of 

 ammonia, although small quantities of free 

 nitrogen (N 2 ) are produced by some species. 

 Free nitrogen is also liberated by the burn- 

 ing (in fires) of organic nitrogen compounds, 

 and in greater quantities by the denitrifying 

 bacteria. 



Denitrifying Bacteria. These soil sapro- 

 phytes utilize carbohydrates present in decay- 

 ing humus. When the soil is well aerated by- 

 cultivation, the denitrifying bacteria depend 

 upon free oxygen for the oxidation of their 

 carbohydrates. But when free oxygen is lack- 

 ing, they decompose nitrate ( — N0 3 ) salts, 

 using this bound form of oxygen as an 

 oxidizing agent. Under these circumstances 

 the denitrifying bacteria liberate free nitro- 

 gen. The liberation of free nitrogen from 

 various nitrogen compounds represents a 

 definite loss of soil fertility, because green 

 plants are not able to use free nitrogen to 

 sustain their growth. Such a liberation of 

 gaseous nitrogen has been designated rather 

 loosely as "a leak in the nitrogen cycle." 



Return of Elements to the Environment. 

 In summary, the complete decay of any 

 material involves a series of different bacteria 

 acting successively upon the various organic 

 substances as they appear in the decompos- 

 ing mass. Some of the decomposition reac- 

 tions are digestive, and others are metabolic 

 in nature, but the final end products are all 

 simple inorganic substances. The carbon of 

 organic compounds emerges from the process 

 mainly as C0 2 , and the hydrogen mainly as 

 FLO. The nitrogen part of various organic 

 compounds appears chieffy as NH 3 , although 

 small quantities of N 2 and of nitrate ( — N0 3 ) 



