COMMUNITY ORGANIZATION: METABOLISM 



497 



extensively by Waksman (1932), The 

 photo-autotrophs, such as the red and the 

 green sulfur bacteria, utibze sunlight to 

 synthesize carbon dioxide and hydro- 

 gen sulfide into organic materials. Photo- 

 synthetic bacteria are chiefly aquatic; Van 

 Niel (1931, 1935, 1936) has studied them. 



In their multiple relations with media, 

 heterotrophs are consumers and autotrophs 

 are producers; both are transformers of raw 

 materials in the food web of the commu- 

 nity. The bacteria may be said to have 

 three chief roles in community metabolism. 



First, they function in decomposing the 

 organic substances continually added to the 

 community as its constituents die. This 

 function is taken over by the heterotrophs. 

 They consume and break down the lifeless 

 bodies of plants and animals. By this activ- 

 ity protoplasms are disintegrated, and 

 much of the organic matter becomes inor- 

 ganic, i.e., is freed for resynthesis. 



Secondly, these inorganic materials may 

 be further oxidized or transformed by 

 chemo-autotrophs or may be used directly 

 by higher plants. In any event, these 

 inorganic compounds are made available 

 for organic synthesis by photosynthetic 

 plants of a given community, whether 

 aquatic or terrestrial. 



Thirdly, both heterotrophs and auto- 

 trophs are available as food for animals; for 

 example, the soil protozoans and zooplank- 

 ton (Baier, 1935). 



The first of these three basic functions 

 is to be considered the most fundamental. 

 There is some question concerning the 

 rank of bacteria as producers. In this latter 

 function they compete with higher plants. 

 This is especially true of the photo-auto- 

 trophs, about which relatively little is 

 known. Our paucity of information on these 

 photosynthetic species is in marked con- 

 trast with the rapidly growing store of fact 

 and theory about the much-cultured hetero- 

 trophs familiar to medical research. Birge 

 and Juday (1922), concluded that bacteria 

 as producers are of relatively small impor- 

 tance in the metabolism of the lake com- 

 munitv as compared with the algal phy- 

 toplankton. 



A true picture of bacterial importance 

 in the metabolism of communities may not 

 be gained from an outline of separate func- 

 tions unless these general functions are 



exemplified by summarizing certain of the 

 chief processes at work. 



The role of soil bacteria is better under- 

 stood than that of the bacteria of aquatic 

 communities. The bacterial floras of soils 

 are engaged in many fundamental reor- 

 ganizations that are vital to the metabo- 

 lism of terrestrial communities with re- 

 spect to nitrogen, phosphorus, sulfur, and 

 iron. A brief statement of each will serve 

 our general purpose. 



The importance of the nitrogen cycle is 

 well known and has received a great deal 

 of attention (Waksman, 1932; Wilson, 

 1940; Rahn, 1945; Frobisher, 1945). 



Upon the death of a plant or an animal, 

 its protoplasm is disintegrated through the 

 agency of heterotrophic bacteria. More ex- 

 actly (Frobisher, 1945, p. 414) : "As soon 

 as protoplasm ceases to live, and as soon as 

 any organic matter returns to the soil, it 

 begins to undergo spontaneous oxidative 

 changes and also the biological decompo- 

 sition process of decay, which is aerobic 

 decomposition, or putrefaction and fermen- 

 tation, which are anaerobic decomposition 

 of proteins and carbohydrates, respectively. 

 Through these processes the nitrogen and 

 other elements become available to plants. 

 Decomposition results from the action of 

 hordes of bacteria and other creatures 

 found in all soil and in natural waters." 



A part of the residual material of proto- 

 plasmic disintegration is protein. Certain 

 bacteria digest the protein to relatively 

 simple amino acids, and by combination 

 of water with NH2" ions form ammonia. 

 This first series of reactions is known as 

 ammonification. It is fundamental to the 

 well-being of the community. 



Nitrogen in the form of ammonia 

 is combined into ammonium salts in part, 

 and in part is oxidized by such bac- 

 teria as Nitrosomonas to form nitrites (ni- 

 trosification) . If this did not happen, the 

 fixed nitrogen would be lost partially into 

 the atmosphere, just as it is lost from com- 

 post heaps, and might be greatly delayed 

 in returning to the soil system. 



These nitrites are in large part useless 

 to plants in the community until other bac- 

 teria, such as Nitrobacter, oxidize them to 

 nitrates (riitrification in the strict sense). 

 The two processes of nitrosification and ni- 

 trification are at times combined loosely 



