498 



THE COMMUNITY 



under the term "nitrification," but are 

 sufficiently distinct to warrant separation. 



At this stage in the nitrogen cycle the 

 soil nitrates can be utilized by green plants 

 to form plant proteins. 



At the same time, still other bacteria re- 

 duce the nitrites and nitrates to gaseous 

 nitrogen in a fourth reaction chain known 

 as denitrification. Still other bacteria trans- 

 form the free, gaseous nitrogen of the at- 

 mosphere pervading the forest or grass- 

 land community back into amino acids. 

 These amino acids are stored in these nitro- 

 gen-fixing bacteria in a fifth chain of reac- 

 tions, termed nitrogen fixation. Such bac- 

 teria belong to two groups, both of which 

 are residents of the subterranean stratum 

 of terrestrial communities. They either are 

 free-living in the soil, or live symbiotically 

 upon the root systems of legumes. In 

 either case, as these bacteria die, the stored 

 amino acids are available for ammonifica- 

 tion. 



These five sets of reactions are concerned 

 with the production of raw materials of 

 plant proteins. Such bacterial activities in 

 the community metabolism are analogous 

 to enzyme chains in organismal metabolism. 



In this connection it must be remem- 

 bered that about 1000 pounds of atmos- 

 pheric nitrogen are fixed annually by light- 

 ning for each square mile of the earth's 

 surface (p. 190). This annual increment 

 of nitrogen undoubtedly affects bacterial 

 activity in communities. Just how important 

 this annual nitrogen addition is in the 

 metabolism of communities is not known. 

 Nevertheless, in view of the problem of the 

 availability of dissolved organic substances 

 in the sea, any fixed inorganic nitrogen 

 falling into the ocean, where it may be uti- 

 lized by phytoplankton, even in consider- 

 ably less amounts than cited, may be of 

 great importance in the nitrogen cycle. 

 Furthermore, nitrates, nitrites, and am- 

 monia are carried into the sea in substan- 

 tial amounts by rivers. For example, the 

 Mississippi river carries some 361,000 

 metric tons of nitrate nitrogen annually 

 into the Gulf of Mexico (calculated from 

 Clarke, 1924). 



Phosphorus is also an essential element 

 in the residue of decomposing protoplasms. 

 It is finally resolved into phosphoric acid by 

 soil bacteria in a series of reactions that 

 may be called phosphatization and is 



stored in the soil in the form of phosphates 

 of aluminum, calcium, iron, and magne- 

 sium. Again such phosphates are protein- 

 building blocks in the metabolism of the 

 community. 



Soil bacteria are also engaged in less 

 well-defined systems of oxidation-reduction. 

 One is the transformation of iron com- 

 pounds, in some cases the oxidation of fer- 

 rous to ferric iron. In this instance the 

 bacteria obtain energy that enables them 

 to synthesize their sugars; hence they are 

 autotrophic. At other times deficiency of 

 soil iron in alkaline areas may result directly 

 from bacterial action or indirectly by the 

 production of water-insoluble compounds. 



Sulfur is also an important part of some 

 protein molecules. It is obtained by green 

 plants in the form of soil sulfates. When a 

 plant or an animal dies, the released sulfur 

 is attacked by sulfur heterotrophs to pro- 

 duce hydrogen sulfide, which is then oxi- 

 dized into sulfur dioxide by other sulfur 

 bacteria. Still other bacteria oxidize the 

 sulfur dioxide into sulfuric acid. This acid 

 reacts molecule by molecule with a va- 

 riety of soil bases to form highly important 

 compounds. One of these bases is calcium, 

 which unites with sulfuric acid to form cal- 

 cium sulfate, which can be utilized directly 

 by green plants. This complex chain of 

 reactions to produce sulfates is known as 

 sulfofication. In apposition to this process 

 is a converse series of reactions known as 

 desulfofication, in which bacteria reduce 

 the soil sulfates to hydrogen sulfide. This 

 latter process results, temporarily at least, 

 in a depletion of available soil nutrients. 



The foregoing summary of four impor- 

 tant, separate series of bacterial activities 

 is but a small part of the total biochemical 

 reactions that take place continuously in 

 the subterranean strata of grassland and 

 forest communities. A more detailed ac- 

 count of bacterial activity may be obtained 

 from such treatises as that of Waksman 

 (1932) and Frobisher (1945), but the 

 essential matter for consideration here is 

 the point of view. 



These really vital bacterial activities are 

 outlined in most modem texts on general 

 biology and general botany, often in har- 

 mony with the subject matter (Transeau, 

 Sampson, and Tiffany, 1940). They are 

 much less widely recognized in texts on 

 general ecology or in lectures upon this 



