298 Nutrients 



community. We have seen earlier in this chapter that for both ter- 

 restrial and aquatic plants the nutrients that are likely to run short 

 —particularly phosphate and nitrate— are derived in large measure 

 from the decomposition of dead organisms. The destructive part of 

 the biological cycle is thus revealed to be just as critical as the con- 

 structive part in keeping the wheel of life turning. This idea can 

 perhaps be expressed no more graphically than in the words of a 

 student who wrote on an examination: "If it were not for the decom- 

 position process, the whole world would become choked with the 

 dead bodies of plants and animals, and this shocking situation would 

 bring all life to an end." 



Processes of Decomposition and Transformation 



The release of the nutrients that have been built into the bodies of 

 plants and animals involves two steps: first, the organic matter must 

 be decomposed into soluble form and subsequently into inorganic 

 form, and second, the resulting inorganic material must be trans- 

 formed into compounds that can be absorbed by phototrophic plants. 

 Carbohydrates decompose into carbon dioxide and water; fats break 

 down into these materials and also release phosphate. The decom- 

 position of protein is more elaborate, involving hydrolysis into pro- 

 teoses, peptones, and polypeptides, and then break down into amino 

 acids, ammonia, carbon dioxide, and water in addition to minor con- 

 stituents. Special complexes present in living organisms, such as 

 cellulose, hemicellulose, chitin, agar, and bone, decompose much more 

 slowly but eventually to the same products. 



Living organisms are required to carry out practically every one 

 of these steps in decomposition. Fungi are especially active in de- 

 composing organic matter on land; bacteria are prominent in damp 

 soil and in the water environment. These microorganisms not only 

 work on the surfaces of solid material but also attack dissolved or- 

 ganic matter. 



The necessity for the transformation of the products of decomposi- 

 tion from one inorganic form to another is especially apparent in 

 nitrogen compounds. Since most plants can obtain their nitrogen 

 only in the form of nitrate, the ammonia or ammonium salts resulting 

 from the decomposition of protein must be oxidized first to nitrite 

 and then to nitrate. These transformations, comprising the process 

 of nitrification, are brought about in both the land and the water en- 

 vironments by the activities of autotrophic ( chemotrophic ) bacteria: 

 the oxidation of ammonia to nitrite is accomplished by bacteria of the 



