758 ANIMALS AND THEIK ENVIRONMENT 



urea, uric acid, creatinine and ammonia-and decay bacteria convert 

 these into ammonia. Most of the ammonia is converted by nitrite bac- 

 teria to nitrites and this in turn is converted by nitrate bacteria into 

 nitrates, thus completing the cycle (Fig. 37.3). Denitrifying bacteria 

 convert some of the ammonia to atmospheric nitrogen. Atmospheric 

 nitrogen can be converted to amino acids and other organic nitrogen 

 compounds by some algae (Nostoc) and by the soil bacteria Azotobacter 

 and Clostridium. Other bacteria of the genus Rhizobimn, though un- 

 able to fix atmospheric nitrogen by themselves, can carry out this process 

 when in combination with cells from the roots of legumes such as peas 

 and beans. The bacteria invade the roots and stimulate the formation 

 of root nodules, a sort of benign tumor. The combination of legume 

 cell and bacteria is able to fix nitrogen, something neither can do alone. 

 For this reason legumes are often planted to restore soil fertility by 

 increasing the content of fixed nitrogen. Nodule bacteria may fix as 

 much as 5 pounds of nitrogen per acre per year and soil bacteria as 

 much as 6 pounds per acre per year. Atmospheric nitrogen can also be 

 fixed by electrical energy, either by lightning or by man-made electricity. 

 Although 80 per cent of the gases in the atmosphere is nitrogen, no 

 animals and only these few plants can utilize it in this form. When 

 the bodies of the nitrogen-fixing bacteria are decayed, the amino acids 

 are metabolized to ammonia and this in turn is converted by the nitrite 

 and nitrate bacteria to complete the cycle. 



331. The Water Cycle 



The seas are the world's great reservoir of water. The sun's heat 

 vaporizes water, forms clouds, and these are blown over the land, where 

 they are cooled enough to precipitate the water as rain or snow. Some 

 of the precipitated water soaks into the ground, some runs off the 

 surface into streams and goes directly back to the sea. The ground water 

 is returned to the surface by springs, by pumps, and by the activities of 

 the roots and stems of plants. Water inevitably ends up in the sea, but 

 it may become incorporated into the bodies of several successive organ- 

 isms en route. The energy to run the cycle— the heat needed to evaporate 

 water— comes from sunlight. 



332. Mineral Cycles 



As water runs over rocks it gradually wears away the surface and 

 carries off a variety of minerals, some in solution and some in suspen- 

 sion. Some of these minerals, such as the phosphates, sulfates, and 

 other salts of calcium, magnesium, sodium and potassium, are essential 

 for the growth of plants and animals. Phosphorus, an essential com- 

 ponent of many of the compounds found in protoplasm, enters plants 

 as inorganic phosphate and is converted to a variety of organic phos- 

 phates. Animals obtain their phosphorus as inorganic phosphate in the 

 water they drink or as inorganic and organic phosphates in the food 

 they eat. The phosphorus cycle is not completely balanced, for phos- 



