FOOD OF MICROORGANISMS 223 



for all green plants is carbon dioxide (COa). Animals cannot use this, 

 for they all require complex compounds, such as carbohydrates, fats 

 or amino-acids. Bacteria exist between the plants and animals in 

 this respect. Some bacteria have already been mentioned (page 201) 

 as being able to use carbon dioxide (CO 2 ), as the only source of carbon; 

 they are the mineral-oxidizing species. Such bacteria are called 

 autotrophic in their relation to carbon, since they use it in the inorganic 

 form. A bacterium feeding on carbon, as such, would be called 

 prototrophic; bacteria of this class are said to exist. The vast majority 

 of microorganisms are heterotrophic, using carbon in organic form, 

 Organic acids and sugars are excellent sources of carbon for micro- 

 organisms, although proteins and their decomposition products seem 

 to be equally satisfactory as construction material. 



SOURCES OF NITROGEN. The sources of nitrogen are equally varied; 

 the green plants use nitrates; animals must have a number of different 

 amino-acids; the microorganisms again are found between plants and 

 animals. We know autotrophic bacteria, and especially molds and 

 yeasts which can grow with nitrates or ammonium salts as the only 

 source of nitrogen. There are three groups of prototrophic bacteria 

 in their relation to nitrogen the B. amylobacter group, the Ps. radicicola 

 group and the Azotobacter group. These bacteria are of the greatest 

 importance to agriculture; soil fertility depends, to a large extent, 

 upon the last two groups, for they take nitrogen gas from the surround- 

 ing air, form their own protoplasm from it, and thus increase the 

 amount of chemically combined nitrogen in the soil. Details of their 

 relation to soil fertility can be found in Chap. Ill, page 400. The 

 majority of bacteria are heterotrophic, requiring organic nitrogen. Urea 

 is not well adapted for this purpose; amino-acids or the peptones from 

 which amino-acids are derived are the best compounds for most 

 organisms. Asparagin is very commonly used if for some reason 

 peptones are to be omitted. 



SOURCES OF HYDROGEN AND OXYGEN. The sources of hydrogen are 

 hardly ever discussed with bacteria since hydrogen bears such a close 

 and peculiar relation in water and organic food supplies. The ulti- 

 mate association of hydrogen with oxygen in the molecule of water 

 (H 2 O) and with carbon in organic substances (CH 4 ) establishes its 

 importance in all life processes. There are many prototrophic bacteria, 

 using oxygen as such; others are able to reduce such compounds as 



