OXYGEN REQUIREMENTS 69 



and proteins, but usually not those compounds which contain only 

 carbon and hydrogen, such as methane and its homologues. 



Sulphur. Sulphur is required by all bacteria possibly for the for- 

 mation of the proteinaceous material of their bodies. In addition 

 to this, some organisms use it as a source of energy. For instance, 

 the Beggiatoa sometimes use two to four times their own weight of 

 hydrogen sulphid in a day, under which conditions the sulphur grains 

 may be seen in the cell-protoplasm and may be looked upon as an 

 intermediate stage in the oxidation process, the reaction proceeding 

 as follows: 



2H 2 S + O 2 = 2H 2 O + 2S 

 2S + 3O 2 + 2H 2 O = 2 



Some bacteria may get their required sulphur from sulphates, sul- 

 phites or thiosulphates, but probably the great majority of them 

 obtain it from the proteins. 



Phosphorus. Phosphorus is used by bacteria in large quantities, 

 being essential for the building of the nucleoproteins and phospho- 

 proteins in which the unicellular organisms are especially rich. 

 The form and quantities required by the organisms vary greatly 

 with the species. The Azotobacter are able to utilize it from most 

 organic and inorganic sources, some, however, being much more 

 valuable than others. 



Potassium. Potassium is essential to the higher plants and cannot 

 be replaced entirely by related substances, yet Gerlach and Vogel 

 early reached the conclusion that potassium and magnesium are 

 not essential to Azotobacter. Their results, however, were con- 

 sidered for a long time to be erroneous. But if these elements are 

 essential to Azotobacter it must be in extremely small quantities. 

 Potassium does, however, favor their development, and is probably 

 valuable, if not essential, to all bacteria. Most inorganic potassium 

 compounds can be utilized. 



Other Inorganic Substances. The other inorganic constituents 

 are required by bacteria only in small quantities and are obtained 

 from either organic or inorganic compounds, depending upon the 

 specific organism. 



Oxygen Requirements. Bacteria, like all other plants and animals, 

 require oxygen in their life activity. The various classes of organ- 

 isms are not indifferent as to the form in which they obtain their 

 oxygen. One great class requires that their oxygen be furnished 

 free; to these is given the name "aerobic." Another requires their 

 oxygen in the combined form; they are called "anaerobic." Some 

 organisms grow best in the presence of free oxygen but may become 

 adapted to combined oxygen; these are known as "facultative 

 anaerobes." Others grow best in the absence of free oxygen but 

 may fceconie adapted to it; they are known as "facultative aerobes." 



