172 TRANSFORMATION OF MINERAL SUBSTANCES 



In these reductions the sulfur and sulfate act as oxidizing 

 substances, and serve, under these anaerobic conditions, in 

 much the same capacity as oxygen itself does in an aerobic 

 environment. 



In case conditions are changed, after the formation of sulfide, 

 so as to permit the entrance of free oxygen, thus creating aerobic 

 conditions, the sulfide does not persist but is oxidized again to 

 sulfate. In certain lakes and seas, as well as in a number of cura- 

 tive muds, the two processes, namely, the oxidation of hydrogen 

 sufide to sulfate and the reduction of sulfate to hydrogen sulfide, 

 may go on side by side. In the lower layers of the lake or sea 

 where free oxygen is scarce, the reduction process predominates. 

 The hydrogen sulfide once formed moves upward in the convection 

 currents or as bubbles of gas, and at or near the surface of the 

 lake, on coming in contact with the oxygen of the air, it is oxidized 

 to sulfate by specific oxidizing bacteria. The sulfate may diffuse 

 downward again, where it is again reduced to sulfide under the 

 anaerobic conditions. 



The reduction of sulfur to sulfide may be brought about by a 

 great number of different bacteria, but the reduction of sulfate is 

 limited to very few organisms, although these are widely dis- 

 tributed. Spirillum desulfuricans is the name generally applied 

 to the reducing form which is found in fresh water and soils 

 (see Fig. 16). 



As this short discussion indicates, there are striking similarities 

 between the transformations of various compounds of sulfur and 

 nitrogen through the agency of microorganisms. Organic com- 

 pounds of nitrogen decompose to form ammonia, while the sulfur 

 compounds lead to hydrogen sulfide. Ammonia is changed to 

 nitrite and nitrate, while sulfide is oxidized to sulfur, sulfate, and 

 numerous incompletely oxidized inorganic substances. These 

 oxidations are all the result of the action of specific autotrophic 

 bacteria. Both nitrates and sulfates may be reduced by anaerobic 

 bacteria, leading to a variety of products including ammonia 

 and hydrogen sulfide. Although a process like nitrogen fixation 

 is not found in sulfur transformations, the reactions associated 

 with oxidation and reduction of elementary sulfur are similar in 

 certain respects. Inorganic and organic nitrogen compounds 

 furnish different microorganisms with their requirements for this 

 element, and similar sulfur compounds serve a like purpose in 



