RESPIRATION AND FERMENTATION 257 



in nature was considered. The oxidation of hydrogen sulphide 

 proceeds according to the following equation: H 2 S + 2O2 = 

 H2SO4 + 115 cal. 



As in the case of nitrification, so here the heat energy given in 

 calories is utilized by the bacteria, not only to obtain the neces- 

 sary energy for their vital processes, but also for the decomposition 

 of carbon dioxide. Consequently, these bacteria do not need 

 organic substances. Contrary to the nitrification bacteria, the 

 sulphur organisms can grow without detriment in the presence of 

 considerable amounts of organic substances in the nutrient 

 medium. 



The sulphur bacteria represent a rather varied group of organ- 

 isms. They include single-celled as well as threadlike forms. 

 Some of them contain a bright reddish purple pigment, called 

 "bacteriopurpurin." Hence, they have received the name of 

 purple bacteria. Engelmann has advanced the opinion that this 

 pigment is a substitute for chlorophyll and is effective in the decom- 

 posing of carbon dioxide. In fact, light, which generally sup- 

 presses the development of bacteria, has a favorable influence upon 

 the purple bacteria. Possibly the oxygen eliminated in the decom- 

 position of CO2 facilitates the oxidation of hydrogen sulphide by 

 these organisms. This permits them to live in water which is 

 saturated with hydrogen sulphide and poor in oxygen, and to a 

 degree excludes the possibility of development of other microbes. 

 Besides the nitrifying and sulphur bacteria there exist large 

 numbers of microorganisms whose respiration proceeds at the ex- 

 pense of mineral substances. Of particular significance are the iron 

 bacteria, first described by Winogradsky, which convert ferrous 

 into ferric salts. The most important of these are the threadlike 

 Leptothrix, Crenothrix, and other related forms. They play an 

 important role in nature. The so-called bog ores, immense accu- 

 mulations of oxidized iron at the bottom of bogs and lakes, are the 

 results of their activity. Some of these bacteria are able also to 

 transform organic substances. They are not so absolutely auto- 

 thropic as the nitrifying organisms. Besides ferrous salts, many 

 of them also can oxidize manganese compounds. 



In nature, especially in bogs, there are widely distributed 

 bacteria which can obtain energy through the oxidation of marsh 

 gas (methane) or of hydrogen. The chemistry of this oxidation 

 and the metabolism of these organisms are at present very little 



