48 PHYSIOLOGY OF NUTRITION 



increases. One hundred liters of water, collected at the depths given, contained 

 the following amounts of hydrogen sulphide. 



Depth in the Black Sea, H2S content per 100 l. 



meters '-'•• 



215 33 



432 222 



2040 555 



2525 6ss 



In the mud of the sea-bottom are therefore going on various kinds of fermenta- 

 tion, which are accompanied by the ehmination of hydrogen sulphide." Only 

 because of the presence of sulphur bacteria is the hydrogen sulphide prevented 

 from reaching the upper layers of water. 



Nitrifying and sulphur bacteria use ammonia and hydrogen sulphide, which 

 are injurious to other organisms, and aid in preventing the accumulation of these 

 substances upon the surface of the earth; oxidizing them to nitric and sulphuric 



FtG. 30. — Part of a membrane of sulphur bacteria, magnified 11 times. (After Yegunov.) 



acids, they bring these substances again into the general circulation of materials 

 in nature. 



Besides ammonia and hydrogen sulphide, hydrogen is also produced in 

 large amounts by the decomposition of complex organic compounds, and yet 

 it is present only in minimal quantities in the atmosphere. According to various 

 determinations, the amount of hydrogen in the air varies between 0.0003 ^^^ 

 o.oi per cent. It therefore appears that processes must occur on the earth, 

 by which hydrogen is combined and so started anew in the general circulation 

 of materials. 



The researches of Kaserer' have shown that there are special bacteria that 

 utilize hydrogen. Viewed from the standpoint of thermo-chemistry, hydrogen 

 represents the best nutrient substance. Its heat of combustion is eight times 

 that of starch; a gram of starch gives out during combustion but 4.0 kg.-cal., of 

 heat, while a gram of hydrogen gives out 34.6 kg.-cal. (see page xxii). Certain 



' Kaserer, Hermann, Die Oxydation des Wasserstoffes durch Mikroorganismen. Centralbl. Bakt. II 

 16: 681-696, 769-77S. 1906. Lebedeff, A. F., Ueber die Assimilation des Kohlenstoffes bei wasserstoff. 

 oxydierenden Bakterien. Ber. Deutsch. Bot. Ges. 27: 598-602. 1909. Nabokich, A. J., andLebede£f, 

 A. F., Ueber die Oxydation des WasserstofEes durch Bakterien. Centralbl. Bakt. //, 17: 350-355. 1907. 



» This deduction is of course not strictly accurate; although perhaps most of the hydrogen 

 sulphide, ammonia and hydrogen in nature is of organic origin, these substances are also pro- 

 duced, to some extent at least, quite independently of organisms. — Ed. 



