264 BACTERIA IX THE SEA [PART III 



substance is thus obtained by the organism. But in the case of 

 the fermentation of sugar by the wine yeast this complete utilisa- 

 tion of the sugar by the organism does not take place, for a certain 

 portion of the former (say one half) is oxidised as far as alcohol, and 

 about one half suffers complete oxidation to carbon dioxide. Many 

 minor products are of course obtained in small quantities. So also 

 with putrefaction. A great number of molecules are oxidised, or 

 otherwise broken down, and in these incomplete decompositions the 

 micro-organism obtains its energy by, in one sense, a wasteful 

 reaction. Many different bacteria and other organisms are con- 

 cerned in putrefaction and fermentation, and the one may begin 

 where the other leaves off. So in the end the products are the 

 completely resolved ones, ammonia, carbon dioxide, water, &c. 



The sulphur bacteria. In some parts of the sea, as for 

 instance in the " dead grounds " of the Bay of Kiel, in some parts 

 of the Black Sea, and perhaps in parts of some of the Norwegian 

 fjords, where the water circulation is defective, and where there 

 may be a deficiency of oxygen, very remarkable bacteria are to be 

 found. These are the sulphur bacteria, the occurrence of which is 

 not, however, confined to these habitats. In the places I have 

 mentioned sulphuretted hydrogen is evolved fi'om the decomposition 

 of dead organic matter, and this sulphuretted hydrogen, to us a 

 vilely smelling and poisonous gas, is utilised as food substance by 

 the bacteria. Such a microbe as Beggiatoa takes in the SH2 and 

 oxidises it so that the sulphur is deposited in the cells of the 

 bacterial colony, and the hydrogen appears as water. This is the 

 form of assimilation of the organisms. Then some of the sulphur 

 thus resulting from the decomposition of the SHg is oxidised to 

 sulphuric acid. This is the form of respiration of the organism. 

 It requires some source of nitrogen for the formation of its living 

 proteid and this it obtains ft-om the minute quantities of nitrates 

 and nitrites which exist in solution in the water in which it lives. 

 But it requires very little nitrogen compound, for whereas a higher 

 animal may require to oxidise some of the living nitrogenous tissue 

 of its own body in order to obtain its energy, the sulphur bacterium 

 oxidises the sulphur stored in its cells as the result of the assimila- 

 tion of the SHo. Thus the proteid part of the cell is protected 



