CH. Xl] BACTERIA IX THE SEA 265 



from waste, and the minimal quantity of nitrogenous food-stuff 

 suffices. 



This process is exactly reversed by the desulphurising bacteria, 

 of which one species — Spirillum desulphuricans — is known. This 

 organism is able to break down sulphates in its cells, liberating the 

 sulphur in the form of sulphuretted hydrogen. 



The nitrifying bacteria. Thus the dead matter of organisms 

 is broken down by the vital activity of the bacteria. In the case 

 of putrefactive decomposition the process is carried out in a number 

 of stages, but the final result is the conversion of the complex pro- 

 teid substance into ammonia. In the case of the fermentation 

 process the fats and carbohydrates are so broken down that the 

 elements of these substances finally appear as carbon dioxide and 

 water. But this is not all. The ammonia of the putrefactive pro- 

 cess is further transformed and is oxidised by the vital activity of 

 the nitrifying bacteria. 



We know most of the details of this process now but it is only 

 as the result of one of the very finest pieces of biological work 

 that have ever been carried out. Winogradsky's work has now 

 become classical and perhaps some day his investigations on nitri- 

 fication will be translated into English and presented to the 

 student of biology as a model of an investigation and an ideals 

 Previously it had been assumed that the accumulations of nitrogen 

 salts — the nitre beds of Chili, for instance — had been formed by 

 the oxidation of dead organic matter, by the oxygen of the atmo- 

 sphere. Winogradsky, after a long series of investigations, in the 

 course of which fundamental methods of investigation had to be 

 evolved, shewed that the transformation of the organic matter was 

 the result of bacterial activity. 



Only three genera of nitrifying bacteria are known but they 

 are of world-wide distribution. They are Niti^osomonas, Niti^o- 

 cocciis, and Nitrohacter. Nitrosojuonas, which (see Fig. 31) is a 

 small oval bacillus furnished with a long cilium, oxidises ammonia 

 to nitrous acid. Nitrohacter is a minute, non-motile bacillus and 

 its function is to oxidise nitrous to nitric acid. These two 



1 There is a good summary of Winogradsky's investigations on nitrifying 

 bacteria in Jost's Plant Physiology. 



