228 METABOLISM 



The amount of organic carbon introduced along with the nitro-bacteria 

 was immeasurably small, so that practically all that was obtained must have 

 been produced in the culture. 



This very small but absolutely certain increase of organic substance must 

 have taken place at the cost either of the carbon-dioxide of the air or of the 

 carbonate in the solution. As Godlewski showed (1895), this latter source is 

 insufficient, and nitrification does not go on when the carbon-dioxide of the 

 air is shut out. The carbonate further can only be conceived of as a source 

 of carbon-dioxide to the minV^-bacteria, since the nitrous acid which appears 

 must of course decompose the carbonate ; in the further oxidation of the 

 nitrite, however, no carbon-dioxide would come off free. 



Winogradsky's quantitative analyses have also proved what had already 

 been suspected by Heraeus (1886) and Huppe (1887), that the nitro-bacteria 

 are able to form organic materials out of carbon-dioxide ; in other words, they are 

 in this respect, like green plants, autotrophic. We have already seen, that the 

 formation of organic material from carbon-dioxide is essentially associated 

 with expenditure of energy, and that the sunlight provides this energy for 

 carbon assimilation in the case of the green plant. The case is different with 

 the nitro-bacteria, for they assimilate the carbon-dioxide in the dark, if only they 

 be provided with ammonia or nitrite, which they oxidize with the aid of oxygen. 

 It would appear that the energy obtained by the oxidation of ammonia takes 

 the place of the energy of sunlight in the case of the green plant, and hence it 

 becomes explicable why Winogradsky found a definite relation subsisting 

 between the amount of organic substances formed and the amount of ammonia 

 oxidized. On an average 35-4 mg. of nitrogen must be oxidized for every mg. 

 of organically combined carbon formed. The individual determinations vary 

 but little from this average : — 



These statements of Winogradsky date from the time when he was 

 unacquainted with the differences between the nitrite- and nitrate-bacteria, 

 and the results he obtained were put down to their collective activity. He 

 would probably have considerably modified his views had he experimented with 

 pure cultures of one or the other, for they exhibit great differences in their capaci- 

 ties for oxidizing nitrogen. In a culture of the nitrite-bacterium the daily amount 

 of nitrogen oxidized gradually rises from 3-0 mg. on the fifth day, to 20 mg. 

 four weeks later, while the most energetic nitrate organism is capable of oxidiz- 

 ing not more than 10 mg. of nitrogen per day. It is only natural that the 

 energy, not only of oxidation but also of assimilation, in the two forms should 

 be quite different. 



Further investigations are yet needed to elucidate fully the whole problem. 

 For example, we are as yet quite in the dark how carbon assimilation is effected, 

 and what is the first product of assimilation. It is by no means essential 

 that the process should be carried out in the same way as in the green plant, 

 viz. by the decomposition of carbon- dioxide and the evolution of oxygen. 

 Winogradsky, in fact, has advanced arguments against the possibility of 

 such a method. He observed that if oxygen is given off in this assimilation 

 process, it must be capable of maintaining nitrification just as well as respira- 

 tion in the green plant may be maintained by the oxygen released in carbon 

 assirnilation. Winogradsky, however, did not observe the quantitative 

 relation existing between the nitrogen oxidation and the carbon assimilation 

 in the nitro-bacteria, which differs entirely from that between respiration 

 and assimilation in the green plant ; in the case of the nitro-bacteria the 



