SULPHUR AND NITROGEN BACTERIA 229 



oxygen termed in assimilation is quite insufficient to supply what is wanted 

 for nitrification, while in a green plant assimilation results in the formation of 

 far more oxygen than can be used in respiration. It is possible that a splitting 

 off of oxygen from carbon-dioxide and a formation of carbohydrate does 

 occur in the nitro-bacteria, but, according to Winogradsky, urea may also be 

 formed directly by the union of carbon-dioxide and ammonia, from which 

 might arise the other organic compounds found in nitro-bacteria. The be- 

 haviour of the nitro-bacteria in presence of urea, however, does not support 

 this hypothesis. 



Respiration forms another important problem which still requires 

 elucidation. Do the nitro-bacteria remain content with oxidizing ammonia 

 into nitrous acid, or do they use up organic material they themselves have 

 manufactured ? This cannot be easily answered, but it is of interest in relation 

 to our general summary of respiration. PflDger and Detmer's conception of 

 respiration as taking place in the protoplasm, that it is protoplasm that 

 undergoes respiration, and that the reserves are used to regenerate it has 

 already been discussed (p. 204). This regeneration can be effected by carbo- 

 hydrate, but not by ammonia. Were it possible to prove that the nitro-bacteria 

 respired no organic material, this hypothesis of Pfluger would be definitely 

 established, but it is scarcely possible to bring forward such evidence 

 although it may perhaps be possible to do so in the case of other organisms. 



Having now discussed the nitro-bacteria, the relations established for 

 the colourless sulphur-bacteria appear in altogether a new light. Not 

 only are we able to note a perfect analogy between the respiration 

 of ammonia on the one hand and sulphuretted hydrogen on the other, but 

 also the evil effects of supplying organic nutrients to Beggiatoa. It is in 

 the highest degree probable (Winogradsky, 1890, p. 275) that both sulphur- 

 and iron-bacteria are autotrophic, and that they grow better when organic 

 materials are completely excluded than when they are present. One 

 wonders why this experiment has not long since been carried out. Then the 

 energy which is obtained by the oxidation of inorganic substances in the case 

 of sulphiu- and iron-bacteria is perhaps used up only in the assimilation of 

 carbon, and just as in the case of the nitro-bacteria we have to inquire whether 

 or not a respiration of organic substance takes place here also. 



Recently Nathansohn (1902) has pubhshed some observations on a new 

 group of sulphur-bacteria. Details as to the occurrence of these forms are 

 as yet entirely wanting, and studies on their physiological behaviour require 

 confirmation also. In consequence of their great importance we may add here 

 a few words on the subject of these Bacteria, without waiting for a confirma- 

 tion of Nathansohn's work [Omelianski, 1904]. They do not oxidize 

 sulphuretted hydrogen, but thiosulphate, and form therefrom sulphuric acid 

 and tetrathionic acid. Further, they are not sensitive, like nitro-bacteria and 

 Beggiatoa, to the addition of organic material, and hence it is possible to prove 

 that they are entirely unable to oxidize organic substances, e. g. sugar ; they pro- 

 duce no carbon-dioxide, and have no normal respiration. On the contrary, 

 carbon-dioxide is an essential food-stuff, since they form organic substance 

 from it. If Nathansohn's experiments are correct, then these forms afford 

 proof of the fact that there are organisms which respire inorganic material only. 



Let us now return to the nitro-bacteria. The formation of organic material 

 out of carbon-dioxide is not the final point of interest in their physiology. 

 It is of importance also to note their behaviour to such organic compounds as 

 are presented to them from without. As we have already remarked, the 

 culture of these microbes was retarded in Winogradsky's experiments by the 

 addition of potassium tartrate and gelatine. A short time previously Wino- 

 gradsky, in conjunction with Omelianski (1899), investigated the effect of 



