106 BACTERIA AND THE NITROGEN CYCLE 



The nitrate bacteria (Nitrobacter, Fig. 23, c] are minute non-motile 

 rods (0-5 /u. by 0-25 /u) that grow without causing turbidity in the medium. 

 They form delicate pellicles on the floor and sides of the culture glass. 

 Spores are unknown. 



As might be expected in the case of organisms with oxidizing functions, 

 all the nitrifying bacteria are aerobic. They require no light, and yet 

 in spite of this are able to assimilate the CO 2 of the atmosphere. In 

 three experiments, where the nutritive solution contained originally 6 mgr. 

 CO 2 in the form of MgCO 3 , the cultures after seven weeks' growth showed 

 37-6 mgr., 26 mgr., and 17-5 mgr. CO 2 respectively. This was subsequently 

 proved by Godlewski (73) to have been absorbed entirely from the air 

 without light and without chlorophyll. As already mentioned, the nitrogen 

 was derived from the ammonia, or from the nitrous acid. A little nitrogen 

 is even set free. 



The energy necessary for the processes of life is gained by the nitrifying 

 bacteria from the oxidation of the ammonia, or the nitrous acid. The 

 materials from which they build up their cells are therefore inorganic 

 compounds of the very simplest character, carbon dioxide and ammonia or 

 nitrous acid, with a few mineral salts. They are thus prototrophic in the 

 strictest sense of the word, for a simpler synthesis of proteids than theirs 

 is scarcely conceivable. 



Facts have recently come to light which show that green plants do 

 not get the full benefit of all the nitrates formed in the soil by the nitrify- 

 ing bacteria (74). Nitrate-reducing bacteria have been discovered which 

 destroy that which the nitrifiers have built up. Several species of these 

 denitrifying organisms have been isolated from manure. They grow on 

 a medium containing 0-3 per cent. NaNO 3 , sugar, and the necessary salts. 

 Of the nitrogen offered to them, 82-7 per cent, is set free (in some species 

 as much as 99 per cent.), the rest being used up in their cell-substance. 



Although it is probable that the importance of these denitrifying 

 organisms in agriculture is very small, it cannot be denied that they occur, 

 and they certainly mar to some extent the clearness of our conception of 

 the processes by which nitrogen circulates through organic creation. 



Just as the nitrifying organisms have their converse in the denitrifying, 

 so have the sulphur bacteria theirs in the desulphurizing species. A spiril- 

 lum (S. desidfuricans) has been isolated from ditches and sewers, which sets 

 free the SH 2 from sulphates. The complete life-history of this organism is 

 not known (75). 



There can be little doubt that many other biochemical processes are 

 going on in nature besides those we have been considering, and mineralogical 

 chemistry will have to turn its attention to bacteria. Possibly we shall 

 even find prototrophic species which attack silicates. The reader may be 

 left to ponder further on the subject. 



