54 BIOLOGY AND TECHNIQUE 



ANABOLIC OR SYNTHETIC ACTIVITIES OF BACTERIA 



Nitrogen Fixation by Bacteria. — ^The constant withdrawal of nitroge- 

 nous substances from the soil by innumerable plants would soon lead to 

 total depletion were it not for certain forces continually at work re- 

 plenishing the supply out of the large store of free nitrogen in the atmos- 

 phere. This important function of returning nitrogen to the soil in 

 suitable form for consumption by the plants is performed largely by 

 bacteria. 



It is well known that specimens of agricultural soil when allowed to 

 stand for any length of time without further interference wUl increase 

 in nitrogenous content, but that similar specimens, if sterilized, will 

 show no such increase.^ The obvious conclusion to be drawn from this 

 phenomenon is that some living factor in the unsterilized soil has aided 

 in increasing the nitrogen supply. Light was thrown upon this problem 

 when Winogradsky,^ in 1893, discovered a microorganism in soil which 

 possessed the power of assimilating large quantities of nitrogen from 

 the air. This bacterium, which he named " Clostridium Pasteurianum," 

 is an obligatory anaerobe which in nature always occurs in symbiosis 

 with two other facultatively anaerobic microorganisms. In sym- 

 biosis with these, it can be cultivated under aerobic conditions and thus 

 grows readily in the upper well-aerated layers of the soil. 



Although, until now, no other bacteria with equally well-developed 

 nitrogen-fixing powers have been discovered, yet it is more than likely 

 that Clostridium Pasteurianum is not the only microorganism endowed 

 with this function. In fact, Penicillium glaucum and Aspergillus niger, 

 two molds, and two other bacteria described by Winogradsky, have been 

 shown to possess this power slightly, but in an incomparably less marked 

 degree than Clostridium Pasteurianum.^ According to the calculations 

 of Sachse,* unsterilized soil may, under experimental conditions, gain 

 as much as 25 milligrams of nitrogen in a season, a statement which 

 permits the calculation of a gain of twelve kilograms of nitrogen per 

 acre annually.' It is very unlikely, however, that such gains actually 

 occur in nature, where nitrogen-fixation and nitrogen-loss usually 

 occur side by side. 



' Berthelot, Compt. rend, de la soc. de biol., cxvi, 1893. 



2 Winogradsky, Compt. rend, de la soc. de bid., cxvi, 1893, ibid., t. cxviii, 1894. 



3 Tacke, Landwirtsch. Jahresber., xviii, 1889. 

 'Socfeie, "Agr. Chem.," 1883. 



s Pfeffer, Pflugers Physiologic, p. 395. 



