218 



Nitrogen Fixation— Symbiotic 



Physiology of Nodule Bacteria 



The various strains of Rhizohium are strictly aerobic. They are 

 unable to fix atmospheric nitrogen when grown in artificial media. 

 Different carbohydrates can be used as sources of energy, maltose, 

 sucrose, glucose, and mannitol being best; cellulose, pectin, or starch 

 cannot be utilized. Laurent found that Rhizobium can be cultivated 

 on nitrogen-free media containing 0.1 per cent KH2PO4, 0.01 per 

 cent MgS04, and 5-10 per cent of an available energy source. Beije- 

 rinck insisted, however, that a source of nitrogen is also required. 

 Some of these strains produce considerable acidity, whereas others 

 do not, the acid producers giving rise to peritrichous flagellation. 

 Some of the strains grow very fast, others very slowly, requiring 3-4 

 weeks. The slow growers produce gum (Table 49). 



Table 49. Rate of Fixation of Nitrogen by Various Leguminous Plants 

 Fixation of Atmospheric Nitrogen 



The optimum reaction for the growth of Rhizobium is /;H 5.5-7.0, 

 with limiting reactions of pll 3.2-5.0, on the acid side, and pH 9.0- 

 10.0, on the alkaline. Maze was the first to draw attention to the 

 fact that the nodule bacteria comprise both acid-resistant and acid- 



