45 



E. G. Mulder: Copper and Molybdenum 



Molybdenum:— In the experiments on copper with Asfergillus 

 niger the nutrient solution was purified by shaking with charcoal. 

 In a second series of experiments the copper impurity of the nutri- 

 ent salt was removed by recrystallizing the salts. In a medium 

 prepared with these salts and provided with copper the growth of 

 Aspergillus was very poor. Obviously the medium purified with 

 charcoal, though free from copper, contained some substance 

 which was removed by recrystallization of the nutrient salts and 

 the sugar. As Steinberg (7) in 1936 had shown that Aspergillus 



O36 



g32 

 630 



KJ-2£> 

 24 

 Z2 

 30 

 18 

 16 

 14 

 12 

 10 

 d 

 6- 



4 • 

 2 



0.5/tg of Cu. 



pcrjocc of /^nutrient 



' ^ solution 



^'•noCu 

 / added 



o—j 



2 4 e S 10 12 14 



dovjS of incubation 



Textfigure 5.— EfiFect of copper on the production of 

 acetic acid by Acetohacter aceti. 



niger requires small amounts of molybdenum when cultivated in 

 a medium with nitrate as the sole source of nitrogen, it was sug- 

 gested that molybdenum deficiency was the cause of the poor 

 growth of Aspergillus. A third series of experiments was started 

 with recrystallized salts, this time with a trace of molybdenum 

 added to some of the cultures. In this medium a normal develop- 



