914 ADVENTURES IN RADIOISOTOPE RESEARCH 



MuLLiNS and Brooks (1939) placed cells of Nitella coronata first in 

 a solution containing radioactive potassium (^-Iv) and later in solutions 

 of different chlorides. Sodium and lithium were found to be much less 

 effective in removing labelled potassium than potassium itself. We 

 investigated the exodus of labelled zinc into the culture solution both 

 in the absence and presence of zinc in the solution in which the mould 

 containing labelled zinc was placed. 



In these experiments Neurospora grown for 5 days at 28°, after repeated 

 washing with distilled water, was divided into five approximately equal 

 parts. One part was kept as a control, two parts immersed separately 

 in 150 ml. culture solution containing no zinc for 24 hr at 12°, while 

 two parts were kept under similar conditions in a culture solution con- 

 taining 30 pgm. zinc. 



Table 2. — Migration of Labelled Zinc from Neurospora into the Culture 



Solution 



Sample I Activity 



Control 100 



Neurospora after being kept in zinc-free solution for 24 hr at 12° 88, 84 



Neurospora after being kept in zinc-containing solution for 24 hr at 12° ... 74, 75 



Zinc-free solution after 24 hr 9, 10 



Zinc-containing solution after 24 hr 14, 19 



Penultimate wash water i 1 



In all our experiments more labelled zinc left the mould when placed 

 in a solution containing zinc. Table 2 shows the results of a typical 

 experiment. 



Uptake of cobalt by Neurospora 



We compared the uptake of Co++ (using ^^Co as an indicator) with 

 that of zinc by Neurospora. Four moulds were grown for 5 days at 28° 

 in the presence of 25 jugm. of cobalt in the culture solution. They were 

 found to take up 0.013, 0.011, 0.011 and 0.011% of the cobalt content 

 of the solution per mgm. of their dry weight (i.e. 0.0033, 0.0028, 0.0028 

 and 0.0028 f^gm.). The cobalt uptake corresponds to only about one- 

 twentieth of the uptake of zinc under similar conditions, showing that 

 the moulds do not possess such an effective mechanism for fixing cobalt 

 as they do for zinc. 



Summary 



1. By making use of labelled zinc (^^Zn) the amount of zinc necessary to 

 obtain maximal growth, of Neurospora crassa was studied. 



