SECTION V 



RELATION OF MICROORGANISMS TO THE IRON CYCLE 



Exercise i 



A Method for Growing Crenothrix and Spirophyllum 



1. Clean and sterilize a Berkefeld filter. 



2. Connect the filter to the city water-supply and allow 

 the water to run slowly for twenty-four hours. 



3. Remove the metal cap from the filter and place in a 

 large beaker of iron solution (m. 61). 



4. Incubate in the ice-box or at 15° to 20° C. 



5. At regular two-day intervals examine the deposit 

 on the sides of the filter. 



6 If bacteria are found, test for iron. Add a few drops 

 of a 5 per cent, hydrochloric acid solution and a 4 per cent, 

 potassium ferrocyanid solution. In the presence of ferric 

 salts an intense blue color is formed, 



7. In order to stain the higher forms of iron bacteria 

 it is well to remove the deposit of iron by treating with a 

 S per cent, hydrochloric acid solution. 



Exercise 2 

 Iron Precipitating Bacteria 



1. Shake 20 grams of field soil with 200 c.c. of water. 

 Dilute until i c.c. equals i : 100,000. 



2. Pour plates with medium 64. 



3. Incubate the plates for several weeks at 28^ C. 



4. Note the precipitation of iron compounds around 

 certain colonies. 



84 



