CULTURE FOR PHYSIOLOGICAL RESEARCH 49 



50 X 10^ cells per ml. It is likely that for cells grown under dif- 

 ierent conditions the optical density is not always the same func- 

 tion of the population. In any event the method provides a rapid 

 means of following growth with a precision adequate for many 

 problems. We are now using this technique as a screening meth- 

 od to find out some of the more obvious nutritional character- 

 istics for a number of different algal species. Techniques similar 

 in principle, but differing in detail, have been reported by Rodhe 

 (1948) and Osterlind (1949). 



A second special design (Myers and Johnston, 1949) is a 

 modification of the so-called roll culture often used in microbio- 

 logical work. A ten-liter serum botde containing 1000 ml. of 

 algal suspension was used as the culture vessel. Here the objective 

 was to obtain extensive data on one culture, in particular to study 

 metabolism during growth. It was possible to study the gas ex- 

 change by gas analysis and to make chemical analyses on the 

 culture medium and on the cells produced. By such study it was 

 possible to show that Chlorella pyrenoidosa Link excretes only 

 a very small amount of organic matter. About 95% of the carbon 

 and nitrogen used up during photosynthetic growth could be 

 recovered in the cells produced. 



A third design (Myers and Clark, 1944) we have called a 

 continuous culture apparatus. The objective here can best be 

 explained with reference to the characteristic growth curve of a 

 microorganism. This is commonly interpreted as describing 

 growth under some standard or specified cconditions. Actually, 

 however, conditions in the cellular environment do not remain 

 constant throughout the course of the growth curve. In an algal 

 culture increased numbers of cells cause a mutual shading with 

 a decrease in the effective light intensity per cell. Nitrate is rapid- 

 ly taken up by the cells with attendant increase in pH. Fortunately 

 all such variables are functions of the population so that if we can 

 hold the population constant by diluting the culture as fast as it 

 grows, then we can also hold constant all the variables which nor- 

 mally accompany the growth curve. 



The central glass chamber is essentially a vertical doubly- 

 jacketed condenser built of three concentric glass tubes. Thermo- 



