LARGE-SCALE CHLORELLA CULTURE 71 



appreciable reduction in yield is incurred. If conditions can be 

 found that will permit the same production at high densities, 

 certain obvious savings in equipment and processing will result. 

 It has been one of our objectives to determine conditions that will 

 permit the same production under more dense culture conditions, 

 but to date, we have not been successful. 



Turbulence 



A necessary condition for optimum growth is turbulence or 

 agitation. This serves three primary purposes: i) to keep cells in 

 suspension and, therefore, at uniform density; 2) to maintain 

 equilibrium conditions between the necessary nutrients and each 

 cell; and 3) to remove cells under high light intensity conditions 

 to lower light intensities to prevent their injury and thus main- 

 tain them at maximum growth rates. This can be obtained by 

 mechanical or gaseous agitation or by turbulent flow conditions 

 of the culture medium. 



Other Conditions 



Temperature must be maintained at 20° to 25° C. plus or 

 minus a few degrees for maximum production. During periods 

 at night with no illumination we have not observed adverse 

 effects after allowing the temperature to fall as low as 7° C. 



A closed and sterile system is necessary to maintain an un- 

 contaminated culture. Because there are indications that certain 

 molds and bacteria will cause a considerable decrease in growth 

 rates, such a sterile system is certainly necessary in a large-scale 

 process at present. Considerable work and years of experimenta- 

 tion might provide a system that would operate without aseptic 

 techniques. Even when such a system is devised, however, con- 

 taminants not previously encountered might destroy a culture 

 and bring about costly expenditures. Therefore, not only must 

 a closed sterile system be used, but also one in which contamina- 

 tion can be localized and the system easily cleaned and resteril- 

 ized without sacrificing a large amount of the culture or incur- 

 ring high labor costs. 



