68 P. M. COOK 



the yield for clear days was 0.351 grams per liter. The total cul- 

 ture intercepts two square feet of sunlight on a vertical plane and 

 89% of the incident sunlight is transmitted into the column. 

 Actual growth rates for this equipment are considerably higher 

 than die artificial light column due to the higher intensity of 

 sunlight during the day. 



Some data have also been obtained for different values of the 



following variables: 



Carbon dioxide - air mixtures for aeration; aeration rates and 

 turbulence; temperature; light; mineral nutrients and pH; water 

 and micro-nutrients; contaminants. 



The optimum nutrient and culture conditions for large-scale 

 growth as determined by us do not vary seriously from the pres- 

 ent common practices of Chlorella culture. Five per cent carbon 

 dioxide in air appears to be the best aeration mixture. There was 

 no detectable change in rate of growdi when the carbon dioxide 

 was varied ± 1% in our apparatus. An optimum temperature 

 of 25° C. confirms some results given in the literature. An aera- 

 tion rate of 3 cubic feet per hour or more in the 4-inch column 

 is optimum, but this, as will be discussed later, is apparently 

 dependent upon the turbulence necessary rather than the carbon 

 dioxide required. The mineral nutrients are optimum over a 

 wide range and agreement is found with the work of Spoehr 

 and Milner (1949) and many others. By using water from differ- 

 ent sources we obtained different rates of growth even after sup- 

 plementing with Arnon's A-4 and B-7 micro-nutrient solutions. 

 We have not yet been able to detect what other micro-nutrient, 

 or what quantities of it, influenced this variation in the growth 

 of Chlorella. In one case, it is quite definite that it was the pres- 

 ence of one micro-nutrient in too high a concentration, rather 

 than a lack of it, that had an adverse effect on production. The 

 water which permitted the most rapid growth was always used 

 for study of other factors. 



Our cultures have become contaminated on several occasions 

 and in some cases this may have caused lower growth rates than 

 normal. A continuous culture, however, has now been main- 

 tained for over two months without contamination. There is no 



