40 GERLOFF, FITZGERALD, AND SKOOG 



indicate that maximum algal growth could be produced with 

 a N/P ratio in the nutrient solution of at least 30/1. 



As with phosphate, the sulfate concentration could be low- 

 ered to one-fourth that in the basic medium solution without 

 decreasing growth. But the basic solution contained 3.3 ppm. 

 sulfur and only 1.8 ppm. phosphorus so that for maximum 

 growth this organism required almost twice as much sulfur as 

 phosphorus in the medium. 



Of the cations, potassium was required in the largest amounts, 

 for growth decreased when the potassium concentration was less 

 than 2.25 ppm. or one-half the amount in the basic solution. The 

 magnesium concentration could be lowered to 0.13 ppm., which 

 is one-twentieth the concentration in Treatment C, without sig- 

 nificantly decreasing the amount of growth. Inasmuch as there 

 is usually a high calcium concentration in the eutrophic lakes 

 where blue-green algae are most abundant, it seems surprising 

 that the growth of Coccochloris Peniocystis did not decrease when 

 calcium salts were omitted from the nutrient solution. This does 

 not mean that calcium was completely absent, for a small amount 

 was introduced with the one milliliter inoculum and was present 

 as impurities in other than calcium compounds added to the cul- 

 ture medium. 



The results in Table 5 also show that, with ferric citrate as 

 the iron source, the iron concentration could be much lower than 

 the 0.56 ppm. in the basic solution. In fact, growth did not de- 

 crease until the iron concentration was less than 0.03 ppm., or 

 one-twentieth the amount present in Treatment C. 



The above results give the minimum concentrations of indi- 

 vidual elements required to produce optimal yields when each is 

 varied singly. There is reason to believe, however, that a nutrient 

 solution containing only these minimum concentrations of the 

 nutrients might not produce maximum yields. For example, it 

 has been established that for higher plants there is an optimum 

 range of osmotic or total salt concentration. Furthermore, both 

 the extent of uptake and the actual requirement for each ion will 

 be determined in part by the concentrations of other ions in the 

 nutrient solution. From the data in Table 5, a solution contain- 



