Some Relationships of Phy to plankton to Environment 



283 



controls were set aside and the required nutrients added to the 

 test flasks to bring the final dilutions of the test substances to 

 1 X 10-4, I X 10-5 and 1 X 10-^ g/liter. The cultures were in- 

 cubated in a water bath at 20° C, and illuminated by fluorescent 

 lamps giving 300 lux. Counts were made at intervals, and the 

 oxygen given off by photosynthesis was estimated by polarog- 

 raphy under nitrogen, using a rotating platinum electrode. Re- 

 sults:— Figures 4 and 5 are graplis of tjqpical results. In these 

 cases asparagin caused an increase in the rate of reproduction of 

 both Dunaliella and Skeletonema, but in the latter, the death 

 rate was later accelerated. This may have been due to exhaustion 

 of nutrients. With Diinaliella, asparagin increased the total oxygen 

 production, but with Skeletonema, any increase is probably not 

 significant. Of the substances tested, urea did not have any ap- 

 preciable effect on any of the test organisms, cysteine caused 

 increases in growth rate and oxygen production in Dunaliella and 

 Skeletonema, but decreases in Thalassiosira. Asparagin, inulin 

 and galactose gave increases in all cultures; arginine was bene- 



DUNALIELLA MARINA 



X ASPARAGIN POPULATION 



O ASPARAGIN OXYGEN 



O CONTROL POPULATION 



A CONTROL OXYGEN 



DARK CONTROL POPULATION 



Fig. 4. Effect of asparagin on oxygen production and growth of Dunaliella 



manna. 



