476 LIGHT AND LIFE 



This requirement is met by red algae, which contain chlorophyll a, 

 but not chlorophyll /;. The main auxiliary pigments are the phyco- 

 cyanin, absorbing around 625 ni/x, and the phycoerythrin, absorbing 

 from 565 to 500 ni/i,. The red alga, Porphyridiuni cruentum, was 

 grown over fluorescent white light tubes in the medium described 

 by M. Brody and Emerson (1) . Gradual removal of light absorbed 

 by the phycobilins was obtained by filtering the incident white light 

 through cuvettes containing aqueous phycobilin extracts of increas- 

 ing concentration. As a light source, a 225-watt ribbon-filament, in- 

 candescent lamp was used. Care was taken to remove, as far as possi- 

 ble, the infrared radiation (not absorbed by the algae) , by using a 

 6-inch water layer and two American Optical IR-absorbing glass filters. 

 A correction was applied to account for the remainder of the radiation 

 beyond 720 m^^i. 



The phycobilin extracts were prepared by storing thalli of Porphyra 

 perforata in distilled water in the dark at room temperature (about 

 23 °C) for at least five days. In this time the cells died and the 

 phycobilins, but not the chlorophyll and the carotenoids, were re- 

 leased. The extract was partly purified by filtering through a Seitz 

 filter, centrifuging, and repeated precipitation with ammonium sul- 

 fate. The final preparation was highly concentrated, but clear, show- 

 ing only very little light-scattering. Our thanks are due to Dr. 

 L. R. Blinks for providing us with the algal material, and to Mr. 

 C. Cederstrand for measuring true (scattering- free) absorption of the 

 Porphyridiiim suspensions in an integrating sphere set-up. 



Photosynthetic oxygen production was measured manometrically, 

 while the incident energy was determined with a bolometer, as de- 

 scribed by Emerson and Chalmers (1) . 



Results and Disclssion 



The results of six consecutive experiments are shown in Fig. 1, in 

 which quantum yield in percentages of that in full light is plotted 

 against the fraction of total incident light absorbed by the phycobilin 

 filter. The curve consists of three sections. In the first section, the 

 quantum yield does not change as the absorption of the phycobilin 

 filter is raised from to 20%. Further increase of the filter density 

 up to 80% absorption leads to a linear decline of the quantum yield. 

 When the concentration is increased still further, the quantum yield 

 declines more steej^ly, until, at about 95% absorption, photosynthesis 

 ceases to be measurable (—0.24 ± 0.25 fxl Oo/hour) . However, the 

 Emerson effect can still be observed: if a background of orange light 



