THE PHOTOTROPHIC ASSIMILATION OF CARBON 27 



particularly clear examples of accessory pigments partici- 

 pating in photosynthesis. In Chroococcus the quantum yield 

 at 600 m/t is about the same as at 660 to 680 m^<, where 

 nearly all the light absorption is due to chlorophyll a, in 

 spite of the absorption by phycocyanin at this wavelength 

 being about six times that of chlorophyll aJ'^ Thus the 



/oo 



THALLUS ABSORPTION 

 ACTION SPECTRUM 

 EXTRACTED PHYCOERYTHRIN 



400 440 480 620 560 600 640 680 720 760 

 WAVE LENGTH IN MjU 



FIG. 7. Absorption and action spectra of Porphyra nereocystis. The 

 action spectrum for photosynthesis corresponds more closely 

 to the absorption spectrum of the water extract of the alga, 

 which contains phycoerythrin as the principal phycobilin 

 pigment, than to the absorption curves of chlorophylls and 

 carotenoids (after ref. 139). 



photosynthetic efficiency of phycocyanin- in Chroococcus 

 must equal that of chlorophyll. The phycobilins in Rhodo- 

 phyceae are effective in the same way and in certain species, 

 e.g. Porphyra nereocystis, there is the curious situation that 

 light absorbed by these pigments is utilized more efficiently 

 than that absorbed by the chlorophylls themselves. ^^^ This 

 is shown in Fig. 7, in which the photosynthetic efficiency 

 at wavelengths at which all the absorption is by chlorophylls 



