28 THE METABOLISM OF ALGAE 



is seen to be much less than that at wavelengths where the 

 absorption is almost entirely due to phycobilins. 



Light absorbed by chlorophyll b in ChlorellaP^ and by 

 chlorophyll c in Navicula^^^ appears to be equally available 

 for photosynthesis with that absorbed by chlorophyll a. 



Further information about the part played by accessory 

 pigments in photosynthesis comes from studies of fluores- 

 cence. Fluorescence occurs when radiant energy absorbed 

 by a substance is re-emitted almost instantaneously as 

 light. This light is necessarily of a longer wavelength than 

 that which excites the fluorescence and is of a characteristic 

 wavelength for a given pigment. The yield of fluorescence, 

 i.e. the ratio of the energy emitted to that absorbed, gives 

 information regarding the fate of the excitation energy. In 

 living Nitzschia closterium the yield of chlorophyll fluores- 

 cence is the same whether it is excited by red light (600 m//.), 

 absorbed exclusively by chlorophyll, or by blue-green light 

 (470 m/i), three-quarters of which is probably absorbed by 

 carotenoids."^ Similar studies with Gigartina, Iridaea,^^^ 

 Porphyrtdium,''^' ^^^ Porphyra and Oscillatoria^^ have estab- 

 lished that light absorbed by phycobilins can also excite 

 fluorescence of chlorophyll a. This evidence of a transfer- 

 ence of energy from accessory pigments to chlorophyll 

 suggests that light absorbed by the former pigments is not 

 utilized directly in the photochemical reaction. It appears, 

 in fact, that all light energy used in photosynthesis must 

 pass through chlorophyll «.''*' ^^^ Thus in Chlorella only 

 chlorophyll a fluoresces and its fluorescence is excited by 

 light absorbed by chlorophyll h."^^ Since chlorophyll a is 

 the principal photosynthetic pigment in all the algal classes 

 this provides evidence that the photochemical reaction is 

 of the same nature in all algae. However, it has been found 

 that light absorbed by phycobilins is sometimes more 

 effective in exciting the fluorescence of chlorophyll a than 

 is light absorbed by this chlorophyll itself and that light 

 absorbed by chlorophyll a excites the fluorescence of 

 another pigment which is perhaps chlorophyll dJ^ These 

 findings, which fit in with the observation mentioned above 

 that light absorbed by phycobilins is more efl"ective in 



