1186 THE LIGHT FACTOR. III. COLOR CHAP. 30 



Levring (1947) determined "action spectra" of photosynthesis of a 

 number of marine algae in filtered sunlight. (Only qualitative results can 

 be expected from such measurements, because of the relatively high light 

 intensity and the relatively strong absorption of the thalli). He found 

 evidence of particularly strong photosynthetic efficiency (high ratio yield/ 

 absorption) in green light in ten species of red algae, and concluded that 

 the red phycobilin pigment is as active (if not more active) than the green 

 chlorophyll. Combining these results with those of his measurements of 

 spectral distribution of light in different depths, he concluded that because 

 of the presence of the red pigment, the Rhodophyceae utilize the blue-green 

 light deep under the sea better than the Chlorophyceae, as postulated by the 

 Engelmann-Gaidukov theory. He agreed, however, that adaptation to 

 low light intensity is an alternative way of adjustment to life in great 

 depths; an important element of it is low respiration. 



Thus, even more uniformly than in the case of brown algae, the crude 

 observations on the relative efiiciency of photosynthesis of red algae in light 

 of different color support the assumption that the accessory pigments of 

 these organisms are active sensitizers in photosynthesis, and that Engel- 

 mann's theory of chromatic adaptation was fundamentally correct. And 

 one may ask oneself, how could it have been otherwise? Would it not be 

 strange if the appearance of orange or red pigments in deep-water algae 

 would be only a coincidence, and these pigments were helpless in performing 

 the task so obviously set to the plants by the character of the "light field" 

 in which they live — to catch and utilize for their maintenance and propaga- 

 tion radiations in the middle of the visible spectnim, which are the only 

 ones to reach them in some intensity? 



It may be argued that not all deep-water algae are red, some green 

 algae being encountered in great depth. In other words, algae can sur- 

 vive without phycobilins in the greatest depths where life occurs. How- 

 ever, this in itself is not a convincing argument against Engelmann's theory. 

 Algae could adapt themselves to great depths in two ways: by reducing 

 respiration to a level permitting growth even in extremely Aveak, and weakly 

 absorbed light; and by adjusting their pigment systems to enhance light 

 absorption. The fact that the first adjustment has been sufficient for 

 some green species does not invalidate the hypothesis that red algae have 

 also used chromatic adaptation for the same purpose. 



Another objection to Engelmann's theory is that many red algae live 

 on or near the surface and that phycobilins are found in blue-green algae, 

 which are surface organisms. It is known, however, that red algae often 

 tend to lose their phycobilin and become green when exposed to sunlight 

 (c/. Vol. I, Chap. 15); and even if many of them (as well as the blue-green 

 algae) apparently find their phycobihn content useful, or at least not harm- 



