THE PHOTOTROPHIC ASSIMILATION OF CARBON 29 



photosynthesis than that absorbed by chlorophyll, can be 

 explained if it is assumed that chlorophyll a exists in two 

 states of combination, one active in photosynthesis, to 

 which energy transference from phycobilins is possible, and 

 another, inactive in photosynthesis, from which energy trans- 

 ference to the unidentified fluorescent pigment occurs.^^s 



After it was first postulated by Engelmann,^^' ^^ the 

 existence of chromatic adaptation among algae was for a 

 long time the subject of controversy (see 107, 247, 248 for 

 references). This problem, of w^hether or not the char- 

 acteristic pigmentation of particular forms is of biological 

 advantage to them, is now largely resolved by the findings 

 that have been outlined in the preceding paragraphs. 

 Chromatic adaptation is most clearly exemplified by the 

 algae of the littoral region of the sea, in which, as a general 

 rule, red algae occupy the lowest zone and green algae the 

 highest, whilst brown algae are to be found in the inter- 

 mediate position. Whereas yellow light predominates in the 

 sunlight reaching the earth's surface, blue-green light is 

 transmitted to the greatest extent by clear seawater. The 

 pigmentation of the littoral algae is thus complementary to 

 the quality of light characteristic of the positions which 

 they occupy and will consequently secure maximum light 

 absorption. Since it has been shown that the energy 

 absorbed by the principal accessory pigments is utilized in 

 photosynthesis it is clear that the pigmentation of the dif- 

 ferent algae is such as to enable each to make the maximum 

 use of the light energy available in its characteristic habitat. 

 Chromatic adaptation of this sort is genetically determined, 

 but there is evidence that, in some species at least, onto- 

 genetic adaptation can also occur.^^^' ^^^ However, the 

 zonation of algae to be expected from this simple view of 

 chromatic adaptation may be blurred by several circum- 

 stances. Thus changes in the proportions of individual 

 photosynthetic pigments may occur in response to variation 

 in intensity as well as in quality of light or more eflFective 

 absorption of particular wavelengths may be obtained 

 by increase in total concentration of pigments or by in- 

 crease in thallus thickness rather than by changes in the 



