SPECTRA OF PHOTOSYNTHETIC PIGMENTS 301 



inactive absorption at the red peak, where the curves were matched in 

 height. 



The data of Fig. 6-76 of Emerson and Lewis for Chlorella photosynthesis 

 and absorption were measured in three separate experiments that have 

 been brought together here by factors to make complete curves. The 

 participation of chlorophyll b is indicated by the shape of the action 

 curve near 650 m/i, and that of some carotenoids by the shoulder at 

 480 m/x. Some inactive carotenoid absorption presumably causes the 

 separation of the two curves below 500 m^t. Both in this figure and in 

 that for spinach chloroplasts the action falls far below the absorption on 

 the long-wave-length side of the chlorophyll red band. This widespread 

 phenomenon, which has come to be known as the "Emerson effect," may 

 be due to the energy of a quantum dropping below a minimum necessary 

 value at about 690 m/x. 



It is easy to demonstrate the participation of pigments that absorb in 

 regions of the spectrum where others absorb very little or of pigments 

 that are relatively efficient, such as the phycobilins. It is difficult, on 

 the other hand, to detect the participation of pigments which are present 

 in smaller quantity (i.e., chlorophyll c), which absorb in parts of the spec- 

 trum where other components absorb a greater share of the light, or 

 which are relatively inefficient (e.g., chlorophyll a in Rhodophyceae). 

 If inactive pigments are present in an appreciable concentration, they can 

 distort the shape of the action curve, owing to their "internal-filtering" 

 action, so that it is hard to interpret the action spectrum in terms of the 

 active components. An example of this is seen in the action spectrum 

 for protochlorophyll transformation to chlorophyll (Koski et al., 1951), 

 where the action spectrum for an albino lacking in carotenoids is com- 

 pared with the action spectrum for normal seedlings. 



Adequate corrections for internal filtering would in general greatly 

 facilitate the interpretation of action spectra. Suitably corrected action 



1951.) (b) Chlorella photosynthesis and absorption. In this figure adjustments 

 were made at the break points of the original data to bring the several sets together to a 

 complete curve. {Data from Emerson and Lewis, I9i3.) (c) Ulva taeniata. Curve I, 

 thallus absorption converted to adjusted optical-density units. Curve II, optical 

 density of the active components calculated from Eq. (6-9). Curve III represents 

 an attempt to derive the absorption curve of the active carotenoids in Ulva. It was 

 obtained by subtracting from curve II the calculated absorption of the active pigments 

 of Chroococcus from Fig. 6-86, which appears to be due, in the blue region, nearly 

 exclusively to chlorophyll a. Curve IV is a similar approximation to the absorption 

 spectrum of chlorophyll b in the 570- to 650-mju region. It was obtained by sub- 

 tracting the absorption of the active pigments of Coilodesme, which lacks chlorophyll b, 

 from curve II. {d) Coilodesme, a brown alga. Curve I, thallus absorption converted 

 to adjusted optical-density units. Curve II, absorption curve for the active com- 

 ponents calculated from Eq. (6-9). Curve III, absorption curve of active carotenoids, 

 which was obtained by subtracting the curve of Chroococcus from the blue end of the 

 Coilodesme absorption spectrum. {Haxo and Blinks, 1950.) 



