1178 THE LIGHT FACTOR. III. COLOR CHAP. 30 



ments, and 2.46 times larger referred to chlorophyll alone; while, according to the size 

 of the quanta, it should be smaller by a factor of 0.68. 



In the case of brown algae {Laminaria digitata) a similar calculation (based on Mont- 

 fort's own measurements) gave energy yield ratios (related to the combined absorption 

 by all pigments) of 0.86 and 1.18 (for two different combinations of filters), as compared 

 with the quantum size ratios of 0.64 and 0.77, respectively, again indicating a higher 

 quantum yield in the blue-violet than in the red. At their face value, Montfort's figures 

 appear to indicate that the carotenoids of the diatoms and brown algae are several times 

 more efficient as sensitizers than chlorophyll! 



Mothes, Baatz, and Sagromsky (1939), Baatz (1941) and Sagromsky (1943) have 

 described observations of the rate of photosynthesis in filtered red and blue light of 

 equal intensity (in energy units). They found for these two rates, a ratio of 1 : 1.2 in the 

 diatom Chaetoceras simplicia centrosperma, as against 1 : 0.7 in two unicellular green algae, 

 and attributed the relatively better utilization of blue light by the brown cells to the 

 presence of fucoxanthol. These experiments were more satisfactory than those of Mont- 

 font in that imicellular algae were used rather than thick thalli; but it was equally un- 

 satisfactory in the use of broad spectral regions, and even less satisfactory in the absence 

 of any absorption measurements, which would permit approximate allocation of ab- 

 sorption to the several pigments. Mothes and co-workers pointed out the difficulty 

 of the latter problem, caused by the difference of the carotenoid spectrum in vivo from 

 that in vitro. (This difference is clearly indicated by the change of color from brown to 

 green caused by placing brown algae in hot water — a treatment which, they assumed, 

 disrupts the molecular association of carotenoids with chlorophyll and proteins.) 



6. Quantum Yield and Action Spectrum of Red and Blue Algae. 



Role of Phycobilins 



The history of our knowledge of the role of phycobilins in the sensitiza- 

 tion of photosynthesis in red and blue algae is similar to that of the role of 

 carotenoids in brown algae. Here too we find the — we now know, correct — 

 guess by Engelmann, made as early as 1883, that the phycobilins are active 

 sensitizers of photosynthesis, as well as a series of vague and indecisive 

 observations and calculations of several authors, mostly tending to confirm 

 this guess, and finally, quantitative analyses of the quantum yield as a 

 function of wave length, carried out by Emerson and Lewis (1941, 1942), 

 and Haxo and Blinks (1950), which brought convincing confirmation of 

 Engelmann's concept. As in the previous section, we will discuss the 

 more recent and reliable experiments first. 



Table 29. Ill showed that Emerson and Lewis (1941), in comparing 

 the quantum yield of different plants in yellow sodium light, found no 

 difference between green plants and the blue-green algae Chroococcus, al- 

 though in the latter, far more than half the absorption in the yellow re- 

 gion was due to phycocyanine. This result of a preliminary observation 

 was in itself a more convincing proof of the activity of phycobilins in photo- 

 synthesis than could be derived from all the extensive earlier discussions 

 of this problem. It was confirmed and amplified by the same authors in 



