117G THE LIGHT FACTOR. III. COLOR CHAP. 30 



If fucoxanthol and other carotenoids are to a certain extent active as 

 sensitizers in green plants and brown algae, the mechanism of their par- 

 ticipation is likely to be based on a transfer of energy to chlorophyll, rather 

 than on a direct interaction with the oxidation-reduction system. This 

 hypothesis was suggested by Engelmann over fifty years ago; its first 

 direct confirmation came from the experiments of Button, Manning and 

 Duggar on the excitation of chlorophyll fluorescence by light absorbed by 

 the carotenoids, wliich were described in cluipter 24 (page 814). They 

 were carried out with the same organism {Nitzschia clodcrium) that was 

 also used for the measurement of the quantum yield of photosynthesis. 

 The quantitative results of this study also are in need of re-examination for 

 possible effects of chlorophyll c. 



We will now describe in brief the experiments in light of indefinite (and probably 

 partly .saturating) intensity, which can be aiiduced in support of the hypothesis that 

 the carotenoids of brown algae actively participate in photosynthesis. Conditions in 

 these organisms appeared somewhat more favorable for coriect guessing than in green 

 plants, because of the absence of chlorophyll b, and consequent enhanced importance 

 of the carotenoids for the absorption in the region between 450 and 500 m/u; liere again 

 the contribution of chlorophyll c requires consideration. 



As early as 1884, Engelmann found that brown algae (Melosira, Navicula, Pinnul- 

 aria) illuminated by sunlight or gas light, produced the largest amount of oxygen in the 

 green part of the spectrum, and concluded that the "orange pigment" of these algae must 

 participate in sensitization. 



Fifty years later, Montfort (1934) compared the rate of photosynthesis in white 

 light of a certain standard intensity with its rate in orange-red light. While greeji algae 

 {Ulva lactuca) were equally efficient with both kinds of illuminations, brown algae, 

 Diclyota dichotoma, Alaria and Desmarestia, produced in orange-red light only one half 

 the oxygen liberated in wliite light. Montfort interpreted this as an indication that 

 Phaeophyceae have a liigher relative efficiency in blue and violet light, and attributed 

 this to the presence of fucoxanthol. Later, IMontfort (1936) and his co-worker Schmidt 

 (1937) found that the removal of blue and violet rays from white light depressed the 

 rate of photosynthesis in brown algae {Diclyota and Laminaria) much more strongly 

 than in the green Ulva. They calculated the ratio P/A (photosynthesis per unit ab- 

 sorbed energy) for different colors, using incident light of equal intensity in all spectral 

 regions. 



An arbitrary selection from the confusing abundance of their material is shown in 

 Table 30. VI. Not all the figures in this table may be strictly comparable, but they 

 show the trend of the results. 



In green algae, the decrease in P/I from the red to the green and the renewed in- 

 crease in the blue reflected more or less clearly the changes in absorption and in the size 

 of quanta. (These two factors cooperate to make the yield in green light smaller than 

 in the red; but become antagonistic in the blue.) In brown algae — with the exception 

 of Fucus, which Montfort classified as a "xanthophyll alga" (whereas he designated 

 Laminaria and Diclyota as "fucoxanthol algae") — the yields were invariably 30 or 40% 

 higher in the blue than in the red, and would have become twice as high if related 

 to the absorption by chlorophyll alone. In the green, too, the P/I values were relatively 

 higher than they should be according to the absorption of chlorophyll and the size of the 



