308 RADIATION BIOLOGY 



sis and chlorophyll fluorescence show up when the process limiting the rate 

 of photosynthesis is closely connected with the energy transfer from the 

 chlorophyll. This obtains when the rate-limiting process influences the 

 available concentration of energy acceptor. The erroneous assumption 

 has often been made that a limitation of the rate of photosynthesis neces- 

 sarily inhibits the energy transfer. Limitation of photosynthesis has been 

 found, however, to be accompanied by either unchanged, decreased, or 

 even enhanced transfer of energy. In these cases unchanged, increased, 

 or decreased fluorescence yields, respectively, are observed. 



The earotenoid fucoxanthin in diatoms was the first pigment for which 

 energy transfer to chlorophyll a without loss was proved. It has been 

 found that the energy absorbed by fucoxanthin leads to fluorescence of 

 chlorophyll with the same yield as the energy absorbed directly by chloro- 

 phyll (Button and Manning, 1941 ; Button et al., 1943; Wassink and Ker- 

 sten, 1946-1948) . This energy is also equally effective in photosynthesis. 



Extensive work of Buysens (1951) indicates that a similar situation 

 exists in many other cases. By a quantitative study of fluorescence yield 

 in relation to the wave length of the incident light ("action spectra for 

 fluorescence ") and of fluorescence spectra, he showed that energy is trans- 

 ferred from chlorophyll b to chlorophyll a and from certain carotenoids 

 of purple bacteria to the bacteriochlorophyll-protein complexes. It is of 

 special interest that in Chromatium, strain B (the strain used generally 

 by the Utrecht Biophysical Group; cf. Wassink et al., 1942), energy trans- 

 fer could be demonstrated from the bacteriochlorophyll-protein complexes 

 with higher excitation levels to the complex with the lowest excitation 

 level (the longest wave-length absorption). According to Buysens, the 

 eflficiency of the carotenoids for exciting fluorescence of the bacteriochloro- 

 phyll-protein complex (with absorption at about 890 m^) is of the order 

 of 40 per cent. In Chlorella he found transfer from chlorophyll b to 

 chlorophyll a with an eflficiency of almost 100 per cent. 



A very peculiar situation has been recorded in red algae. Blinks et al. 

 (1949) and Haxo and Blinks (1950), in detailed and extensive studies, 

 showed that in these algae chlorophyll and the carotenoids were much 

 less efficient for oxygen evolution than the phycobihns. For the light 

 absorbed chiefly by the bilins, an efficiency of about yi-z (one molecule of 

 oxygen evolved for 12 quanta absorbed) was found as compared with an 

 efficiency of only Ho to V50 for the fight absorbed by chlorophyll or the 

 carotenoids. Most of the energy absorbed by the chlorophyll-carotenoid 

 complex in these algae is apparently wasted (Fig. 5-8). Buysens (1951) 

 also presents some very interesting observations on this energy transfer. 

 The fluorescence of chlorophyll a per (juantum absorbed by phycoerythrin 

 (in Porphyra) is stronger than its fluorescence per quantum absorbed by 

 itself ! The same was found in other species. An unknown pigment with 

 a fluorescence maximum at about 725 m/x, however, showed the reverse 



