368 RADIATION BIOLOGY 



Duysens ( 1951a, b, 1952) has made measurements of the energy trans- 

 fer from phycoerythrin to phycocyanin and to chlorophyll as well as from 

 phycocyanin to chlorophyll in red algae. The efficiency for the first 

 transfer appeared to be probably greater than 80 per cent; the efhcien- 

 eies for the transfer from the phycobilins to chlorophyll were found to 

 be equal. 



In the unicellular red alga Porphyridwm cnientum the fluorescence 

 spectra have been measured for various incident wave lengths. Figure 

 6-10 shows a family of fluorescence curves obtained by using ecjual quanta 

 of incident light of various wave lengths to excite fluorescence in a sus- 

 pension of Porphyridium cruentum Naeg. The sizes of the curves have 

 been adjusted so that the curves can be compared directly on the basis of 

 equal numbers of incident quanta of the different wave lengths. These 

 and other comparable curves were resolved into the fluorescence spectra 

 of the individual in vivo pigments, as illustrated in Fig. 6-lOrf. The 

 chlorophyll fluorescence excited by 546 m/i is 3.8 times as intense as that 

 excited by an equal number of quanta at 436 m/x. The efi"ectiveness 

 curves derived from the above data for the excitation of chlorophyll, 

 phycoerythrin, and phycocyanin fluorescence in the live cells are shown 

 in Fig. 6-lOa (French and Young, 1952). The action spectra show that 

 in algae illuminated with blue light there is some chlorophyll fluorescence 

 due to light absorbed directly by chlorophyll, as evidenced by the small 

 peak around 440 m/x in the chlorophyll excitation spectrum. We attrib- 

 ute the small size of this peak to internal filtering by carotenoid pig- 

 ments.^ The rise in these three curves from 450 to 500 m/x matches the 

 absorption band of phycoerythrin in these algae (Fig. 6-9c). This is 

 taken to mean that energy absorbed by phycoerythrin can be transferred 

 to chlorophyll and that phycocyanin may be an intermediate in this 

 transfer process. 



Energy transfer has also been demonstrated by Duysens (1951a, b, 

 1952) from carotenoids to bacteriochlorophyll in Chromatium. Figure 

 6-15c shows the action vspectrum for fluorescence and phototaxis in this 

 organism. 



In resolving the fluorescence excitation spectra into their individual 

 components, it is necessary to know whether the fluorescence yield of 

 each separate pigment is independent of the wave length of the incident 

 light. Organisms containing only one pigment would be valuable in this 

 type of analysis. Such organisms have not become available yet (the 

 one reported case being the Ws mutant of Zea mays L., which contains 

 other pigments than protochlorophyll in only very minute traces) . Inf or- 



' Duysens (1961b) finds, however, that, even after a correction for internal filtering 

 by carotenoids is applied, the fluorescence of chlorophyll excited by blue light directly 

 absorbed by chlorophyll is smaller than the fluorescence excited by transfer from the 

 phycobilin. 



