384 RADIATION BIOLOGY 



Fig. 6-146. The color change of the hve bacteria was found by Van Niel 

 (1947) to be due to the conversion of the yellow pigment, which gives the 

 bacteria a brownish appearance, to a red pigment upon exposure to 

 oxygen. The total carotenoid content remained constant during this 

 conversion. 



The purple bacteria are unusually valuable organisms for studies of the 

 absorption spectra of pigments in the natural state because of the wide 

 separation of the bands of the two different classes of pigments. Such 

 measurements could well be used in the interpretation of the state of 

 photosynthetic pigments in their functional environment. 



ACTION SPECTRA OF PHOTOSYNTHESIS, PHOTOTAXIS, 

 AND FLUORESCENCE EXCITATION IN PURPLE BACTERIA 



The photosynthesis and growth of purple bacteria in projected spectra 

 was found by Buder (1919) to take place at the absorption bands of 

 bacteriochlorophyll and also at the bands due to carotenoid pigments. 

 French's measurements (1937b) of photosynthesis of Rhodo spirillum 

 ruhrum, however, showed that light absorbed by the bacteriochlorophyll 

 was more effectively used than those wave lengths in the region of 

 carotenoid absorption. It was therefore concluded that photosynthesis 

 was done by bacteriochlorophyll but not by the bacterial carotenoids. 

 These data are shown in Fig. Q-12d and compared with the absorption 

 spectrum of an aqueous urea extract of these bacteria. Recent work on 

 the action spectra for phototaxis and on photosynthesis and fluorescence 

 excitation in this species by Manten (1948) and Thomas (1950) has 

 cleared up the previous discrepancy. It has now been found that, 

 although the hght of wave lengths in the carotenoid absorption region is 

 less effective than that absorbed by bacteriochlorophyll, there are very 

 definite action peaks in this region which must be taken to indicate the 

 participation of a small fraction of the carotenoids. The predominant 

 carotenoid appears inactive, but others, present in lower concentration, 

 are certainly effective. These results emphasize the need for measure- 

 ments at many closely spaced wave lengths in the determination of action 

 spectra. Figure 6-12d also shows Manten's action spectrum for photo- 

 taxis. The broad bands used for the photosynthesis measurements in 

 the infrared account for the deviation of these two points from Manten's 

 phototaxis curve. Manten concludes: "Thus it is about certain that in 

 addition to bacteriochlorophyll, a carotenoid less abundant than spi- 

 rilloxanthin and similar to rhodopin or rhodopurpurin is responsible for 

 the absorption of the phototactically active light." 



The photosynthesis measurements in Rhodospirillum ruhrum by 

 Thomas (1950) also show the carotenoid peaks in the action spectrum. 

 In Fig. 6-15a we have plotted these data, as well as part of the phototaxis 

 and absorption curves, on a larger scale, bringing all curves together at 



