370 RADIATION BIOLOGY 



phyll a fluorescence with exciting light of 4358 A is approximately 0.6 

 and, with 6685 A, 0.8 of the yield with 5780 A. A lower yield at the 

 longer wave lengths (i.e., 690 iiim) has been known for some time and 

 has been demonstrated for other compounds as well as chlorophyll 

 (Pringsheim, 1949). This whole subject needs further investigation. 



The transfer of energy in solutions of these pigments is another field 

 that has not been sufficiently explored. Watson and Livingston (1950) 

 have reported energy transfer from chlorophyll b to chlorophyll a in 

 equimolar solutions at molarities greater than 2 X 10-^. The molar con- 

 centration of chlorophyll in living leaves of some species has been calcu- 

 lated to be 0.2 (Aronoff, 1950), so that the transfer of energy is certainly 

 theoretically possible at this concentration. Duysens (1951b) also found 

 the transfer of energy from chlorophyll b to chlorophyll a in solution. 



SPECTRA OF PHOTOSYNTHETIC BACTERIA 



Photosynthetic bacteria are organisms that are of relatively little 

 quantitative importance in the carbon cycle of nature. These bacteria 

 are, however, of great interest as experimental material for the investi- 

 gation of photosynthesis. They are fairly widespread, being found in 

 mud and in ditch water, but rarely occur in large quantity. Since these 

 organisms have a photosynthetic mechanism that differs in some major 

 respects from that of higher plants and since their pigments are different 

 from those in higher plants, they have provided excellent material for 

 the application of the principles of comparative biochemistry to the study 

 of photosynthesis. Their study has therefore contributed to a more 

 generalized understanding of the photosynthetic process (Van Niel, 1941, 

 1944). The photosynthesis of these bacteria differs from that of higher 

 plants in that they use other compounds than water for reductants, 

 namely, hydrogen sulfide, thiosulfate, hydrogen, or other inorganic or 

 organic compounds. The pigments present are analogous to, but not 

 identical with, those in leaves of green plants. The chlorophyll com- 

 ponent in the purple bacteria is called "bacteriochlorophyll" and in green 

 bacteria, "bacterioviridin." The carotenoids also differ in chemical 

 structure from that of higher plants. For the following reasons the 

 spectral absorption characteristics of these bacteria have made them 

 interesting material for the study of the function of photosynthetic pig- 

 ments: (1) since bacteriochlorophyll absorbs wave lengths longer than 

 those absorbed by chlorophyll a, extension of the study of absorption, 

 fluorescence, and action spectra into the near infrared has been possible; 

 (2) bacteriochlorophyll exhibits different absorption properties in various 

 strains of purple bacteria, thus providing a means for the study of the 

 physical state of this photosynthetic pigment in living organisms; (3) the 

 absorption maxima of the different pigments are well separated in bac- 



