SPECTRA OF PHOTOSYNTHETIC PIGMENTS 371 



teria as compared with leaves and algae, thus simplifying the interpre- 

 tation of both absorption and action spectra. 



ABSORPTION SPECTRA OF PHOTOSYNTHETIC BACTERIA AND 



OF THEIR PIGMENTS 



Before taking up the purple bacteria, let us first consider an intermedi- 

 ate form, the green bacteria. These contain a pigment somewhat similar 

 to chlorophyll a but not identical with it. The absorption spectra of the 

 acetone extract and of a suspension of green bacteria are shown in Fig. 

 6-11. It is obvious from this figure that the carotenoid content of this 

 organism is very small. A comparison of the absorption spectrum of the 

 acetone extract with that of the suspension of the bacteria themselves 

 shows a very large shift in the wave length of the red peak of bacterio- 

 viridin when it is extracted from the living bacteria. The position of 

 this peak, about 740 m/x in the living bacteria, falls between the red 

 absorption maximum, 680 nifi, of green leaves, and the 800- to 900-m/i 

 region in which the purple bacteria absorb energy and carry on photo- 

 synthesis. A detailed study of the quantum yield of photosynthesis in 

 the green bacteria has been made by Larsen et al. (1952), to whom we 

 are indebted for Fig. 6-11. 



The absorption spectra of many species of purple bacteria in the near 

 infrared have been examined by Katz and Wassink (1939), Wassink et al. 

 (1939), Wassink and Manten (1942), and Giesberger (1947). Most of 

 these measurements were made by pressing the cells in thin layers between 

 glass plates. The lack of an appropriate scattering correction in these 

 spectra introduces appreciable distortion in shape, but nevertheless the 

 band position is very well defined, and even the relative peak heights in 

 adjacent wave-length regions were obtained by this means. Bacterio- 

 chlorophyll bands were found in several different regions of the spectrum. 

 Bands at 890 875 m/x, at 860-840 myu, and at 803-795 m^ have different 

 relative intensities in different species. All the species investigated, how- 

 ever, had an absorption band at 590 m^ which was due to bacterio- 

 chlorophyll. According to Duysens (personal communication, 1952). 

 an absorption band of bacteriochlorophyll occurs in the near-ultraviolet 

 region. This region, however, is not represented in the spectra of the 

 afore-mentioned authors. A most striking fact was found by Wassink 

 and his collaborators (1939) when bacteriochlorophyll was extracted from 

 these different species of purple bacteria by organic solvents. Whatever 

 the position of the absorption bands in the bacteria, the bacteriochloro- 

 phyll extracted from all these species was spectroscopically identical. 

 It is presumed that the position of the bands in the Hving cells is due to 

 combination of the bacteriochlorophyll with some other cell substance, 

 probably a protein, and that several different proteins or different modes 



