438 



LIGHT AND LIFE 



Rhodopseudomonas spheroides 24IC 

 grown 48 hours in the light 



— anaerobic minus aerobic (dark) 

 Reference =450 nn^i anaerobic dark nninus anaerobic light 



-I 1 — I 1 — [ 1 r— [ — r*-i 1 — I 1 — [ — r 



450 480 510 540 570 

 Wavelength in millinnicrons 



Fig. 2. Difference spectra of /?. sjylwroides grown under illumination. The 

 niea.siireinent.s were made in the double-beam spectrophotometer at the wavelengths 

 indicated; the reference wavelength was 450 m^u. The bacteria were used directly 

 from the culture bottle and contained 1.3 mg protein per ml. 



(i cytochrome absorption bands are obscured by the large changes in 

 the carotenoid absorption spectrum. Small changes in the a absorption 

 bands of the cytochromes can be seen in the difference spectra of 

 R. spheroides (Fig. 2) around 550-560 m^, where they are superim- 

 posed upon the carotenoid changes. Thus the cytochrome reactions 

 can be seen most clearly in the region of the Soret bands. The dif- 

 ference spectra of Fig. 2 show that different cytochrome pigments are 

 oxidized on illiunination, as compared with those oxidized on addi- 

 tion of oxygen in the dark. This relationship is shown more clearly 

 in Fig. 3, which plots the difference spectra of R. rubrum in the region 

 of the Soret cytochrome peaks inider conditions where the intensity 

 of illumination was greater than that used in Fig. 1 (12). The 

 cytochromes oxidized on aeration of bacteria in the dark would be 

 those of the "respiratory chain," over which substrates are oxidized 

 by molecular oxygen. The bacteria can grow either under illumina- 



