1302 THE PIGMENT FACTOR CHAP. 32 



Chromatium: 



BChl "800" -> BChl "850" -> BChl "890": 100% 



Carotenoids -* BChl "850": '-'40% 



Rhodospirillum (molischianum or rubrum): 



Carotenoids -* BChl "890" : ^-40% 



In the species R. riibrum — as contrasted to other purple bacteria — 

 a striking difference between the absorption spectrum and the action spec- 

 trum of phofotaxis was noted by Manten (1948), and interpreted as proof of 

 phototactic inactivity of one of their most abundant carotenoids, spirillox- 

 anthol. The action spectrum of photosynthesis of Rhodospirillum rubrum 

 is in this respect similar to that of phototaxis (Thomas 1950) ; thus, spiril- 

 loxanthol seems to be inactive also in photosynthesis. The efficiency of 

 energy transfer to BChl "890" (as determined from fluorescence measure- 

 ments) is, however, according to Duysens, about the same for spirilloxan- 

 thol as for other carotenoids (namely, about 40%). Duysens suggested 

 that two types of carotene-bacteriochlorophyll complexes occur in Rhodo- 

 spirillum rubrum. The complexes containing spirilloxanthol (which pre- 

 dominate in older cultures) are inactive (both in photosynthesis and in 

 phototaxis), while complexes containing other carotenoids (mainly, rho- 

 dopol), which predominate in younger cells, are photobiologically active; 

 the fluorescence of BChl "890" is excited with the same efficiency by the 

 carotenoids in both complexes. Later, Clayton found spirilloxanthol to 

 be effective in phototaxis of another strain of Rhodospirillum rubrum; 

 Duysens suggested that this may have been due to the use of much lower 

 light intensities than those used by Manten. 



Thomas and Goedheer (1953) again compared the action spectra of photosynthesis 

 and phototaxis in Rhodospirillum rubrum, and confirmed the earlier observation (Thomas 

 1950) that, while the absence of the spirilloxanthol band is common to both of them, the 

 two spectra differ in that the action spectrum of photosynthesis indicates a higher ef- 

 ficiency of the "active" carotenoids than the action spectrum of phototaxis. An explan- 

 ation was suggested for this difference in terms of a two-fold photochemical function of 

 carotenoids — first as energy suppliers to photosynthesis, and second as photocatalysts 

 for the consumption of photosynthetic products (carotenoid-sensitized "photorespira- 

 tion," cf. Emerson and Lewis, chap. 20, p. 568 and Warburg et. al., chap. 37D, section 4). 

 The second process reduces the accumulation of photosynthetic products in violet (com- 

 pared to red) light, and this in turn reduces phototaxis (which, in purple bacteria — but 

 not in algae or higher plants — seems to be stimulated by the products of photosynthesis). 



In green algae (Chlorella), the following transfer efficiencies were calcu- 

 lated by Duysens: 



Chlorophyll b -* chlorophyll a: 100% 



Carotenoids -* chlorophyll a: 44% 



The first figure was obtained by comparison of the yields of fluorescence 

 excited by 670 and 650 m^u — i. e., by a frequency absorbed only by a, and one 

 absorbed about equally by h and a. The second figure was derived from 



