300 RADIATION BIOLOGY 



the molecule, which, of course, strongly alters the system of conjugated 

 double bonds. The rest of the molecule is nearly the same as that of 

 chlorophyll a. A bacteriochlorophyll b is not known so far. 



In green-plant cells the chloroplasts contain chlorophyll a and b and 

 numerous yellow carotenoids. This photosynthetic system is found in 

 green flagellates, green algae, and the Cormophyta. A few deviations 

 from this distribution are known, one of which is the absence of chloro- 

 phyll b in Vaucheria (Seybold et at., 1941). Chlorophyll a is much more 

 widespread, being found in all nonbacterial photosynthetic organisms. 

 It is associated with chlorophyll c (Strain and Manning, 1942; Wassink 

 and Kersten, 1946-1948) in diatoms and brown algae, both of which con- 

 tain the carotenoid fucoxanthin. The close relation between the photo- 

 synthetic pigment systems of these fairly divergent groups of algae is 

 remarkable. A chlorophyll d was found in small amounts in red algae 

 (Manning and Strain, 1943). Strain (1949) reported also on a chloro- 

 phyll e, found along with chlorophyll a in the yellow-green alga Triho- 

 nema spec. The blue-green and red algae contain a phycobilin pig- 

 ment as well as the chlorophyll complex (including carotenoids). It is 

 remarkable that the few groups of photosynthetic bacteria have devel- 

 oped two pigment systems that seem mutually independent and also 

 widely divergent from the pigment system of all "chlorophyll plants." 

 The green bacteria contain bacterioviridin, a compound which is con- 

 sidered to be 2-acetyl (nonvinyl) chlorophyll a (Fischer and Orth, 1940) 

 and which, in extracts, has an absorption spectrum closely similar to that 

 of chlorophyll a (Fig. 5-7). In the living cell its red absorption maxi- 

 mum is shifted about 70 m^u toward the infrared (Fig. 5-4). Sulfur and 

 nonsulfur purple bacteria contain bacteriochlorophyll in various states of 

 association with proteins. The photosynthetic bacteria also contain 

 carotenoids, which, in the purple bacteria, are responsible for the color 

 of the organisms. 



The photosynthetic bacteria, as was mentioned earUer, cannot carry 

 out the reaction summarized in Eq. (5-1). They preferentially substi- 

 tute sulfur compounds for oxygen compounds and in part may use cer- 

 tain organic compounds or molecular hydrogen as reductors. 



All photosynthetic organisms may show a fluorescence spectrally con- 

 nected with the long-wave absorption band of the chlorophyllous pigment. 

 As far as the writer knows, fluorescence of green bacteria has not yet been 

 demonstrated, but no doubt it will be found. It is generally assumed 

 that the average fluorescence wave length represents an excitation level 

 of the pigment system from which the chemical-energy consumption also 

 originates. This implies that smaller energy quanta are at the disposal 

 of the cell in the photosynthetic systems of the bacteria than in those of 

 green plant cells. The supposition that this difl"erence has something to 

 do with the inability of the bacteria to obtain the energy required for 



