650 XIV. PYRROLE PIGMENTS IN EVOLUTION 



support to the hypothesis that the photosynthesis of the green plants, 

 using water as hydrogen donor and evolving oxygen, had its pre- 

 cursor in this period in the photoreduction of carbon dioxide by 

 hydrogen sulfide, molecular hydrogen (974,2326), and other hydrogen 

 donors. This is supported particularly by the experiments of Gaffron, 

 who showed that the photoreduction of carbon dioxide, which occurs 

 alone in purple bacteria, is found side by side with normal photo- 

 synthesis in some green algae {97 J^., 2050); the same has been found 

 for some Cyanophyceae and diatoms {2008). 



There is a parallel to be seen between the presence of anaerobic 

 sulfur metabolism with hydrogenase and bacteriochlorophyll in the 

 more primitive organisms and of aerobic oxygen metabolism with 

 photocatalase and chlorophyll in the green plants. Bacteriochloro- 

 phyll and the pigment of green bacteria which is probably also related 

 to chlorophyll (Fischer and co-workers, 8If.If) may thus be considered 

 evolutionary precursors of chlorophyll. 



Bacteriochlorophyll is a tetrahydroporphyrin compound with an acetyl 

 side chain, chlorophyll a dihydroporphyrin compound with a vinyl side 

 chain instead of the acetyl since chain. Warburg {2928) divided the hematin 

 compounds into three types, red (porphyrin) hemins, green (chlorophyll) 

 hemins, and green-red hemins (derived from porphyrins with carbonyl 

 groups in the side chains). This division has been criticized on chemical 

 grounds in Chapter V, Section 8.1. Warburg assumed that the green-red 

 hemins (to which the prosthetic group of chlorocruorin and perhaps that of 

 the respiratory ferment belong) were the evolutionary prototype of both red 

 and green hemins. In this general form the theory is certainly not correct. 

 There is little likelihood that the respiratory ferment or chlorocruorin has 

 preceded catalase or cytochrome c on the one hand, or bacteriochlorophyll 

 on the other. The relation of chlorophyll and bacteriochlorophyll indicates, 

 however, that the presence of carbonyl groups in the side chain (at least 

 in that which later becomes a vinyl group) is a primitive characteristic. 

 This is probably also correct in the porphyrin (hematin) series, in which 

 chlorocruorin can be considered an evolutionary relic {cf. Chapter VII, 

 Section 8.2.1.). It must not be forgotten, however, that chlorophyll b has 

 also a formyl group as side chain, the latter standing in place of a methyl 

 group, not a vinyl, of chlorophyll a; chlorophyll b is absent in some algae, 

 but present in all higher plants, and that it is an early evolutionary product 

 is thus improbable. It may be mentioned that according to Fischer only 

 one bacteriochlorophyll exists. 



From the geochemical point of view, according to Oparin, the 

 oxygen of the atmosphere was produced in the next period of the 

 earth's history, by the introduction of water as hydrogen donor in 

 photosynthesis. Photoassimilation thus preceded respiration, and 



