598 YOSHIHARU ODA 



occurs in a reaction characterized by a redox potential well below those of the 

 pyridine nucleotide coenzyme systems, and that these cocnz}'mes are probably 

 not reduced by primary acts. In this connection, it is tempting to speculate that 

 the carriers involved in the early stages of electron transport in CO2 assimilation 

 may be similar to those participating in the phosphoroclastic reaction of the 

 Clostridia and in decomposition of formate by the hydrogenlyase complex that 

 will be discussed later. 



While CO2 and H2 were evolved by the activities of anaerobic heterotrophs, 

 N2 gas and sulphate also gradually accumulated on the surface of the Earth, 

 and the new ability of assimilating No gas was born in the kingdom of Hving 

 beings. But this ability may have developed in association with heterotrophic 

 CO2 assimilating process, and both processes may be essentially the same, in 

 spite of the difference in the utilization of either CO2 or Ho as the ultimate 

 hydrogen acceptor. Accordingly, it may be inferred that in the N2 fixation 

 process which occurs in some of the Clostridia, the systems participating in 

 the phosphoroclastic reaction also play the important role in electron 

 transport. 



As sulphate gradually accumulated and the new ability to activate molecular 

 hydrogen developed, sulphate came to be reduced by molecular hydrogen in 

 the environment and, as the results, sulphide and elementary sulphur gradually 

 accumulated. Thus, the so-called stage of chemoreduction [12] came to develop 

 on the Earth. 



Furthermore, as heterotrophic CO2 assimilation and chemical reduction pro- 

 ceeded actively, CO2 began to be deficient in the atmosphere and its partial 

 pressure became lower. Accordingly, the visible parts of the solar spectrum reached 

 the surface of the Earth and made possible the appearance of the assimilative 

 pigments which participate in the CO2 assimilation by light in hving beings. In 

 this way the so-called stage of photoreduction [12], that is, the metabolic patterns 

 of assimilation of CO2 by utilizing radiant energy came to develop. As the 

 living beings acquired these pigments and the capacity to react with hght, 

 photochemical processes appeared in the kingdom of living beings. Some living 

 beings that lived in the stage of chemoreduction substituted some parts of their 

 chemoreduction systems in CO2 assimilation process for the photochemical 

 reduction systems by acquiring new assimilative pigments. The photosynthetic 

 reaction liberating oxygen is a process in association with photochemical reduc- 

 tion, and the former can be reversely transformed to the latter in proper condi- 

 tions. Thus it may be inferred that photosynthesis developed on the basis of 

 photochemical reduction. In the so-called 'photoheterotrophs' such as sulphur 

 purple and nonsulphur purple bacteria, photochemical Ho production appears 

 only after an extended dark anaerobic adaptation period. In addition, a rapid 

 production of H2 by illumination occurs in the presence of suitable organic 

 substrates, when the atmosphere surrounding the organisms does not contain 

 O2, N2; it is also inhibited by NH4+ [6]. Furthermore, photosynthetic auto- 

 trophs such as Scenedesmus slowly produce H2 in the dark and, at a relatively 

 rapid rate, upon illumination. But this metaboUc pattern also appears only after 

 an extended dark anaerobic adaptation period. Once the organism is adapted, 



