The Transitionary Stage from Anaer obiosis to Aerobiosis 599 



it is not only capable of photochemical H2 production but can also reduce CO2 

 with H2 either photosynthetically or by a dark chemoautotrophic process, in 

 which the energy for CO2 assimilation is derived from the oxidation of H2 with 

 O2. The latter type of oxidation is the energy source for growth of autotrophic 

 hydrogen bacteria [15]. Frenkel & Rieger [13] have recently shown that numer- 

 ous other species of green, blue-green, red and brown algae can utihze H2 for 

 the photochemical reduction of CO2. As mentioned above, photosynthesis can 

 be reversely transformed to photochemical reduction by adequate anaerobic 

 conditions. From these observations photochemical reduction may be the more 

 imiversal metaboHc pattern and forms the basis of the photosynthetic process. 

 Here again, the direct precursor of H2 in the Hght-dependent process is unknown; 

 it seems quite possible that water may not be the immediate source, but rather 

 that H2 is evolved from organic intermediates such as pyruvate by mechanisms 

 similar to those operating in H2-producing heterotrophs which was discussed 

 before. The Hght dependency may, in a certain sense, be secondary, that is, 

 Ught could be required for the generation of an organic precursor which is 

 subsequently oxidized in a dark reaction, electrons being transferred to the 

 hydrogenase system. With CO2, however, H2 is not utilized unless light of the 

 proper wavelengths is supphed. When CO2 is the ultimate oxidant, hght may 

 be obhgatory for the generation of oxidized acceptors which are reduced by H2 

 in subsequent dark reactions. 



It is Hkely that photoproduction of H2 has a fundamental significance for the 

 mechanism of electron transport in all types of photosynthetic reactions. From 

 the very existence of light-dependent H2 evolution, it may be inferred that the 

 photochemical generation of electrons, that is, reducing power, occurs in a 

 reaction characterized by a redox potential well below those of the pyridine 

 nucleotide coenzyme systems and that those coenzymes are probably not 're- 

 duced' by primary acts. 



Accordingly, it is tempting to speculate that the carriers involved in the early 

 stage of electron transport in photosynthesis as well as in heterotrophic CO2 

 reduction may be essentially similar to those participating in the phosphoroclastic 

 reaction oï Clostridium spp. and in decomposition of formate by the coli-aerogenes 

 group. 



Concerning the photochemical CO2 assimilation, it is very interesting that 

 the photosynthetic micro-organisms mostly have the ability of nitrogen fixation 

 [6, 14]. These facts show that the No reduction process has developed in con- 

 nection with the CO2 reduction process, and that both processes have been 

 maintained without separation even in the transition from the utilization of 

 chemical energy to that of radiant energy. Here again it is emphasized that a 

 fundamental unity exists in all living forms, and life possesses its Hmitations as 

 well as its organizing potentiaHties in biochemical planning. 



Thus, the evolution of metaboHc systems that originated from anaerobic 

 heterotrophs has developed to the direction of autotrophy and it has led to the 

 appearance of photosynthetic hving beings that is, of free O2. 



The very appearance of free O2 became the basis for the luxuriant develop- 

 ment of aerobic living beings. 



