124 PHOTO- AND CHEMOSYNTHESIS OF BACTERIA CHAP. 5 



oxidation of water, and that the second stage can be changed and other 

 reductants substituted for water without affecting the first one. Chemo- 

 synthesis by autotrophic bacteria makes it plausible that the reduction 

 of carbon dioxide is a nonphotochemical process, which can be brought 

 about by the intermediates of the photochemical oxidation of water (or 

 other reductants), as well as by products of exothermal chemical reactions. 

 These problems will be discussed more extensively in chapters 7 



and 9. 



Another interesting question which arises from the study of the 

 photosynthesis and chemosynthesis of autotrophic bacteria, concerns the 

 role which these processes may have played in the development of life 

 on earth. Prior to van Niel's interpretation of the mechanism of bacterial 

 photosynthesis, the synthesis of organic matter by green plants appeared 

 as a unique process, unrelated to all other biochemical reactions in living 

 organisms. Van Niel's investigations have established the long-missing 

 link between the world of green plants and that of the lower micro- 

 organisms. 



Green plants reduce carbon dioxide in light by means of water; 

 green and purple sulfur bacteria reduce carbon dioxide, also in light, 

 by means of hydrogen sulfide; colorless sulfur bacteria reduce carbon 

 dioxide, by means of hydrogen sulfide, without light. This comparison 

 shows the existence of a hierarchy of autotrophic organisms, and en- 

 courages speculations as to the genetic relationships between them. 



In considering the present state of life on earth, one is struck by the 

 paradox "no life without chlorophyll — no chlorophyll without life." The 

 large-scale formation of organic matter from inorganic materials has as 

 its prerequisite the existence of complex organic molecules, such as 

 chlorophyll and various enzymes, without which photosynthesis appears 

 impossible, but which themselves cannot be synthesized in nature outside 

 the living cell. 



Obviously, photosynthesis could not have started on earth without 

 the previous existence of living matter. The existence of chemauto- 

 trophic and photautotrophic bacteria shows a possible development. 

 It was mentioned on page 82 that the first organic molecules may have 

 arisen on earth by photochemical reactions of inorganic compounds in 

 ultraviolet light, or by the action of electric discharges in the atmosphere. 

 Which of these molecules first acquired the capacity of propagation 

 by self -duplication, which is the first sign of life, we cannot surmise; but 

 we can imagine a continuous " chemosynthetic " development leading 

 from this molecule to autotrophic bacteria. At that time, the earth was 

 less settled in its chemical ways than now, and not only hydrogen sulfide, 

 but also free hydrogen might have been available in the atmosphere. 

 From colorless autotrophic bacteria, the development might have 



