The Evolution of Chemosynthesis 631 



the utilization of organic matter from external sources is hindered in these 

 organisms by special properties of their cellular membrane since 'strict auto- 

 trophs' secrete highly acid products (HNO3, H2SO4). According to some data, 

 their cellular membrane possesses Upophilic properties [3] and appears to be 

 impermeable for organic compounds. 



As has been stated above, transition from heterotrophic metaboHsm to chemo- 

 synthesis occurred repeatedly within various systematic groups of micro- 

 organisms. The same may evidently be said about the photoautotrophic organisms. 

 The very fact of repeated transition from heterotrophic to autotrophic nutrition 

 signifies that the latter type of nutrition was an important advantage for the 

 organisms. This advantage was that the organisms, in the case of such transition, 

 obtained access to unlimited supplies of carbon for the building up of cellular 

 substance. It is well known that heterotrophic bacteria can derive carbon for 

 biosynthesis only by oxidative assimilation of a small part of the particular 

 organic substrate on the oxidation of which they are specialized. It is also known 

 that despite the extensive supplies of organic matter in the world there is a keen 

 struggle between micro-organisms for the substrate. For instance, in silt deposits 

 the organic matter of the bacteria themselves amounts to several per cent of the 

 total quantity of organic matter of the silt. Therefore the liberation from the 

 necessity of ensuring their anaboHsm by oxidative assimilation afforded to the 

 chemoauto trophic bacteria a great advantage over heterotrophic organisms. This 

 is confirmed by our experiments in which the rate of chemosynthesis in silt 

 deposits has been estimated with the aid of ^^C [42]. We found that chemo- 

 synthesis proceeds very intensively even in silt known to be rich in organic 

 matter; up to 10% of the total mass of bacteria inhabiting the upper layer of 

 the silt is renewed daily at the expense of chemosynthesis. These data refute the 

 opinion that in conditions of abundance of organic matter chemoautotrophic 

 bacteria feed heterotrophically, and that their capacity for chemosynthesis is a 

 superfluous accessory function manifested only when they are cultivated on 

 artificial nutrient media. 



How can the biochemical evolution of chemosynthesis be pictured ? It has 

 apparently proceeded according to the scheme common to many other metabolic 

 processes in micro-organisms. Owing to changed conditions of existence to 

 which certain groups of micro-organisms were subjected, some function or other 

 of their metaboHsm developed, became more complex and, rising to a quali- 

 tatively higher level, became the chief function of their metabolism. The possi- 

 bility of such an evolution of chemosynthetic organisms was provided by the 

 fact that their predecessors were biochemically 'highly organized' microbes of 

 the Pseudomonas type, endowed with a powerful enzyme outfit. All mineral 

 substances the oxidation of which provides the source of energy for chemo- 

 synthesis are products of life activity of these heterotrophic bacteria and in 

 some way or other participate in their metabohsm. It is therefore natural that 

 enzyme systems capable of catalysing the transformation of the substances in 

 some direction or other existed in the heterotrophic predecessors of chemo- 

 synthetic bacteria. The direction of the reaction is of no particular importance, 

 since all enzymic reactions are reversible. 



