CHEMOSYNTHESIS 45I 



ised gi'oups of organisms as the chemoautotrophs made their 

 appearance. It was just at this intermediate period, between 

 the prevalence of reducing and oxidising conditions, that 

 there first arose the possibility, in principle, of oxidising the 

 reduced inorganic substances of the crust of the Earth on an 

 extensive scale by means of molecular oxygen. At the period 

 we are dealing with, when free oxygen was beginning to be 

 formed, these oxidative reactions must have been occurring 

 at, literally, every point on the surface of the Earth, for oxidis- 

 able substrates were present everywhere. However, when 

 these reactions took place inorganically they proceeded, 

 relatively speaking, very slowly and the energy which they 

 liberated was lost, being dissipated in the form of heat. 



When there was an acute shortage of exogenous organic 

 compounds those organisms which, during their evolution, 

 had become able to include in their metabolism those re- 

 actions whereby inorganic materials are oxidised, and which 

 had formed in their bodies catalytic mechanisms which 

 hastened these processes and mobilised the energy derived 

 from them for biosynthesis, certainly had a great advantage 

 in the struggle for existence. Their position was therefore 

 secured by natural selection and they were later able to 

 develop extensively. 



At present we usually find organisms capable of a chemo- 

 autotrophic way of life under natural conditions in just those 

 places where the reducing substances of the depths, emerging 

 into the daylight, encounter the molecular oxygen of the 

 atmosphere. For example, reduced compounds of sulphur 

 are easily formed in nature wherever anaerobic conditions 

 prevail. Tremendous amounts of hydrogen sulphide accumu- 

 late in the seas and oceans in places where the water is stag- 

 nant owing to differences in salinity between the surface and 

 deep layers. Considerable amounts of hydrogen sulphide are 

 also concentrated in the water of the petroleum-bearing 

 strata, and also on the surface of the Earth under conditions 

 which lead to the anaerobic decomposition of proteins. 

 Wherever hydrogen sulphide passes from a medium where 

 the conditions are reducing into one where they are oxidis- 

 ing, there we always find the development of thionic or 

 sulphur bacteria. ^^^ Similarly, the nitrifying bacteria carry 



