COMPARATIVE BIOCHEMISTRY 85 



terms of free energy the efficiencies of these two bacteria 

 may then be calculated as 5-9% and 7-4%. (Note: in the 

 calculation of efficiency no allowance has been made for the 

 probable use of part of the energy of oxidation for metabolic 

 processes such as growth.) Similar efficiencies of about 6% 

 have been observed with the sulphur-oxidizing bacteria, e.g. 

 Thiobacilliis spp. 



THE EVOLUTION OF THE PHOTOSYNTHETIC SYSTEM 



Studies with bacteria have excited speculation as to the 

 mode of evolution of the photosynthetic process. It may 

 be supposed that in early times under the very different 

 environmental conditions which then existed organic mole- 

 cules arose spontaneously from inorganic. Eventually a self- 

 reproducing unit was formed which was capable of utilizing 

 the oxidation of certain inorganic compounds for the further 

 production of organic compounds. A pigment system was 

 later developed which allowed of the utilization of solar 

 energy but in the first place the cells were still dependent 

 on the supply of suitable inorganic compounds as hydrogen 

 donors. Finally in green plants a mechanism was developed 

 in which water was the sole hydrogen donor thus attaining 

 a further stage in independence of the external environment. 



Some evidence in support of these ideas may be obtainable 

 from the investigation of mutants. Thus by the use of ultra- 

 violet irradiation mutant strains have been obtained from 

 the obligate autotroph Thiohacillus thiooxidans which require 

 for grov^h the addition of growth factors. The necessary 

 factors are probably one or more of the vitamin B group 

 but the exact requirement could not be established before 

 the strain mutated back to a type not requiring organic 

 growth factors. Thus we have an example of the transition 

 from a heterotroph to an autotroph in a relatively simple 

 step. According to these views, which were developed by 

 van Niel (1949) from the ideas of Oparin and others, the 

 evolution of the photosynthetic system has developed step 

 by step by the acquisition of a more complex mechanism 

 attaining at each stage a greater independence of external 

 reducing substances. 



