86 



growth: synthesis of bacterial protoplasm 



(HCOH). Important members of this group are the sulphur 

 bacteria Thiorhodaceae which are abundant in soil, mud, and 

 sulphuretted waters. Some varieties of the group are purple 

 and some green, the colour in each case being due to a mixture 

 of pigments including a magnesium-porphyrin pigment with 

 the structure of chlorophyll, differing from that of plants 

 only in the nature of certain of the side-chains. 



H H 



COCH 



CH, COOCH, 



I ' 



COOC^H,^ BACTERIAL CHLOROPHYLL 

 20 39 



These organisms reduce CO2 to organic carbon by a linked 

 oxidation of hydrogen sulphide: 



CO2 + 2H2S = (HCOH) + H2O + 2S. 



In the presence of large amounts of hydrogen sulphide, the 

 elementary sulphur is deposited inside the cell in the case of 

 the purple sulphur bacteria, or outside the cell in the case of 

 the green sulphur bacteria. If the supply of hydrogen sulphide 

 is limited, a further oxidation-reduction takes place, the 

 elementary sulphur being oxidised to sulphuric acid : 



2CG2 + H2S + 2H2O = 2 (HCOH) + H2SO4. 



Again the autotrophic organisms are characterised by oxida- 

 tion of an inorganic substrate. In the photosynthetic group 

 the oxidation occurs anaerobically as a means of reducing 



