PHOTOCHEMICAL REACTIONS 445 



Other pigmented bacteria carry out their metabolism 

 along the same lines, but in them hydrogen sulphide, rather 

 than organic substances, acts as a hydrogen donor for the 

 reduction of CO2. These are the so-called purple and green 

 sulphur bacteria {Thiorhodaceae). They were discovered 

 long ago in small bays and lagoons of sea water, well exposed 

 to the sun, in places where the bacteria had access to hydro- 

 gen sulphide. It was later shown that they are widely 

 distributed in the soil and also in slimy pools of fresh and 

 salt water/^'' 



The very interesting researches of van NieP^* on Thio- . 

 rhodaceae showed that, in the light, these bacteria can 

 oxidise hydrogen sulphide in the complete absence of free 

 oxygen but with the simultaneous absorption of co, in 

 amounts corresponding stoichiometrically to an equation, 

 which, for purple sulphur bacteria, is as follows: 



H2S -f 2HoO + 2C02-^2CH20 + H2SO4 



That for green sulphur bacteria is : 



CO2 + SHaS-^CHaO + H2O + 2S 



for these latter bacteria can only oxidise UnS as far as sulphur. 

 According to van NieP^^ the process occurs as follows. 

 Owing to the presence of porphyrin pigments the Thio- 

 rhodaceae absorb sunlight and use its energy for the photo- 

 lysis of water according to the equation 



H2O + hv->H + OH 



The hydrogen of the water reduces CO2 and transforms it 

 into the carbon skeletons of the substances of which the 

 organism is composed (represented schematically in the equa- 

 tions by CHoO). 



According to van Niel the process of photosynthesis in 

 green plants is analogous with that described for Thio- 

 rhodaceae. Here also the agent which directly reduces co, is 

 the hydrogen of water. The only difference is that the 

 hydroxyl radicals liberated by photolysis are not used up in 

 oxidising some hydrogen donor (HoS or some organic com- 

 pound) within the organism. Owing to the presence of 



