360 UNITY AND DIVERSITY IN BIOCHEMISTRY 



C. Photosynthesis in the Purple, Brown, and Green Bacteria 



The Rhodobacteriaceae are an extensive family of bacteria found in soil 

 and water. Following Van Niel, they can be subdivided into three groups : 



(a) Chlorobacteriaceae. Green sulphur bacteria not requiring organic 

 growth factors. 



{h) Thiorhodaceae. Purple sulphur bacteria not requiring organic growth 

 factors but able to utilize as hydrogen donors various inorganic sulphur 

 compounds, organic acids, and in certain cases hydrogen itself. 



(c) Athiorhodaceae. Non-sulphur bacteria, purple, red or brown in 

 colour, requiring various growth factors. 



{a) Chlorobacteriaceae 



These are anaerobic, and according to Van Niel, HgS is the substance 

 which reduces the oxidizing radical liberated by photolysis. 

 Sulphur accumulates in the culture in the form of globules. 



[b) Thiorhodaceae 



They are anaerobes which do not grow except in the presence of HgS 

 and light. Like the chlorobacteriaceae they use HgS without oxidizing it 

 since they are anaerobic, and also like them, they are photosynthetic with- 

 out liberating oxygen. They do not release sulphur, but sulphuric acid. 



light 



H2S + 2CO2 + 2H2O > H2SO4 + 2(CH20) 



In the place of HgS, they can also utilize sulphite or thiosulphate. 



light 



4Na2S203 + CO2 + 3H2O > 2Na2S406 + (CH2O) + 4NaOH 



The mechanism of this curious piece of photosynthesis is unknown. 



(c) Athiorhodaceae 



These bacteria assimilate COg in the presence of light, but for this they 

 require certain organic or inorganic compounds. Fatty acids are good 

 substrates for their growth. A number of experimental facts indicate 

 that they act as hydrogen donors, but this idea is still under discussion. 

 The situation is clearer when we consider the utilization of isopropyl 

 alcohol. Here, there is no doubt that the organic compound acts as a 

 hydrogen donor to reduce the [OH] radical leaving acetone behind in the 

 medium. 



The overall equation for the reaction is: 



light 

 CO2 + 2CH3CHOH.CH3 > (CH2O) + H2O + 2CH3CO.CH3 



