PHOTOSYNTHETIC BACTERIA 85 



Iron bacteria can play an important part in the corrosion of 

 iron pipes, etc. 



An organism of particular interest is Thiohacillus thio- 

 oxidans, which utilises as energy source the oxidation of 

 elementary sulphur to sulphuric acid: 



2S + 3O2 + 2H2O = 2H2SO4 + 141-8 Cals. 

 This organism has an exceptionally high tolerance of acid, 

 and is unaffected by a pH value as low as 0-6, while it grows 

 most rapidly at a ^H between 3 and 4. In this case the 

 oxidation of sulphur provides the energy for the assimilation 

 of CO2 and its reduction to cell-carbon. The processes can 

 be separated ; the organism will oxidise a certain amount of S 

 in the absence of COg and then, if exposed to CO2 later, will 

 take up and reduce an amount of COg corresponding to the 

 initial oxidation. The oxidation of S is accompanied by an 

 uptake of inorganic phosphate from the medium and this 

 phosphate is again liberated during CO2 reduction; this 

 suggests that the energy obtained from the oxidation process 

 is stored as a form of " energy-rich " phosphate until it is 

 utilised in the reduction process. Umbreit has isolated 

 adenosine-tri-phosphate from Thiohacillus thio-oxidmis, which 

 suggests that this is the organic phosphate in which the energy 

 is stored, thus Unking the energy-systems of the autotrophic 

 bacteria with those demonstrated in heterotrophic organisms 

 (see Chap. VII). 



PHOTOSYNTHETIC AUTOTROPHES: It is obvious from the 

 nature of their metaboHsm that the chemosynthetic auto- 

 trophes must be strictly aerobic in their habitat. The photo- 

 synthetic autotrophes, on the other hand, are strict anaerobes 

 and obtain the energy for their synthetic activities by photo- 

 chemical utiHsation of hght energy. They obtain their 

 nitrogen from ammonia or nitrate, their carbon from 

 bicarbonate, and reduce the bicarbonate to organic carbon by 

 a linked oxidation of an inorganic substrate. The organic 

 carbon so produced may be of carbohydrate nature in the 

 first instance and can be conveniently represented by 



