THE METABOLISM OF THE SULPHUR BACTERL4 49 



lates nitrates but not nitrites, and reduces them to free nitro- 

 gen. In its sulphur metabohsm hydrogen sulphide, sodium 

 thiosulphate (NagSaOg), and the sodium salt of dithionic acid 

 (NagSaOg) are utilized as sources of energy and changed into 

 the sulphate. 



The metabolism of this organism is interesting because it 

 presents certain peculiar features. It is unable to utilize the 

 energy of the sun as do the chlorophyll plants, and it has no 

 ready-formed stores of supply as have the parasites and 

 saprophytes. But energy must be obtained by it to build up 

 the protoplasmic molecule from simple substances like carbon 

 dioxide, and to carry out all the processes associated with life, 

 namely, movement, reproduction, etc. Finally, the organism 

 is one of the so-called obligate anaerobes, and cannot utilize 

 oxygen when supplied to it at the normal pressure. 



The place of the oxygen of the atmosphere used by aerobic 

 plants is taken by the oxygen liberated by the reduction of 

 the nitrate to free nitrogen. We may follow the energy 

 changes which take place in the metabolism. 



The reduction of the nitrate to the nitrite, according to 

 figures supplied by Waksman, demands an expenditure of 

 energy equal to ^6-6 calories. 



2HNO3 = 2HNO2 + 0., (- 26-6 calories). 



But oxygen is produced, and if utilized to break down a 

 fermentable carbohydrate liberates II2 calories per molecule. 

 The whole process will result in a gain on balance of 



112 — 36-6 = 75-4 calories. 



According to the same worker the change from the nitrite 

 to free nitrogen is an exothermic process and therefore gives a 

 further gain of energy : — 



2HNO2 = N2 + i|0, + H2O (+ 6-8 calories). 



Further the i-| molecules of oxygen can be utilized by the 

 organism with a gain of II2 calories per molecule. Hence 



4 



