8 SULPHUR BACTERIA 



usually there are many kinds of different bacteria in competition 

 and some of them effect oxidation, whilst others effect the 

 reduction of the sulphur element. Also the environmental 

 conditions are continually changing, so that at every change 

 the organisms which for the moment had gained the upper 

 hand in the decomposition of the protoplasmic molecule now 

 find themselves supplanted by other microbes which in their 

 turn must give way to others with fresh changes in the 

 environment. 



The sulphate may be reduced to elementary sulphur if the 

 supply of oxygen be scanty, or possibly even to sulphuretted 

 hydrogen if there be no oxygen present. The elementary 

 sulphur may unite with iron or other metals to form sulphides. 

 Again, the sulphide may be replaced by carbonic acid to form 

 carbonates. Some bacteria oxidize thiosulphates to thio- 

 persulphates. Ultirfiately, however, all the sulphur is com- 

 bined in the form of sulphates, in which condition it cannot 

 be further exploited as a source of energy by bacteria. To 

 return to our metaphor, in the condition of sulphate the 

 accumulator is discharged, and must be connected up with 

 some source of energy if the sulphur is to be made once more 

 available to build up the protoplasmic molecule. This is 

 supplied by the sun through the medium of green plants. 

 The sulphates are absorbed by the roots of green plants, 

 and enter into combination with the carbohydrates built up 

 by the carbon-assimilation of the plants. Eventually the 

 sulphur which has been caught in this stream becomes a part 

 of the protoplasmic molecule once more, and the cycle is 

 complete. 



As the energy of the universe is derived from green plants 

 (and a small number of minute animals), and as all animals arc, 

 directly or indirectly, nourished by green plants, the sulphur 

 in the protoplasmic molecule of such plants may pass into 

 and become incorporated with the protoplasmic molecule of 

 various animals. In such cases the sulphur-containing pro- 

 teins of the plants enter into the composition of the proto- 

 plasmic molecule of the animal. When the animal dies, the 

 fate of the sulphur element, speaking generally, is the same 



