NUTRITION OF THE AUTOTROPHIC BACTERIA 73 



of sulphur within their cells. They are obligate auto- 

 trophs which require hydrogen sulphide for their growth. 

 The hydrogen sulphide is oxidised in two stages, first to 

 sulphur and then to sulphate : — 



HgS + > H2O + S 



2S + 30, + 2HoO > 2H28O4. 



As long as hydrogen sulphide is available the organisms 

 contain globules of sulphur, but as soon as the supply 

 fails the sulphur is oxidised, and on its complete dis- 

 appearance the cell dies. The free access of oxygen and 

 carbon dioxide is necessary for the gro^\i:h of the colourless 

 organisms. 



The purple l^acteria contain the pigment bacterio- 

 purpurin, which is a mixture of two components, one 

 a green chlorophyll -like pigment, bacteriochlorin, and 

 the other a red carotenoid pigment, bacterioerythrin. 

 The bacteriochlorin apparently is the active pigment in 

 the respiration of these organisms, behaving, like the 

 chlorophyll in plants, as a sort of transformer for radiant 

 energy. That is, these bacteria require light as well as 

 carbon dioxide and hydrogen sulphide, but can dispense 

 with free oxygen. It seems possible that the light energy 

 is needed in the reduction of carbon dioxide with forma- 

 tion of, probably, formaldehyde, which is used in the 

 synthetic reactions, and of oxygen, which oxidises the 

 hydrogen sulphide : — 



CO2 ^ 2H2'^ > HCHO ^ HoO ^ 2S. 



These sulphur bacteria are found in fresh and salt water 

 and the mud of lakes and rivers, but not in soils. 



The sulphur oxidising bacteria play an important role 

 in nature in rendering the sulphur of proteins available 

 again to plants as sulphate. They may also be of value 

 in neutralising alkaline soils by the production of sulphuric 

 acid, and probably also convert insoluble phosphates into 

 soluble, available salts. On the other hand, it is possible 



