NUTRITION OF THE AUTOTROPHIC BACTERIA 71 



Nathansohn. It grows under nearly neutral conditions 

 and oxidises thiosulpliate, tetrathionate, or sulphides 

 with separation of sulphur, which is deposited outside 

 the cells. For example, thiosulphate is oxidised as 

 follows : — 



2X3.28203 + 02 > 2XaoS04 + 2S. 



and tetrathionate probably by the reaction : — 



NaaS^Oe+NagCOs + O > 2XaoS04 + C02 +2S. 



The second type, Th. thio-oxidans, was found by 

 Waksman in soils in the neighbourhood of sulphur 

 deposits, but is not ordinarily found in soils which have 

 not been treated with sulj)hur. Th. thio-oxidans produces 

 large quantities of sulphuric acid, and in fact only grows 

 in somewhat strongly acid conditions at pK 2 to 3, 

 corresponding to 5 to 10 per cent, of sulphuric acid. 

 It performs this remarkable function by the direct oxida- 

 tion of sulphur or thiosulphate : — 



2S -f 3O2 4- 2H2O ^2H2S0i 



XagSgOa + HoO + 2O2 ^XagSO^ + H2SO4 



Sulphur is taken into the cell prior to oxidation by 

 solution in fat globules situated at the ends of the organ- 

 isms. The oxidation of sulphur, by which energy is 

 provided, can take place in the absence of carbon dioxide, 

 and the latter can be assimilated in the absence of sulphur 

 oxidation, either aerobically when no free sulphur is 

 available, or anaerobically when oxidation camiot occur, 

 provided that the products of the latter reaction are 

 available in the cell. That is, there must be some 

 " accumulator " mechanism in the cell by which energy 

 is stored, to be used in the subsequent metabolism of 

 carbon dioxide. It has been shown that the oxidation 

 of sulphur is coupled with the conversion of inorganic 

 phosphate from the medium into organic phosphate 

 esters in the cells, and that when carbon dioxide is utilised, 



