THE SULPHUR CYCLE 241 



Van Delden isolated an organism causing the reduction 

 of sulphates in sea water, and found that it closely resembled 

 Spirilhim desulphuricans ; he named it Microspira estuarii. 

 Both these organisms, although as above stated they do not 

 need large quantities of nitrogen, are not inhibited in their 

 growth by organic matter, if they are present in pure culture. 

 Under natural conditions the presence of organic matter 

 facilitates the growth of other organisms, to the detriment of 

 the sulphate reducing spiriUae. 



The reduction of sulphates is of special importance in 

 relation to the discharge of sewage into sea water. There is 

 no doubt that, in absence of sufficient dilution, putrefaction 

 may set in, resulting, in the case of sea water, in sulphuretted 

 hydrogen production ; so that the nuisance may be much 

 greater in the case of discharges into sea water than into 

 fresh water. 



Sulphate reduction has been compared to de-nitrification ; 

 it will be remembered that in the case of the reduction of 

 nitrates the oxygen of the nitrate with the assistance of the 

 de-nitrifying organism combined with the organic matter 

 present. A similar reaction appears to take place in the case 

 of sulphate reduction ; thus in the above described solution, 

 where the chief source at any rate of oxidisable material is 

 sodium lactate, Van Delden suggests the following equation : — 

 2C3H503Na + 3MgS04 



= SMgCOj + NajCOg + 200^ + 2H2O + SH^S 



Experimental evidence supports the above equation fairly 

 well. 



II. Oxidation of Sulphur. — UnUke amjnonia, whose direct 

 oxidation by purely chemical means has been shown to take 

 place to only a limited extent in nature, hydrogen sulphide 

 readily oxidises in a variety of ways. The simplest is the 

 direct oxidation to water and sulphur according to the simple 

 equation : — 



H-jS + O = HjO + S 



