40 SULPHUR BACTERIA 



when decomposed. This apphes in particular to urea and 

 glycocoH. 



(2) Undoubted increase of growth has been observed after 

 addition of organic matter (Mohsch, Nadson, Skene). 



(3) Undoubted use is made of organic matter by the closely 

 related non-sulphur purple bacteria (Molisch). 



(4) Ammonium sulphate can be replaced by calcium nitrate 

 (Nadson, Ellis), suggesting the absence of specificity. 



(5) In Bavendamm's cultures the individuals did not show 

 the same healthy growths which characterize the cultures in 

 nature, suggesting that the growth was weak from the lack of 

 certain ingredients which are supplied cither directly or in- 

 directly by organic matter. 



(6) The same lack of robustness characterized the present 

 writer's cultures of Thioporphyra volutans in a medium from 

 which organic matter was excluded (calcium nitrate being 

 used as the source of nitrogen). 



Whilst individually each of these points is inconclusive, 

 collectively they appear to justify the conclusion that the 

 sulphur bacteria satisfy their nitrogen requirements in more 

 ways than one, and that organic matter plays a subordinate 

 role as a source of nitrogen. This is rendered all the more 

 probable because 



(1) They grow best in waters containing an abundant 



supply of organic matter. 



(2) Their artificial cultures from which organic matter was 



excluded do not show the same robust growths which 

 characterize their development under natural con- 

 ditions. 



(3) The maintenance of growth is dependent on substances 



that are derived from organic matter. 



3. The Source of Carbon. — Particular attention has been 

 paid to this aspect of nutrition by Skene and by Bavendamm. 

 Using ammonium sulphate as the source of nitrogen, and 

 introducing small amounts of various organic compounds, 

 Skene failed to obtain growth with any of the carbon com- 

 pounds with which he experimented. He concluded that they 



