OXIDATION OF SULFUR IN THE SOIL 253 



within or without their cells, which are very small in size (a micron 

 or less in length and 0.5 micron in diameter) and which oxidize 

 sulfur rapidly to sulfuric acid with a very acid reaction. 



ThiobaciUus thioparus was demonstrated by Nathanson in 

 sea water and by Beijerinck in canal water. It was isolated on a 

 medium contahiing sodium thiosulfate as a source of sulfur, in 

 addition to minerals and ammonium chloride (0.01 per cent) 

 and sodium bicarbonate as a source of carbon. In two to three 

 days, the surface of the medium became covered with free sulfur 

 filled with bacteria. This organism is 3 by 0.5^, not forming 

 any spores, is very motile and very sensitive, dying out on the 

 plate in a week. The thiosulfates can be replaced by CaS, H2S 

 and elementary sulfur. 



Jacobsen dissolved the sulfur in sodium sulfide and precipi- 

 tated with dilute hydrochloric acid, washed and dried, then 

 added to a medium containing 100 parts of water, 0.05 K2HPO4, 

 O.O5NH4CI, 0.02 MgClo, 2 of CaCOs or MgCOs and a trace of 

 FeCls (3 parts of NaCl were used in the case of the organism iso- 

 lated from sea water). The cultures were incubated at 30°C. 

 The organism was found to form a film on the surface of the 

 culture, with sulfur granules surrounding the cells; at the end, 

 instead of sulfur, only a slimy bacterial mass was found to remain. 

 Traces of hydrogen sulfide were always found. Pure cultures 

 of the organism were obtained on agar plates, using 0.5 per cent 

 of sodium thiosulfate and some CaCOs. The carbon dioxide 

 is obtained from carbonates, no growth being obtained and no 

 sulfuric acid produced in the absence of carbonates. The or- 

 ganism is autotrophic since it does not require any organic matter 

 for its development ; it is sometimes motile and sometimes non- 

 motile. 



ThiobaciUus denitrificans was isolated by Beijerinck by adding 

 to 100 parts of canal water, 10 parts of powdered sulfur, 0.05 

 KNO3, 0.02Na2CO3, 2CaC03, O.O2K2HPO4 and 0.01 part of MgCl^, 

 and incubating the medium at 30°C. The sulfur was oxidized 

 and growth was accompanied by a reduction of the nitrate to 

 atmospheric nitrogen. The organism was isolated on agar plates 

 and was found to be a motile, short rod, hardly distinguishable 



