OXIDATION OV SULl rii IX THIO SOIL 251 



these go rapidly into solution by the action of (iio acid, thus 

 tencUns to change the reaction towards alkaUno. Cas (P04)2 

 offers the best material for the jiurjwse, because, on dissolving, 

 it gives an acid salt and an insoluble residue (CaSO*2H20). 



Mechanism of sulfur oxidation. The sulfur is oxidized, accord- 

 ing to the following reaction: 



So + 30, + 2H2O = 2H..SO4 + 282 Cal. (1) 



In the presence of tri-calcium ])hosphate: 



Cas (PO^), + 2H,S04 = Ca (H,P04)2 + 2 CaS04 (2) 



Ca (HoPOO^ + H,S04 = 2 H3PO., + CaS04. (3) 



The energy liberated in the oxidation of sulfur is used bj^ 

 the organism for its activities. The acid formed interacts with 

 the neutralizing agents of the medium, giving first mono-cal- 

 cium iihosi)hate, at a pH of about 2.8-3.0, then phosphoric 

 acid. So that, at a condition of equilibrium, we have a mix- 

 tuie of phosphoric and sulfuric acids, and the calcium salts of 

 these acids, the condition of eciuililtriuni depending on the stage 

 of oxidation. 



Taxonomic considerations. The first paper of this series con- 

 tains a study of the five groups of sulfur bacteria. The organism 

 descri])ed in this pa]ier, ThiobaciUus thiooxidnns n. sp., is placed 

 in a fifth group, which includes members morphologically related 

 to the members of the fourth group, but which are distinctly 

 different physiologically. 



Group four includes colorless sulfur bacteria which do not 

 accunuilate sulfur within their cells, but which produce an 

 abimdance of sulfur ( from US and thiosulfates) outside of their 

 cells. This groiqi of bacteria is the one closely related to the 

 organism studied in this jniper and will, therefore, be discussed in 

 greater detail. Ciroup four is represented by two bacteria, 

 ThiobaciUus thioparus (Nathanson) Beijerinck and ThiobaciUus 

 denilrificans Beijerinck. Group five, which is so far represented 

 only by ThiobaciUus Ihiuoxidans n. sp., will include colorless 

 sulfur-oxidizing bacteria which do not accumulate sulfur either 



