PRODUCTS FROM NITROGENOUS COMPOUNDS 93 



Or the nitrate may be completely reduced with the liberation of 

 gaseous nitrogen, thus completely removing it from the soil. 



2Ca(NO 3 ) 2 = 2CaO + 2N 2 + 5O 2 



Sulphates may in a similar manner be reduced to hydrogen sulphid. 



H 2 S0 4 H 2 S + 2O 2 



Hence, water containing calcium sulphate if shut off from air may 

 give rise to that ill-smelling gas, hydrogen sulphid. 



Pigments. Many bacteria produce pigments, among which are 

 practically all the colors of the spectrum violet, indigo blue (B. 

 molaceus, B. janthinus, B. cyanogenes, B. pyocyaneus), green (B. 

 fluorescens) , yellow (Staphylococcus aureus, Sarcina lutea), orange 

 (Sarcina aurantiacd), and red (B. prodigiosus) . Usually oxygen is 

 essential to the production of pigments and their intensity varies, 

 depending upon the media upon which the organism is grown. 



The phenomenon of pigment production has long attracted the 

 attention of bacteriologists, and many attempts have been made to 

 explain their occurrence; but so far none of the explanations would 

 seem to be wholly satisfactory. The pigment seems to be of no 

 material advantage to the organism, for colorless strains may be 

 cultivated which possess all of the properties of the original strain 

 with the exception of pigment production. There is no evidence 

 that they protect the organism against light, nor is there anything 

 that would lead to the belief that (analogous to hemoglobin) they 

 form a loose combination with the oxygen which under certain 

 circumstances may be liberated. The pigment does not make it 

 possible for the organisms to assimilate carbon dioxid as does the 

 chlorophyll of the higher plants in the majority of cases. The best 

 evidence, therefore, points to the conclusion that they are mere 

 by-products that have no particular meaning to the organism. 



Beijerinck divides chromogenic bacteria into three classes: 



1. Chromophorous bacteria, in which the pigment remains within 

 the cell and has a certain biological significance analogous to the 

 chlorophyll of higher plants. To this class belong the green bacteria 

 and the red sulphur bacteria, or^purple bacteria. 



2. Chromoparous, or true pigment-forming bacteria, which set free 

 the pigment as a useless excretion, either as a color-body or as a 

 leuco-body which becomes colored through the action of atmospheric 

 oxygen. The cells themselves are colorless and may under certain 

 conditions cease to produce pigments. To this class belong B. 

 prodigios'ws and others. 



3. Parachrome bacteria which form their pigment as an excretory 

 product but retain it within their body, as B. janthinus and others. 



The chemical nature of pigments is not well understood, but it is 

 known that they differ in solubility and are usually classified 

 according to solubility in water, alcohol, chloroform, ether, benzol, 



