232 Transformation of Mineral Substances 



water; (c) as sulfates, which are added to the soil in certain mineral 

 fertilizers, such as superphosphates and gypsum. 



The sulfur content of plant materials varies from a small fraction 

 of 1 per cent, as in rye straw, to almost 1 per cent, as in turnip tops. 



Sulfur and sulfur compounds in the soil are subject to numerous 

 transformations resulting directly or indirectly from the activities of 

 different groups of microorganisms. When plant and animal resi- 

 dues undergo decomposition, whether in the soil or in the compost, 

 the sulfur-bearing constituents, notably the proteins and certain glu- 

 cosides, are hydrolyzed. The proteins give rise to the sulfur-bearing 

 amino acid cystine; the glucosides yield mustard oil and other sul- 

 fur compounds. 



The chemical structure of two typical sulfur-bearing compounds 

 may be presented as follows: 



dl2 S b 0x12 



I I 



NH, HC CH NH2 



I I 



HOOC COOH 



Cystine 

 CH2SO0OH 



I 

 CH2NH2 



Taurine 



In recent studies on antibiotic products of microbial metabolism, 

 sulfur has also been found as an essential constituent of such im- 

 portant compounds as penicillin. 



These compounds can be attacked by various bacteria and other 

 microorganisms. Only a small part of the sulfur is consumed by 

 these organisms. Most of it is liberated as hydrogen sulfide. Under 

 anaerobic conditions, other sulfur compounds, such as mercaptans, 

 may also be produced. These are largely responsible for the pungent 

 odor which is always present when animal residues and certain 

 protein-rich materials undergo decomposition in absence of avail- 

 able oxygen. Under aerobic conditions, both hydrogen sulfide and 

 mercaptans are rapidly oxidized further to sulfates. 



Among the bacteria responsible for the transformation of sulfur 

 in nature, those that are able to oxidize the elemental sulfur and 

 simple compounds of sulfur are of particular interest. These bacteria 

 are known as "sulfur bacteria." They belong to different morpho- 

 logical and physiological groups, some being small, a micron or so 



