INTRODUCTION 7 



in the flora of three sulphur wells in West Galicia. In these 

 the following grouping prevails : — 



1. First Zone.— Rich in HgS ; yellow-green Cyanophycece 

 and purple bacteria, particularly in the case of the latter, the 

 more motile forms. Absence of Beggiatoa, Diatomacece, and 

 Chlorophycece. 



2. Second Zone. — Less rich in H2S (0-4 gram per 10 

 kilogram H2O). Fewer of Cyanophycece, a few Diatomacece, 

 and no Beggiatoa. 



3. Third Zone. — Very small amount of HgS. Rich in 

 Diatomacecs, BeggiatoacecE, Chlorophycece and non-thermoph- 

 ilous Cyanophycece. Complete absence of purple bacteria. 



The Sulphur Cycle in Nature. 



Plant and animal remains are attacked by hordes of 

 saprophytic bacteria, and, after death, the protoplasmic 

 molecule undergoes a number of transformations. Such a 

 molecule may be compared to an accumulator as a storehouse 

 of energy. The bacteria which nature uses as scavengers 

 liberate some of the stored up energy in the molecule, and 

 at the same time obtain material for their sustenance. In 

 essentials the protoplasmic molecule is composed of a complex 

 of carbon, hydrogen, oxygen, nitrogen, sulphur, and phos- 

 phorus atoms. With each transformation the level of energy 

 is brought lower, and the molecule becomes less complex 

 in structure. The final stage is reached when the original 

 component atoms of the protoplasmic molecule have become 

 changed to carbon dioxide, water, nitrates, sulphates, and 

 phosphates. After complete oxidation the protoplasmic 

 molecule has descended to its nadir position as a source of 

 energy. 



Sulphur is liberated as a constituent of the amino-acids 

 cystine and the associated cysteine, and sulphuretted hydrogen 

 is formed, either directly or indirectly, from one or other of 

 these compounds. This sulphide is then absorbed by the 

 sulphur bacteria, and the sulphur ultimately appears in the 

 surrounding water in the form of sulphates. The develop- 

 ment of sulphates is not, however, a simple progression, for 



