82 PLANT PHYSIOLOGY 



The sulphur bacteria (Beggiatoa and Thiothrix) which are found 

 in bogs, sulphur springs, and in the ocean, play a similar part in 

 the sulphur cycle and help to keep the sulphur in an available shape 

 to be used by higher organisms. These bacteria oxidize hydrogen 

 sulphide to sulphur and water: 



2H 2 S+0 2 = 2H 2 0+S 2 +122.2 cal. 



Here there seems little doubt that the bacteria, which seem so 

 morphologically simple and are yet so physiologically complex, 

 are able to carry on chemosynthesis and then to use the sulphur 

 which results and is stored up inside the cell as a source of energy, 

 oxidizing it later in the presence of water to sulphuric acid: 



S 2 +2H 2 0+30 2 = 2H 2 S0 4 +284.4 cal. 



This oxidation takes place only in the presence of sufficient calcium 

 salts to neutralize the sulphuric acid produced, thus forming 

 calcium sulphate: CaC0 3 +H 2 S04 = H 2 C0 3 +CaS04. The car- 

 bonate or carbonic acid released during this neutralization process 

 is then used up by the bacteria to form carbon compounds. These 

 forms are especially important in the ocean where the quantities 

 of organic matter which fall to the ocean floor are gradually de- 

 composed, with the elimination of large quantities of hydrogen 

 sulphide, which, if it reached the upper layers of water, would 

 poison them and make them unfit for life. This the sulphur bacteria 

 prevent. Also in the soil are sulphur bacteria which can oxidize 

 the sulphur of proteins to elementary sulphur, which is either 

 eliminated as such or oxidized to sulphuric acid. Thiobacillus 

 thio-oxidans is one such form which has been isolated in pure cul- 

 tures and which can oxidize sulphur to sulphuric acid. It grows 

 best in a very acid solution (pH 2.5) and will even grow in Normal 

 sulphuric acid. With the energy derived from this oxidation it 

 can take the carbon from carbon dioxide, the nitrogen from 

 ammonia, and thus make its own foods. It may, therefore, repre- 

 sent one of the earliest forms of life on the earth. 



Although hydrogen is produced in large amounts, it is found in 

 the air in only small quantities. One explanation of this has been 

 given by Kaserer (1906) and Niklewski (1910), who have showed 

 that there are certain bacteria, which can oxidize hydrogen. From 

 a chemical viewpoint, hydrogen is the best possible source of energy 

 because of its high heat of combustion (34.6 kg. cal. per gram), 



