SULPHUR 179 



later classified as Microspira desulphuricans which possessed the 

 power of reducing sulphates. Another sulphate-reducing organism 

 is Msp aestuarii. These organisms act only in the presence of 

 organic matter: 



MSO 4 +'' 2C = 2CO 2 + MS 



The true sulphur bacteria possess a directly opposite physiological 

 action to the reducing bacteria. There are two genera of the 

 true sulphur bacteria recognized Beggiatoa and Thiothrix. Beggia- 

 toa is filamentous, motile, and morphologically resembles the blue- 

 green alga, Oscillaria. Thiothrix is not filamentous nor motile and 

 possesses a sheath and forms spores. The sulphur bacteria contain 

 in their protoplasm highly refractive inclusions of amorphous sulphur. 

 According to Winogradsky, a single Beggiatoa thread used in a day 

 two to four times their own weight of hydrogen sulphid with the 

 production of sulphur: 



4H 2 S + 2O 2 = 4H 2 O + 4S 



The sulphur seen within the cell protoplasm is to be looked upon as 

 an intermediate state in the oxidation process, for if the organisms be 

 transferred to fresh water these soon disappear with the formation 

 of sulphuric acid: 



2S + 3O 2 + 2H 2 O = 2H 2 SO 4 



This reacts with a base, usually calcium carbonate, with the forma- 

 tion of calcium sulphate : 



CaCO 3 + H 2 SO 4 = CaSO 4 + H 2 O + CO 2 



There are also organisms in soil that can oxidize sulphur to sulphuric 

 acid which in turn would act as a solvent for plant-food. Moreover, 

 small quantities of sulphur added to a soil will increase ammonifica- 

 tion. It is likely that much of the benefit resulting from sulphur 

 fertilization is due to these factors. 



Brown has recently shown the power of oxidizing sulphur to vary 

 with different soils. Aeration and optimum moisture favor it, 

 whereas the addition of carbohydrates, depresses the process. He has 

 elaborated a method of measuring the speed of sulphur oxidation in 

 soils and given to it the name of sulphofication. 



According to Lafar, the importance of the sulphur bacteria in the 

 economy of nature is unmistakable. In cooperation with the sulphate- 

 reducing bacteria they insure that the sulphur cycle pursues an un- 

 interrupted course, the elements being taken up by the higher plants 

 in the condition of sulphates and deposited in the cells in the form of 

 organic compounds from which, in the course of putrefaction, sulphur 

 is liberated as hydrogen sulphid, and finally reconverted into sulphates 

 by the sulphur bacteria. It then recommences its course through the 

 higher plants. 



