THE METABOLISM OF THE SULPHUR BACTERL4 43 



organisms Thiothrix and Beggiatoa were isolated by Keil 

 in 191 2 their oxygen requirements were not ascertained 

 in pure culture experiments. They are, however, niicro- 

 aerophilic, as are the coloured forms, so far as may be 

 judged from the manner of their growth and their physio- 

 logical requirements. 



Observations on Thioporphyra volutans. — No specific ex- 

 periments have been made to determine the physiological 

 requirements of this organism, but it was observed that its 

 growth in various mixed cultures invariably reached its 

 optimum when the supply of oxygen had been nearlv ex- 

 hausted by the multiplication of the saprophytic bacteria. It 

 thus thrives best under micro-aerophilic conditions. On one 

 occasion, however, it was found to be growing in abundance 

 on wooden piles only some six inches or so below the sur- 

 face in the open water of the Clyde. Hence one member of 

 the coloured sulphur bacteria at any rate is not hindered 

 in its growth by the presence of an abundant supply of 

 oxygen. 



It may thus be concluded that the majority of the sulphur 

 bacteria, both coloured and colourless, are micro-aerophilic, 

 but that one at least [Thioporphyra volutans), and probably 

 more, thrive in highly oxygenated waters ; and it is highly 

 improbable that these do not absorb oxygen in quantity. 

 A diversity in the oxygen requirement is to be expected of 

 organisms that thrive under such diverse conditions as the 

 various members of the sulphur bacteria. 



5. Mineral Matters. — In all plants the presence of extremely 

 small quantities of various elements, over and above those 

 that enter into the constitution of the protoplasmic molecule, 

 is necessary for healthy growth. Some may be required in 

 fairly large quantities, as, for example, calcium when used to 

 combine with the oxalic acid formed in the leaf during meta- 

 bolism. 



In cultivating bacteria it is usually found advantageous 

 to add to the culture, not pure water, but a solution containing 

 various inorganic salts in solution. The mixture which has 

 hitherto been used with the greatest success for the sulphur 



