42 SULPHUR BACTERIA 



concentration as in the atmosphere, and that they throve 

 abundantly in a medium if it was almost devoid of oxygen. 

 When cultivated in drop cultures the bacteria retreated to 

 the centre of the drop, and remained there so long as the 

 oxygen pressure was not changed. Winogradsky ascer- 

 tained that while oxygen at ordinary pressure was actually 

 harmful, it was necessary in very small amounts. He did 

 not experiment with organisms growing in media completely 

 free from oxygen. 



Nadson advanced the opinion that sulphuretted hydrogen 

 was assimilated for protection against the poisonous effects 

 of oxygen rather than for purposes of nourishment. He 

 regarded the sulphur bacteria as obligate anaerobes. Molisch 

 and Skene, like Winogradsky, considered that a small amount 

 of oxygen was necessary, and maintained that without it the 

 bacteria could not oxidize sulphuretted hydrogen first to 

 sulphur and then to the sulphate. Winogradsky and Skene 

 thought that they had found sufficient of this gas to satisfy 

 the needs of the sulphur bacteria in the carbon assimilation 

 of small green organisms that were always found in natural 

 waters in the neighbourhood of the sulphur bacteria. This 

 idea is not convincing, for the sulphur bacteria are fre- 

 quently found in natural waters in which the supply of green 

 organisms is either non-existent or is inadequate for such a 

 purpose. 



Bavendamm, working with pure cultures, found that 

 Lamprocystis and Chromatiiim throve well in a medium com- 

 pletely devoid of oxygen, but that both these organisms 

 developed at their best when supplied with oxygen at a pressure 

 of 5 -2 mm. of mercury. 



The conclusions of Bavendamm may be taken as final for 

 the coloured sulphur bacteria, which resemble nearly all, if 

 not all, so-called obligate anaerobes in that, whilst they are 

 able to grow in a medium completely devoid of oxygen, yet 

 assimilate this gas if it be presented to them in a sufficiently 

 dilute form. 



The above remarks apply only to the relationship of 

 the coloured forms to oxygen. Although the colourless 



