68 BIOLOGY AND TECHNIQUE 



or thiobacteria. Red, purple, and colorless, these bacteria all possess 

 the power of utilizing sulphuretted hydrogen and by its oxidation 

 into free sulphur obtain the energy necessary for their metabolic 

 processes. The colorless sulphur bacteria, the Beggiatoa and 

 Thiothrices, usually appear as threads or chains which, in media 

 containing sufficient H 2 S, are usually well-stocked with minute 

 globules of sulphur. If found upon decomposing organic matter, 

 they often cover this as a grayish mold-like layer. The red sulphur 

 bacteria, of which numerous species have been described by Wino- 

 gradsky, may appear as actively motile spirilla (thiospirillum) or 

 as short, thick bacillary forms. 



The physiology of all the sulphur bacteria, and especially of the 

 colored varieties, is of the greatest interest in that these micro- 

 organisms are among the few members of the bacterial group which 

 behave metabolically like the green plants. The higher organic 

 substances play little or no part in the nutrition of these micro- 

 organisms. Strictly aerobic, the colorless thiobacteria are inde- 

 pendent of sunlight, while the red and purple varieties exhibit their 

 physiological dependence upon light by accumulating under natural 

 conditions in well-lighted spots. Both varieties possess equally the 

 power of oxidizing sulphuretted hydrogen as a source of energy. 

 The sulphur is then stored as elemental sulphur within the bacterial 

 body and when a lack of food stuffs sets in, the store of sulphur 

 can be further oxidized into sulphurous or sulphuric anhydrides. 

 With this sole source of energy, these bacteria are capable of flourish- 

 ing aerobically, while an absence of H 2 S, even in the presence of 

 organic food stuffs, leads to a rapid disappearance of their sulphur 

 contents and an inability to develop. 



In the case of the colored thiobacteria, the red pigment appears 

 to fulfil, to some extent, a function comparable to that of the 

 chlorophyll of the green plants. 



Engelmann, 33 who has studied this pigment spectroscopically, has 

 found that besides absorbing the red spectral rays there is an ab- 

 sorption of rays on the ultra-red end of the spectrum. The absorp- 

 tion of the red rays between the lines B and C of the spectrum, 

 and of violet rays at the line F, is the same as that of the absorption 

 spectrum of chlorophyll, and it is in the zone of these rays that the 

 physiological effects of cholorophyll are most active. In addition 



** Engelmann, Bot. Zeit., 1888. 



