THE BIOLOGICAL ACTIVITIES OF BACTERIA 61 



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

 colored varieties, is of the greatest interest in that these microorganisms 

 are among the few members of the bacterial group which behave meta- 

 bolically like the green plants. The higher organic substances play lit- 

 tle or no part in the nutrition of these microorganisms. Strictly aerobic, 

 the colorless thiobacteria are independent 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 flourishing aerobically, while an absence of H^, 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, 1 who has studied this pigment spectroscopically, has 

 found that besides absorbing the red spectral rays there is an absorption 

 of rays on the ultra-red end of the spectrum. The absorption 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 chlorophyll are most active. In addition to these absorp- 

 tion bands, the bacteriopurpurin of the red sulphur bacteria shows 

 absorption of the invisible ultra red rays of the spectrum. 



Engelmann, with a microspectroscope, projected a spectrum into 

 a miscroscopic field in which green algse or, in the case under discussion, 

 red sulphur bacteria had been placed. Other sources of light were, of 

 course, excluded. By adding emulsions of strictly aerobic bacteria to 

 such preparations, an accumulation of microorganisms was observed 

 at those points in the spectrum at which most oxygen was liberated. In 

 the case both of chlorophyll and of the red sulphur bacteria such areas 

 of bacterial accumulation (in oxygen liberation) occurred in the zones 

 of the absorption bands mentioned above. 



Engelmann, Bot. Zeit., 1888. 



