NUTRITION OF THE AUTOTROPHIC BACTERIA 69 



bacteria, are facultative autotrophs. Evidence is ac- 

 cumulating that those organisms which are now regarded 

 as obligate autotrophs may all be capable of growi^h 

 in the presence of organic matter if the proper conditions 

 can be discovered. 



The autotrophic bacteria show a wide variety of 

 morphology, ranging from coccal forms and rod forms 

 to multicellular filamentous forms. The photosynthetic 

 autotrophs contain pigments, for example the bacterio- 

 purpurin of the purple sulphur bacteria, which act as 

 respiratory pigments much like the chlorophyll of green 

 plants . 



We can classify the autotrophic bacteria on the basis 

 of their metabolic activities, as follows : — 



A. Oxidise Nitrogen Compounds. 



1. Ammonia to nitrite, e.g. Nitrosomonas, Nitro- 



sococcus. 



2. Nitrite to nitrate, e.g. Nitrobacter. 



B. Oxidise Sulphur or Sulphur Compounds. 



1. Simple bacteria, e.g. Thiobacillus. 



(a) Obligate autotrophs — 

 (i) Aerobic 



(a) Nearly neutral conditions, 



e.g. Th. thioparus. 

 (jS) Acid conditions, e.g. Th. 

 thio-oxidans. 

 (ii) Anaerobic, e.g., Th. denitrificans. 

 (6) Facultative autotrophs. 



2. Higher bacteria (complex morphology). 



(a) Colourless, e.g. Beggiatat, T hi othrix, etc. 



{b) Red or purple pigmented, e.g. ThiQ-' 

 cystis, etc. 



