Chapter 5 

 PHOTOSYNTHESIS AND CHEMOSYNTHESIS OF BACTERIA 



A. Bacterial Photosynthesis* 

 1. Types of Autotrophic Bacteria 



Most bacteria are heterotrophic organisms, that is, unlike the auto- 

 trophic green plants, they are unable to synthesize their organic matter 

 directly from carbon dioxide, but require organic nutrients, in common 

 with animals and fungi. They live either in host organisms as parasites 

 or in media containing organic decay products. 



However, there are two groups of bacteria which are exceptions to 

 this rule. The first group, which consists of photautotrophic bacteria, 

 is capable of reducing carbon dioxide to organic matter in light, using 

 hydrogen sulfide, thiosulfate, hydrogen or other inorganic or organic 

 reductants (but not water, as do the higher green plants). Green and 

 purple sulfur bacteria are representatives of this group; they thrive in 

 sulfide-containing media, and most of them are more or less strictly 

 anaerobic. 



A second group is formed by chemautotrophic bacteria, colorless 

 organisms which reduce carbon dioxide to organic matter in the dark, 

 by coupling this reaction with different energy-releasing chemical proc- 

 esses. They live in media containing oxidizable substances (sulfide, 

 ferrous iron, methane, etc.) and generally need oxygen (although some 

 can use nitrate instead). 



The photosynthetic activity of purple bacteria was discovered by 

 Engelmann in 1883. At first, he merely noticed their phototropism 

 which is similar to that of the motile green algae. Under the microscope, 

 the purple bacteria could be observed gathering in a beam of light. 

 Later (1888), Engelmann proved that these bacteria cannot develop in 

 absence of light. If a spectrum is thrown on a culture of purple bacteria, 

 they grow only in the absorption bands of the green ''bacteriochloro- 

 phyll" which is found in all of them, together with a variable assortment 

 of carotenoids (c/. Eymers and Wassink 1938). The strongest absorption 

 band of bacteriochlorophyll lies in the near infrared. Engelmann (1888) 

 pointed out that here for the first time, invisible light appeared to be 

 active in photosynthesis; later, Dangeard (1921, 1927) confirmed this 



* Bibliography, page 125. 



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