TYPES OF AUTOTROPHIC BACTERIA 101 



Molisch (1907) disagreed with both Engelmann and Vinogradsky, 

 He thought that purple bacteria are not autotrophic at all, but photo- 

 heterotrophic, i. e., that they require organic nutrients but are capable of 

 assimilating them only in light. This confused situation was clarified by 

 van Niel and coworkers in several important papers (van Niel 1930, 1931 ; 

 van Niel and Muller 1931; Muller 1933, Roelefson 1934, van Niel 1935, 

 19361' 2, 1937; Foster 1940; review by van Niel 1941). Significant con- 

 tributions to this field also were made by Gaffron (1933, 1934, 1935i'2) 

 as well as by French (1936, 19371-2), Wessler and French (1939), Eymers 

 and Wassink (1938), Nakamura (1937i'2, 1938i'2, 1939) and Sapozhnikov 

 (1937). 



The two main results of van Niel's investigations were as follows : 



(1) Engelmann, Vinogradsky and Molisch all observed correctly, but 

 used different organisms. There are two kinds of sulfur bacteria: pig- 

 mented, pJiotmdotrophic sulfur bacteria (Engelmann) ; and nonpigmented, 

 chemautotrophic sulfur bacteria (Vinogradsky). In addition, there is a 

 second kind of pigmented bacteria, the heterotrophic purple bacteria 

 (Molisch). 



{2) In the photosynthesizing sulfur bacteria the oxidation of hydrogen 

 sulfide is not an independent process, coupled with normal photosynthesis 

 through the intermediary of free oxygen, but is a part of the photo- 

 synthetic mechanism itself. The photosynthesis of these bacteria differs 

 from that of the higher plants, in that hydrogen sulfide takes the place of 

 water as reductant. (Gaffron suggested that this type of photochemical 

 metabolism be designated as photoreduction rather than photosynthesis.) 

 However, hydrogen sulfide is not the only reductant which the purple bac- 

 teria can use; in contrast to the photosynthesis of the higher plants, their 

 metabolism is highly adaptable. Some species prefer certain specific re- 

 ductants; but others can use indiscriminantly a large variety of hydrogen 

 donors. Therefore, only a tentative classification of pigmented bacteria 

 according to their normal photosynthetic function is possible. Table 5.1, 

 taken from van Niel (1941), shows the three main classes: green sulfur 

 bacteria, purple sulfur bacteria (Thiorhodaceae) , and purple "nonsulfur" 

 bacteira (Athiorhodaceae) . (Franck and Gaffron designated them as green, 

 red and purple bacteria, respectively; actually the color of both Thio- 

 rhodaceae and Athiorhodaceae can be purple, bright red or brown, de- 

 pending on the nature of the carotenoids associated with the green 

 "bacteriochlorophyll.") The pigment of green bacteria is the so-called 

 " bacterioviridin " (page 445), whose structure probably is intermediate 

 between those of bacteriochlorophyll and ordinary chlorophyll. 



Table 5.1 shows the variety of compounds which purple sulfur bacteria 

 can use for the reduction of carbon dioxide. Sapozhnikov (1937) found 

 that selenium can be substituted for sulfur. The purple nonsulfur 



