100 PHOTO- AND CHEMOSYNTHESIS OF BACTERIA CHAP. 5 



and demonstrated that purple bacteria can develop in complete darkness 

 if they are exposed to infrared radiation. Engelmann thought that 

 purple bacteria are normal photosynthesizing organisms, although he 

 was unable to prove, even by means of the extremely oxygen-sensitive 

 motile bacteria, that they produce oxygen in light. He suggested that 

 all oxygen formed by purple bacteria is immediately utilized for the 

 oxidation of sulfide to sulfur. 



The inability of purple bacteria to produce oxygen was confirmed by 

 Molisch (1907) and van Niel (1931), by means of the even more sensitive 

 luminous bacteria of Beijerinck. On the other hand, Czurda (1936), 

 who observed the oxidation of leuco dyes by purple bacteria in light, and 

 Nakamura (1937), who noticed the decrease in their oxygen consumption 

 in light, interpreted these results as indirect evidence of a photochemical 

 production of oxygen. Van Niel (1941) suggested that the first observa- 

 tion can be explained by the utilization of the leuco dye as reductant in 

 photosynthesis, while the second one proves merely that the respiration 

 of purple bacteria is inhibited by hght (c/. page 111). Van Niel exposed 

 dense suspensions of purple bacteria, mixed with luminous bacteria, to 

 prolonged illumination in closed bottles, without ever being able to 

 detect the slightest traces of oxygen. The indirect arguments of Czurda 

 and Nakamura do not avail against these direct proofs, as was later 

 conceded by Czurda (1937). 



While Engelmann thought that purple bacteria are normal photo- 

 synthesizing organisms, whose oxygen output is used up by a secondary 

 dark metabolic process, Vinogradsky (1887, 1888) saw in this dark 

 metabolism the main source of organic matter in the bacteria. He 

 based this view on analogies with the colorless chemautotrophic sulfur 

 bacteria, which derive the energy required for organic synthesis, from 

 the chemical oxidation of sulfide by oxygen. 



If Vinogradsky's conception was correct, why should light be at all 

 necessary for the development of purple bacteria? Vinogradsky, and 

 Skene (1914) offered the following explanation. Purple bacteria thrive 

 only under anaerobic conditions ; they are thus unable to use atmospheric 

 oxygen for the oxidation of sulfide. Vinogradsky and Skene surmised 

 that purple bacteria live in symbiosis with green photosynthesizing 

 bacteria, the latter supplying them with oxygen of such low partial 

 pressure as not to disturb their anaerobic metabolism. However, this 

 hypothesis had to be abandoned when pure cultures of purple bacteria 

 were obtained and found capable of independent growth in light. Buder 

 (1919, 1920) suggested that photosynthesis is carried out by the purple 

 bacteria themselves, to supply the small quantities of oxygen they 

 require for the oxidation of sulfide. He maintained that the latter is 

 their main source of metabolic energy. 



