106 PHOTO- AND CHEMOSYNTHESIS OF BACTERIA CHAP. 5 



sulfur), or is unlikely to yield it (as in the case of the bisulfite ion, HSOs"), 

 will be demonstrated in chapter 9 (page 220). 



Van Niel's generalized concept of bacterial photosynthesis adds an 

 important argument in favor of the "intermolecular oxidation-reduction" 

 theory of normal photosynthesis, and against the Willstatter-Stoll 

 "internal rearrangement" theory. The easily verifiable fact that in the 

 photosynthesis of sulfur bacteria, the reduction of carbon dioxide leads 

 to the production of sulfur, and not of one part of sulfur and two parts 

 of oxygen (or of sulfur dioxide) is analogous to the fact — proved with 

 much more difficulty by the radioactive isotope experiments of Ruben, 

 Randall, Kamen and Hyde (page 55) — that all oxygen in ordinary photo- 

 synthesis originates in water. 



3. Combined Photosynthesis and Heterotrophic Assimilation 

 of Photoheterotrophic Bacteria 



The metabolism of the "photoheterotrophic" bacteria — that is, bac- 

 teria which require light for the assimilation of organic nutrients, seemed 

 at first to be quite different from that of the " photautotrophic " bacteria 

 discussed above. However, van Niel made it plausible that the organic 

 nutrients serve primarily (although not exclusively) as hydrogen donors, 

 so that the generalized equations (5.8), with R now standing for an 

 organic radical, apply to these organisms as well. 



The study of the photosynthesis of heterotrophic bacteria had at first 

 encountered difficulties, because the organisms employed were found to 

 require yeast extracts or peptones, and would not thrive in solutions of 

 pure organic compounds. However, this difficulty was overcome by 

 Gaffron (1933) and Muller (1933). Muller used Thiorhodaceae, which 

 were found capable of subsisting not only in sulfide media but also in fatty 

 acid solutions; while Gaffron (1933, 1935) found that the Athiorhodacea, 

 Rhodovibrio parvus, grown in a yeast extract, can be transferred into a 

 simple organic solution for the study of its photosynthetic activity. 

 Similar observations were made by van Niel (1941) with Spirillum 

 rubrum. In these investigations, fatty acids were used as organic hydro- 

 gen donors. However, the utilization of these compounds goes far 

 beyond the contribution of one or two hydrogen atoms. In fact, 

 Muller and Gaffron found that the acids are completely used up, leaving 

 no organic residue at all. It is formally possible to explain this complete 

 assimilation in terms of photochemical dehydrogenation, by assuming 

 the transfer of all hydrogen atoms to carbon dioxide, and a conversion 

 of all carbon atoms into carbon dioxide (c/. Eq. 5.15). However, this 

 explanation is speculative; at least a part of the carbon atoms could be 

 assimilated directly, without taking the roundabout way through carbon 

 dioxide. The fact that the assimilation occurs only in light, and often 



