114 PHOTO- AND CHEMOSYNTHESIS OF BACTERIA CHAP. 5 



per mg. bacterial nitrogen. This uptake seemed to be reversible and 

 dependent on the concentration of carbon dioxide in the medium (c/. 

 Chapter 8, page 201). Cells which have been allowed to chemosynthesize 

 intensely, subsequently showed a greater capacity for carbon dioxide 

 uptake in absence of sulfur or oxygen than "starved" cells. A short 

 period of "sulfur respiration" restored the capacity for carbon dioxide 

 fixation in "carbon dioxide saturated" cells; while endogenous respira- 

 tion had no such effect. 



The rate of oxygen consumption decreased in the presence of carbon 

 dioxide, but the sum ACO2 + AO2 remained approximately constant. 

 This is an obvious parallel to the relation between respiration and 

 photosynthesis in purple bacteria, described on page 110. 



The initial carbon dioxide uptake was unaffected by 0.01 mole per 

 liter of sodium azide or arsenite (which completely inhibit the uptake of 

 oxygen), but was completely inhibited by 10~* mole per liter of iodo- 

 acetate, which caused only 10% inhibition of the oxygen uptake. (Of 

 course, inhibition of sulfur oxidation must cause a corresponding inhibi- 

 tion of carbon dioxide absorption after the initial saturation period.) A 

 concentration of 0.006 mole per liter of sodium pyruvate inhibited both 

 reactions completely, and similar effects were caused by lactic, fumaric 

 and succinic acids, while citric acid had a weaker influence and malic 

 acid none at all. 



Vogler and Umbreit (1942) inquired into the way in which sulfur 

 oxidation can cause carbon dioxide fixation in a subsequent period of 

 anaerobiosis. They found that during the oxidation period, inorganic 

 phosphate is transferred from the medium into the cells, to be released 

 again during the period of carbon dioxide fixation. Seventy to 80 

 molecules of oxygen are used up for oxidation while one molecule of 

 phosphate is transferred into the cells; 40-50 molecules of carbon dioxide 

 are taken up concomitantly with the release of one phosphate molecule. 

 This seems to indicate a AO2/ACO2 ratio of about 1.5. Table 5. VII shows 

 that this value corresponds to an almost 100% utilization of the free 

 energy of oxidation, if one assumes that all absorbed carbon dioxide is 

 reduced to carbohydrate. This proves that the question (which Vogler 

 and Umbreit considered as "open") of whether the "delayed" carbon 

 dioxide fixation is a reduction to carbohydrate or not, must be answered 

 in the negative. Probably, it is not a reduction at all, but a carhoxylation 

 (or another reversible carbon dioxide absorption), analogous to that 

 which forms the first stage of photosynthesis (c/. Chapter 8). Remark- 

 able, however, is the large amount of carbon dioxide taken up in this 

 way — it seems to be at least ten and perhaps a hundred times larger than 

 the reversible carbon dioxide fixation by green plant cells (if one excludes 

 from the latter the rather incidental alkali-acid buffer equilibrium). 



