120 PHOTO- AND CHEMOSYNTHESIS OF BACTERIA CHAP. 5 



Hydrogen bacteria occupy a unique position in table 5. VII because 

 of their high efficiency. (The high values given for methane bacteria are 

 unreHable, because the gas balance does not agree with any reasonable 

 equation, and the amount of organic matter, as determined by the 

 permanganate method, indicates a much smaller yield. The value given 

 for the delayed chemosynthesis of T. thiooxydans also is of doubtful 

 meaning, as discussed on page 114.) The figures for B. picnoticus are 

 the most reliable of all, having been derived from a series of complete 

 gas analyses by Ruhland (Table 5. VI). The values for the ratio, 

 AH2/AO2, found in these experiments, varied between 2.1 and 2.8 de- 

 pending on the state of the culture. If one neglects the oxygen con- 

 sumption by respiration (a correction for this process would raise the 

 efficiency still higher) a ratio of 2.8 means that for every two hydrogen 

 molecules burned to water, 0.8 molecules of hydrogen are used for the 

 reduction of carbon dioxide. This corresponds to the reduction of 

 0.4 molecules of carbon dioxide, and leads to the maximum ratio, 

 AO2/ACO2 = 2.5, given in table 5. VII, corresponding to the utilization 

 of 32% of available energy and 42% of available free energy. Even if 

 one uses the average value of AH2/AO2 in table 5. VI (~ 2.5), one calcu- 

 lates that only four oxygen molecules are required for the reduction of 

 one molecule of carbon dioxide, and obtains an energy utilization of 20%, 

 and a free energy utilization of 26%. Gaffron found (cf. page 140), for 

 hydrogen-adapted green algae, a maximum ratio of AH2/AO2 = 3, and 

 considered this value as the theoretical maximum, probably valid also 

 for the hydrogen bacteria. 



Burk's (1931) calculation which gave a 100% efficiency for the chemosynthesis of 

 hydrogen bacteria, was based on the assumption that the average ratio (0.52) in the last 

 column of table 5.VI represents, not a confirmation of the theoretical stoichiometric 

 value (0.5) — cf. equation (5.6) — but the quantity of hydrogen actually combusted to 

 water to provide energy for the reduction of one mole carbon dioxide by one mole of 

 water (as if all the rest of hydrogen oxidized by the bacteria had nothing to do with 

 the reduction process!). With this assumption, the result of Burk's calculation became 



a simple consequence of the fact that the free energy of the reaction, 2 H2 + CO2 > 



{CH20i + H2O, is approximately zero. We can see no relation between his elaborate 

 calculations and the problem of the true thermodynamic efficiency of the hydrogen 

 bacteria. (This was noted also by van Niel, 1943.) 



3. Methane-Producing Bacteria and other Cases of Carbon Dioxide 



Absorption by Heterotrophants 



We have described the methane-oxidizing bacteria together with other 

 chemautotrophic species, because their use of methane is similar to the 

 use of hydrogen sulfide, sulfur, thiosulfate, or ammonia by "true" 

 autotrophic bacteria. Many other allegedly "heterotrophic" bacteria 

 can live on one chemicall}^ pure organic substrate, and it is quite possible 



