140 



ANAEROBICALLY ADAPTED ALGAE 



CHAP. 6 



synthesis. A similar observation was made in the case of B. picnoticus 

 (page 117); but there Ruhland attributed the excess oxygen consumption 

 to respiration, i. e., autoxidation of cellular substrates, which proceeds 

 simultaneously with the combustion of hydrogen. A similar explanation 

 is impossible in the case of the hydrogen-adapted algae. In the first 

 place, the ratio AH2/AO2 drops in these algae as low as 1.0 (as against a 

 minimum of 1.8 in hydrogen bacteria). In the second place, respiration 

 is practically absent (as shown by determinations of the carbon dioxide 

 production during the oxyhydrogen reaction) . Gaffron suggested, there- 

 fore, that in the absence of carbon dioxide ox3^gen is reduced only to a 

 peroxide. (However, since a continuous accumulation of peroxide 

 appears impossible, one must assume that its reduction is completed by 



-200 



c-150 



o 



-100 



-50 



120 180 



Time, minutes 



Fig. 12.— Inhibition by glucose of the hydrogen uptake by the 

 oxyhydrogen reaction in Scenedesmus (after Gaffron 1942). 



cellular hydrogen donors— without the latter's being oxidized to carbon 

 dioxide.) 



In presence of carbon dioxide, the ratio AH2/AO2 is between 2 and 3 

 (as shown in Fig. 11), while the ratio AC02/(AH2 - 2 AO2) (compare 

 Table 5. VI) is close to 0.5. This indicates that now all absorbed oxygen 

 is reduced to water, while all absorbed carbon dioxide is converted into 

 carbohydrates. Thus, the reduction of carbon dioxide helps the oxy- 

 hydrogen reaction to run to completion. 



The efficiency of chemosynthesis was measured, on page 117, by the 

 ratios ACO2/AH2 and AH2/AO2. The minimum value of the second 

 ratio, in the presence of carbon dioxide, is 2.0 (no chemosynthesis, but a 

 complete combustion of hydrogen to water), while the majority of 

 experiments gave values between 2.6 and 3, a result similar to that 

 obtained by Ruhland with Bacillus picnoticus (AH2/AO2 < 2.8, cf. Table 



