THE DARK REACTIONS 



139 



begin to consume both hydrogen and oxygen (Gaffron 1940-). Figure 11 

 shows the course of gas absorption in the presence of about 3.8 mm. Hg 

 of oxygen (50 mm. Brodie solution). The total gas absorption ap- 

 proaches, but does not reach 150 mm. (the value which corresponds to 

 the complete absorption of oxygen, with double its volume in hydrogen). 

 Figure 11 also shows that the gas consumption increases above the 

 2 H2 + O2 mark if carbon dioxide is present; and analysis shows that, 

 in this case, carbon dioxide is absorbed together with hydrogen and 

 oxygen. The algae probably now function as chemautotrophic ''hy- 

 drogen bacteria," that is, they couple the combustion of hydrogen with 

 the reduction of carbon dioxide. 



345 



Time, hours 



Fig. 11.— Oxy hydrogen reaction in adapted Scenedesmus in presence and absence 

 of carbon dioxide (after Gaffron 1942). 



Initial oxygen concentration, 50 mm. Brodie solution. 



In contrast to Bacillus picnoticus, described in chapter V, chemo- 

 synthesizing Scenedesmus cells have only a very limited tolerance for 

 oxygen. If, for example, the partial pressure of oxygen is increased to 

 23 mm. Hg (300 mm. Brodie), the rate of the oxyhydrogen reaction 

 decHnes rapidly and "de-adaptation" sets in. B. picnoticus, on the other 

 hand, works well even in pure electrolytic gas. However, other bacteria 

 capable of catalyzing the oxyhydrogen reaction, for example, Acetobacter 

 peroxidans (cf. page 118), are also inhibited by excess oxygen. 



In a renewed study of the oxyhydrogen reaction in adapted algae, 

 Gaffron (1942^) confirmed that in absence of carbon dioxide, the ratio 

 AH2/AO2 often is much smaller than the theoretical value of 2 for water 



