256 bacterioloCtICal chemistry 



atniospheru contains a))out 10 per cent, of carbon 

 dioxide ; many bacteria, such as Esch, coli, fail to grow 

 if steps are taken to remove the carbon dioxide from the 

 medium, by vigorous aeration for example. Until a few 

 years ago, however, it was not realised that carbon 

 dioxide was actually assimilated by heterotrophic bacteria, 

 the reason being that under normal conditions such 

 micro-organisms form carbon dioxide from carbohydrates 

 in larger amounts than they use so that the net production 

 of carbon dioxide masks its assimilation. The first clue 

 was given by the fact that when the propionic acid bac- 

 teria ferment glycerol they do not produce carbon dioxide 

 and it was found that the products of fermentation 

 contained more carbon than could be accounted for by 

 that in the medium initially. Since then, by the use of 

 isotopic " heavy " carbon or radioactive carbon in the 

 carbon compounds of the medium, it has been shown 

 that assimilation of carbon dioxide is a general 

 phenomenon in heterotrophic as well as autotrophic 

 bacteria. The difference appears to be that autotrophic 

 bacteria can make use of it in conjunction with inorganic 

 substances as sources of energy whilst the heterotrophic 

 bacteria require compounds already containing at least 

 one carbon atom in organic linkage. 



Carbon dioxide has been shown to be fixed by barley 

 roots, liver, yeast, Esch. coli, the propionic acid bacteria. 

 Micrococcus lysodeikticus, Aerobacter indologenes, Proteus 

 vulgaris, Str. paracitrovorus, Staph, candidus, CI. welchii, 

 CI. acetobutylicum and CI. aceticum. 



The mechanism proposed by Van Niel to account for 

 the photosynthetic reaction : — 



CO2 + 2H2A > (CH2O) + H2O + 2A 



probably also holds for chemosynthetic reactions. Ruben 

 has suggested that the fixation of carbon dioxide in the 

 dark (that is chemosynthetically) takes place with the 



