78 BACTERIOLOGICAL CHEMISTRY 



In order that the pigment may again become a hydrogen 

 donor it must be reduced at the expense of the donor 

 HgA, with formation of A. The only essentially photo- 

 synthetic step is the activation of the reduced pigment. 

 The reduction of carbon dioxide and of the pigment can 

 occur in the dark. Some sulphur bacteria can reduce 

 carbon dioxide in the dark in the presence of hydrogen 

 suggesting that the mechanism of reduction is the same 

 for photosynthetic and chemosynthetic organisms. In 

 chemosynthetic bacteria the hydrogen donor is not 

 a pigment but some other substance whose oxidation 

 provides the necessary energy so that light activation 

 is unnecessary. These autotrophic processes are not 

 restricted to autotrophs since it has been shown that 

 carbon dioxide may be reduced in the dark and fixed 

 by heterotrophic organisms, probably by the same 

 mechanisms as in the chemosynthetic autotrophs, the 

 energy being provided by dissimilation reactions (see 

 Chapter VII). 



As mentioned on p. 72, the symbol (CHgO) is used 

 to indicate the reduction product of carbon dioxide 

 which may or may not be formaldehyde, although the 

 latter is a probable intermediate. 



Euben suggests that the reactions by which Meihano- 

 bacterium wnelianski reduces carbon dioxide to methane 

 with simultaneous oxidation of an alcohol to the fatty 

 acid (see p. 76) are as follows : — 



RH + phosphate donor ^ ^ Phospho-RH + donor 



Pliospho-RH. + CO2 ^=^ RCOOH + phosphate 



O 



// 



RCOOH + phosphate donor ^ ^ R.C- 0- PO3H2 + donor 



0^ 



// 

 R.C- 0- PO3H2 + 2H ^=^ R.CHO + phosphate 



R.CHO + 6H ?=^ RH + CH4 + HgO . 



