THE CHEMOTROPHIC ASSIMILATION OF CARBON 49 



presence of carbon dioxide shows that for every two mole- 

 cules of hydrogen transferred to oxygen, up to one molecule 

 is used for the reduction of carbon dioxide,^*^ i.e. the 

 over-all equation is: 



6H2+202+COo->(CHoO)+5H20 . . (9) 



However, there is no evidence that algae such as Scenedes- 

 mus carry out this reaction to any considerable extent under 

 natural conditions and Chlamydomonas moewusu, another 

 species in which the oxyhydrogen reaction occurs, is unable 

 to grow in the dark with this reaction as a source of 

 energy.i^* Thus, although, as with photoreduction, this 

 inability to grow does not appear to be due to an inherent 

 unsuitability of the products of assimilation, these examples 

 cannot strictly be classed as chemolithotrophic according 

 to the definition given on p. 16. 



The colourless organisms belonging to the genus Beg- 

 giatoa, to be found in sewage-contaminated water and 

 sulphur springs, have long been known to be chemolitho- 

 trophic but have generally been classed with the bacteria. 

 However, in their morphology and their characteristic 

 ^method of movement these organisms correspond with 

 blue-green algae of the genus Oscillatoria and there can be 

 little doubt that they should be classified as Myxophyceae.^^^ 

 Species of Beggiatoa oxidize hydrogen sulphide to elemen- 

 tary sulphur which is deposited within the cells: 



O2+2H2S— >2H20+2S + i26 k-cals. . . (10) 



When the hydrogen sulphide is exhausted the sulphur is 

 further oxidized to sulphate: 



02+§S+fH20 -> f SO4- - + itH++98 k-cal. . (11) 



The energy released is used for the reduction of carbon 

 dioxide and Beggiatoa spp. are able by this means to grow 

 in the complete absence of organic substrates.^*"^' ^^'^ 



Chemolithotrophy is obviously analogous to photo- 

 trophy, from which it differs apparently only in the source 

 from which energy is derived. Few experimental studies 

 of the mechanism of the process have, however, been made, 

 and it is not known whether a fixation cycle similar to that 



