444 FURTHER EVOLUTION 



by the purple bacteria Athiorhodaceae.^^'^ Externally, from 

 the point of view of its over-all balance, their metabolism is 

 of the ordinary heterotrophic type. In light they can be 

 cultivated under anaerobic conditions but the solution in 

 which they grow must contain organic substances (e.g. butyric 

 acid or other analogous compounds). As the mass of the 

 bacteria increases so the quantity of exogenous organic sub- 

 stances in the surrounding medium decreases correspond- 

 ingly, but the bacteria also discharge a small amount of 

 gaseous CO2 into the atmosphere. 



The whole difference between the Athiorhodaceae and 

 heterotrophs which can grow equally well in the dark or in 

 the light, is that in the light the Athiorhodaceae can use 

 almost all (90 per cent or more) of the exogenous substances 

 for increasing their mass. The only ' waste product ' is CO2 

 which forms only a few parts per hundred of the organic 

 substances used in the growth and development of the 

 bacteria. If we compare this with the outlay of organic 

 substances by ordinary heterotrophs (metabolising in the 

 dark), in which this dissipation consumes the lion's share of 

 the nutrients, we shall see how far more rational is the use 

 of exogenous organic materials by the Athiorhodaceae owing 

 to their having acquired the ability to use the energy of sim- 

 light. 



As we have already said, on over-all balance, the Athiorho- 

 daceae may be regarded as heterotrophs requiring organic 

 substances from the external medium ; but from the point 

 of view of their internal biochemical mechanisms, these 

 bacteria already approach the photoautotrophs. Like all 

 other organisms, they can fix atmospheric co, but in doing 

 so they make use of the increased energy of the light-absorb- 

 ing pigments. Thus the Athiorhodaceae carry out a photo- 

 catalytic transfer of hydrogen, reducing co, and oxidising 

 the exogenous organic substances. J. W. Foster^" succeeded 

 in observing this in the case of the oxidation by these bacteria 

 of secondary alcohols into ketones. As these do not enter 

 into the general metabolism they accumulate in the external 

 medium and can therefore easily be estimated. In other cases 

 such waste products are not formed and the whole metabol- 

 ism is directed to the synthesis of bacterial protoplasm. ^° 



156 



