FIRST HETEROTROPHS AND ANAEROBES 4O5 



This sort of requirement for ready-made, specific, organic 

 substances may be met with under natural conditions in 

 organisms at the most varied levels of the evolutionary scale, 

 not only among obligate heterotrophs, but even in organisms 

 which in all other respects can dispense with organic nut- 

 rients. 



As we have already seen in the case of Hydrogenomonas, 

 even chemoautotrophs sometimes require specific organic 

 nutrients such as vitamins, although in general they may 

 serve as examples of organisms in which there have been set 

 up, during the course of evolution, extremely thoroughgoing 

 mechanisms for the carrying out of diverse syntheses. 



This is true to an even greater extent among the photo- 

 autotrophs, many of which, either during the whole of their 

 life cycle, or at particular stages of it, require exogenous 

 organic substances such as vitamins, growth factors, essential 

 amino acids, etc. This concerns the lower chlorophyll-contain- 

 ing organisms in particular. Thus, for example, some species 

 of green flagellates like Euglena, even when grooving in light, 

 cannot do without amino acids or peptones for building up 

 their bodies, while other species, although they can use min- 

 eral nitrogen, can equally well use amino acids as nutrients. ^^ 



The requirement for vitamins, in particular for vitamin 

 Bi and various gi'owth factors, is very widespread among 

 most of the algae, among the blue-green algae and diatoms 

 as well as among the green forms. ^* 



E. G. Pringsheim^^ has already pointed out that exogenous 

 organic substances must be addecl to pure cultures of algae, 

 and now various organic extracts are always added when 

 growing such cultures (except, of course, when special investi- 

 gations are being carried out). 



The situation in regard to vitamin Bjo in various algae is 

 extremely interesting. Although algae contain a large amount 

 of this vitamin they cannot synthesise it but obtain it from 

 symbiotic bacteria.^® 



Intensive investigations have shown that, in other cases 

 too, the extremely widespread occurrence of the parasitic 

 mode of life among many algae is associated with their 

 requirement for specific organic substances. This applies, in 

 particular, to the symbiosis established in lichens^'^ and the 



