HETEROTROPHIC ASSIMILATION 85 



is evidently not for a specific amino-acid and it appears that 

 the mechanism which is lacking is one which forms amino- 

 groups from ammonia, for example that represented in 

 equation 21. Given an amino-acid which can participate in 

 transamination reactions (equation 22) then synthesis of 

 other amino-acids is possible. Glycine and phenylalanine are 

 not suitable nitrogen sources for Euglena deses^^^ and it may 

 be significant that of these the former apparently does not 

 participate in transamination reactions in other organisms. 

 Chlamydomonas chlamydogama, an obligate phototroph, 

 provides an example of an alga with specific amino-acid 

 requirements, viz. for histidine and aspartic acid.^^^ Here 

 the missing mechanisms lie towards the end of synthetic 

 sequences rather than on the main track of synthesis as in 

 Euglena deses. 



THE VITAMIN REQUIREMENTS OF ALGAE 



If an organism is unable to synthesize for itself a com- 

 ponent of an enzyme system, only minute amounts of the 

 metabolite concerned need to be supplied from exogenous 

 sources to permit growth to take place. Growth factors of 

 this type are generally known as vitamins. 



Most algae are able to synthesize for themselves vitamin 

 Bi or thiamine, the importance of which in metabolism has 

 already been discussed (p. 9), in quantities sufficient for 

 their needs. ^ However, a requirement for this substance has 

 been demonstrated in a number of algae. It may be noted 

 that the amounts of dissolved thiamine present in natural 

 waters are evidently enough to support the growth of these 

 forms.i*^' 177 'Yht thiamine molecule consists of thiazole 

 and pyrimidine portions and synthetic disabilities may result 

 in an organism being unable to synthesize, either or both 

 of these portions or to unite them to give thiamine. A 

 requirement for thiamine as such has not yet been demon- 

 strated in an alga. The thiazole portion only is required by 

 Polytoma ocellatum whereas Euglena gracilis can manufac- 

 ture this for itself but requires an exogenous supply of the 

 pyrimidine portion.^^^ Both the pyrimidine and thiazole 

 portions must be supplied for the growth of Polytomella 



