SECT. 2] BIOASSAY OF TRACE SUBSTANCES 227 



hypoxanthine, cytosine, thymine and uracil as the seven bases and their 

 ribosides and ribotides. Several of the vitamins contain nucleic acid bases as 

 an integral part of their structure, and a few organisms are known to have 

 requirements for one or another base. Potentially, these compounds could have 

 great biological significance. The mutant isolation program has yielded three 

 mutants with requirements in this group. Two of these respond to single com- 

 pounds, namely the purine adenine and the pyrimidine uracil. The third 

 mutant is apparently blocked at an early step in purine synthesis, and will 

 grow when supplied with any one of the four purine bases (adenine, guanine, 

 xanthine or hypoxanthine). This has been described as a "purine" mutant. 

 Assays of sea-water samples have detected uracil and "purine", but never 

 adenine. The routinely obtained pier samples show uracil in about 30% of the 

 samples, with only about 2% occurrence of "purine". In open-ocean pelagic 

 samples, uracil has appeared in about 25% of the samples on two separate 

 cruises, while "purine" has occurred only once in some 45 samples. The sensiti- 

 vity of these mutants is in the same order of magnitude as the amino-acid 

 mutants, 1 (xg/ml. 



Two significant points to be derived from the preceding discussions are : 

 (1) that when bioassays have been run for each of the groups (i.e. vitamins, 

 amino acids, nucleic acid bases) some representatives of each have been 

 detected in every case, and (2) that the occurrence, distribution and concentra- 

 tion of each of the groups is highly variable. One might expect, in retrospect, to 

 find representatives of each of the building blocks of biological material in sea- 

 water. But until a very few years ago, little was known of the composition of 

 the organic material in the sea. It is only with the advent of sensitive and 

 specific assay methods that a clear picture of the true composition is emerging. 



5. Perspectives 



It seems worth considering at this point two directions of development for 

 the bioassay technique. They are the elaboration and expansion in terms of the 

 groups of materials to be assayed and the application of the bioassay to specific 

 problems. In the discussion of vitamins, amino acids, purines and pyrimidines, 

 it was pointed out that the ultimate aim is to be able to assay for all of the 

 known representatives of each of these groups. It is neither necessary nor 

 desirable, however, to confine the bioassay technique to these areas. Provasoli 

 in Chapter 8 has discussed growth-promoting and inhibiting substances. 

 Some recent studies indicate that these substances are not wholly confined 

 to the preceding groups. Bentley (1959) has demonstrated the existence in sea- 

 water of organic micronutrients possessing hormonal activity for marine algae. 

 She believes these are related to the indole auxins found in terrestrial plants. 

 Johnston (1959) has examined sea-water for antimetabolites and has found 

 that they do exist, and, further, that they compete in certain specific metabolic 

 processes. Collier et al. (1950) have demonstrated that carbohydrates may be 



