226 BELSKK [CHAP. 9 



present in the medium, the organisms can grow, but if absent, no growth 

 occurs. Two vitamin mutants have been obtained so far. These require niacin 

 and biotin respectively. Other cases of mutants will be discussed in the sections 

 that follow. 



These mutant strains are subject to the same limitation as the marine 

 bacteria used by Burkholder in regard to their sensitivity. The biotin mutant 

 shows a linear response (densitometry) in the concentration range from 2 rn^xg/ 

 ml to 40 mjxg/ml while the niacin mutant, although not completely charac- 

 terized, can detect amounts less than 1 m[i.g/ml. Interest thus far has been 

 primarily centered upon the presence and distribution of organic micro- 

 nutrients rather than in quantitative aspects. Therefore, only rough estimations 

 of concentration can be made. It is significant to note, however, that of 29 

 separate samples of sea-water from one area which were assayed for biotin 

 16 (55%) showed detectable quantities of the vitamin (Belser, 1958). Fifty per 

 cent of some more recent near-shore samples contained detectable quantities 

 of niacin. Consistent with the observations on distribution of vitamin B12, the 

 biotin assays show the same variabilities in concentration both horizontally 

 and vertically, and much the same sort of random discontinuity. 



3. Amino Acids 



Although many biologists have not attached particular significance to amino 

 acids in sea-water (as evidenced by the emphasis on vitamin assays and the 

 absence of amino-acid assays), the program of mutant isolation in Serratia 

 marinorubra has provided strains with which some of them can be assayed. At 

 present it is possible to assay for ten different amino acids. The sensitivity of 

 response of the mutants is several orders of magnitude less than the vitamin 

 responses, with the lower limits detectable in the neighborhood of 1 [xg/ml. 

 Over the last year and a half, twice-weekly samples have been taken at a 

 station off the Scripps Institution of Oceanography pier. These samples have 

 been subjected to assay, and conform with the vitamin studies in being charac- 

 terized by marked variability. The one amino acid which has been present in 

 50% or more of the samples is isoleucine. Some of the others have occurred at 

 rare intervals. Threonine, tryptophan, glycine, methionine, histidine and 

 arginine have all appeared sporadically. None of the remaining three — cystine, 

 proline and leucine — have occurred in any of these samples. Pelagic water 

 samples obtained on Zig Pac cruise of SIO (see Fig. 1) contained only isoleu- 

 cine, tryptophan and glycine. A few near-shore sediment samples have been 

 analyzed, and were found to contain several amino acids and niacin. 



4. Purines and Pyrimidines 



This group of compounds has not been examined extensively. With the 

 exception of the Serratia program, no assays are being run for purines or 

 pyrimidines at this time. The group comprises adenine, xanthine, guanine, 



