bacterium sp . , while EDTA is unable to satisfy the 

 requirement for both organisms . So far the active 

 substances are all strong chelators able to pene- 

 trate through the cell membrane . But many other 

 penetrable metal chelators are inactive indicating 

 that penetrability and chelating properties are not 

 the only prerequisites for activity . A strong speci- 

 ficity for binding iron and other properties seem es- 

 sential . For instance, the effectiveness of the 

 transport function may require an easy release of 

 iron to the apoproteins of the iron containing en- 

 zymes and this may be achieved in a number of 

 ways. The activity of the ketose- amino acids as 

 iron transport substances may be related to their 

 role in stimulating, and perhaps participating in, 

 haem production and amino acid incorporation in 

 proteins . These new developments suggest means 

 of making better, more flexible media for algae. 

 Hutner et al^. (1951) had briefly mentioned 

 that it is preferable , for the sake of obtaining heavy 

 growth, to have the complexing agent serve as an 

 auxiliary substrate. There is no evidence that his- 

 tidine nor glycylglycine are utilized to any extent 

 as substrates by Synura , Woloszynskia ,and Volvox 

 because these organisms are unable to utilize exo- 

 genous carbon sources under our experimental con- 

 ditions . Perhaps this is an advantage. If the chel- 

 ator employed is utilized as the sole substrate or is 

 a needed building block, metal toxicities may re- 

 sult because the chelator may be utilized more 



rapidly than the metals. This obstacle may how- 

 ever be circumvented by offering several substrates 

 or other building blocks along the same pathway of 

 synthesis, so as to balance the rate of uptake of 

 the metabolizable chelator. Another procedure 

 would be to employ several chelators; some marine 

 algae seem to prefer media chelated jointly by 

 EDTA and nitrilotriacetic acid (Provasoli et al^. , 

 1957). 



Though far more experiments are needed to 

 find more versatile fresh-water media, the use of 

 penetrating pH buffers endowed with chelating 

 properties like histidine and glycylglycine seem to 

 offer great advantages for the most important pH 

 range (pH 6.0-8.5). Tracer studies are required 

 again to tell the extent to which these buffer- 

 chelators penetrate . 



The supply of trace metals in the acid range 

 offers a different set of problems . At pH below 5 the 

 heavy metals are quite soluble. A chelator is not 

 needed to prevent precipitates , but may still be very 

 useful as a metal buffer to prevent toxicities and to 

 stabilize the metal pool. EDTA and other chelators 

 having acidic coordinating groups offer little prom- 

 ise: their chelating power decreases with increas- 

 ing acidity because the hydrogen ions compete 

 more and more favorably with the heavy metal for 

 the coordinating group. Very little has been done 

 with highly soluble, penetrating but non-toxic 

 chelators having a predominance of basic groups. 



References 



Aaronson, S. and H. Baker. 1959 . A comparative biochemical study of two species of Ochromonas . 

 J. Protozool. 5: 282-84. 



Burton, M. O. 1957. Characteristic of bacteria requiring terregens factors. Canad . J. Microbiol. 

 3: 107-12. 



Cirillo, V. P. 195 7. Long-term adaptation to fatty acids by the phytoflagellate, Polytoma uvella . 

 J. Protozool. 4: 60-62 . 



Chu , S. P. 1942. The influence of the mineral composition of the medium on the growth of planktonic 

 algae. I. Methods and culture media. J. Ecol . 30: 284-3 25. 



Demain, A. L., and D. Hendlin. 1959. Iron transport compounds as growth stimulators for Microbac - 

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Droop, M. R. 1958. Optimum relative and actual ionic concentrations for growth of some euryhaline 

 algae. Proc . Int. Assoc. Limnol. 13: 722-30. 



Droop, M. R. 1959. Chemical and ecological consideration in the design of synthetic culture media 

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