Chapter 12 



Axenic Cultivation of a Pelagic Diatom 



Giorgio Soli 



x\ttempts to cultivate pelagic diatoms axenically in a defined 

 medium were started because bacteria-free organisms appeared 

 necessary for carrying out studies of the effect of marine phyto- 

 plankton on the propagation of sound waves in the sea, with 

 particular attention to absorption and scattering phenomena. 



Many difficulties beset the cultivation of pelagic diatoms, 

 in a defined medium free of bacteria. The reasons for these dif- 

 ficulties seem to be rather complex. 



Bacteria may exercise a beneficial action on the diatoms, 

 by slowly buffering the medium, by influencing heavy metal 

 solubility, or by lowering the oxidation-reduction potential of 

 the medium during the early phases of diatom growth (1). 

 However powerful buffers like TRIS ( 2-amino-2- ( hydroxymeth- 

 yl)-l, 3- propanediol), chelating agents and substances which 

 lower the oxidation-reduction potential, do not appear to sub- 

 stitute for bacterial action. It is possible that the bacteria affect 

 diatom growth by furnishing active metabolites and by changing 

 the physical-chemical environment in early stages. 



Antibiotics are usually employed for obtaining axenic cul- 

 tures of diatoms and dinoflagellates. These microorganisms tip- 

 pear to be very sensitive to doses of antibiotics which are ef- 

 fective in inhibiting completely bacterial growth. Therefore, the 

 selection of the right antibacterial agent, as well as the exposure 

 time, should be points of particular attention. The culture med- 

 ium should also contain all those vitamins and growth factors 

 which the bacteria are presumably furnishing. 



This line of thought was followed in the attempt to obtain 

 sterile cultures of pelagic diatoms. During the course of this work, 

 the possibility of a syntrophic relationsliip between the bacteria 



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