564 Marine Microbiology 



means an increase of that fraction of Sk available for consumption 

 ( Sr-S ) . Thus, growth occurs again, and a new steady state estab- 

 lishes. Now Sr can be lowered until a new value for Do is reached. 

 In the sea, a factor similar to the experimental dilution rate must 

 be anticipated which - along with Sr - also determines the relative 

 minimal requirement of a nutrient for bacterial growth. 



When the experiments were modified, using natural sea- 

 water as the medium and raising the dilution rate, the resulting 

 Do is believed to characterize the growth relationships between 

 the organisms tested and the total effect of nutrients present. The 

 Spirillum did not show the ability of multiplying at a rate of one 

 generation per ca. 24 hours; even in the sample of extremely pol- 

 luted sea water from Naples Harbour. Presence of growth inhibi- 

 ting material is not likely, because growth is easily stimulated 

 by adding nutrients in concentrations derived from the experi- 

 ments above. 



Considering the experimental conditions, it can be suggested 

 that growth of the particular organism used in this study is re- 

 stricted to environments of higher substrate concentrations and 

 probably spatially limited to a zone surrounding decaying partic- 

 ulate plant material. In support of this assumption, all enrich- 

 ments from sea water without addition of pieces of decaying sea- 

 weed proved unsuccessful. 



The communication of these preliminary data is intended 

 to show that experimental reproduction of growth conditions in 

 the chemostat similar to those in sea water may allow close ap- 

 proach to true ecological relationships between marine bacteria 

 and environmental factors. 



ACKNOWLEDGMENT 



The work was supported by the Deutsche Forschungs- 

 gemeinschaft. Experiments using continuous flow of natural sea 

 water were done at the Zoological Station at Naples. 



SUMMARY 



Threshold concentrations of the nutrients limiting growth of 

 marine bacteria in seawater are determined by internal factors 

 (growth constants) and external (i.e., environmental) factors. 



