Studies on the Ecology of a Marine Spirillum 559 



specific growth rate of the organisms. Although there are con- 

 siderable data about high substrate concentrations and growth 

 rates, little has been done to assess possible effects which may 

 occur at low population densities in dilute media. 



In the present experiments, the growth-limiting substrate 

 has been supplied in decreasing concentrations, the dilution rate 

 being kept unchanged. Ecological approaches were made by 

 applying the known growth constants to experiments using nat- 

 ural seawater as the medium, the total effect of nutrients being 

 the unknown factor. 



EXPERIMENTAL 



As a suitable organism for continuous culture, a marine 

 Spirillum was selected which was found not to grow on the walls 

 of the flask. It was isolated from decaying seaweed by enrich- 

 ment with Ca-lactate in liquid culture and final streaking on a 

 corresponding agar medium. It closely resembles Sp. curvatum as 

 described by Williams and Rittenberg ( 14 ) , differing, however, 

 in not being able to use hexoses and such simple nitrogen 

 sources as ammonia, nitrate, and urea. 



The base medium was a 3 per cent solution of re-crystallized 

 commercial sea salt at pH 7.8 with the addition of 0.005 per cent 

 phosphate from a buffer solution. Using Na-lactate as the carbon 

 source and 1-asparagine as the nitrogen source, the C/N ratio 

 was set at 60, a level at which the nitrogen source was foimd to 

 be definitely growth limiting. In its highest concentration 

 (starting concentration) the medium contained 46 mMol Na- 

 lactate/L and 1 mMol 1-asparagine/L. The absolute concentra- 

 tions of asparagine are given below. Decreasing nutrient concen- 

 trations were prepared by a stepwise dilution of the initial me- 

 dium by alternating factors of 2 and 5, i.e., 1.0 - 0.5 - 0.1 etc. 

 mMol asparagine/L (respectively 28 - 14 - 2.8 etc., mg aspara- 

 gine-N/L). 



Continuous culture took place in an apparatus similar to that 

 of Novick and Szilard (11). Technical details will be reported 

 elsewhere. The same apparatus was used for measuring the 

 growth constants /'m (maximal growth rate) and Ks (substrate 

 concentration at which the specific growth rate reaches half of its 



