562 Marine Microbiology 



finally indicated re-establishment of a steady state. This under- 

 shoot phenomenon may be explained by growth rate hysteresis 

 ( 12 ) occurring at transient state. It may be attributed to techni- 

 cal properties of open culture systems and will not be discussed 

 in the present paper. In the presence of a substrate concentration 

 of 0.14 mg asparagine-N/I., slow wash-out appeared. In these 

 experiments, wallgrowth seemed unlikely, although complete 

 wash-out never resulted in sterility of the culture vessel. 



In experiments using natural sea water as culture medium, no 

 steady state was observed in the range of dilutions tested, whicli 

 went down to a rate of 0.03 ]ir~^ (a theoretical doubling time of 

 ca. 24 hr). The corresponding batcli culture showed appreciable 

 growth only when supplemented with 1 mg asparagine-N/L. 



DISCUSSION 



Monod's (9) fundamental description of the relationship 

 between growth rate and substrate concentration and its mathe- 

 matical treatment by Herbert et al. (4) has been adopted here 

 to compute the theoretical value of the critical dilution rate ( Dc ) 

 at which wash-out appears. Under steady state conditions, the 

 relationship between Dc and Sr depends on the growth constants 

 /j-m and Ks: 



Dc = /.n. (1) 



Ks + S« 



From this equation the importance of the ratio Sr/Ks is evi- 

 dent. When Sii is lowered stepwise - enough time being allowed 

 for readjustment of a new steady state - the concentration of the 

 substrate consumed in the culture, Sr-S (S being the substrate 

 concentration in the effluent) also decreases, because S is nearly 

 independent of Sr. As a result, the cell concentration decreases, 

 compensating for the lower substrate concentration available 

 for consumption. This continues until Sr becomes too low to main- 

 tain any cell concentration at the given growth rate. At tliat 

 point, D becomes D. and wash-out appears. 



In the present experiment, D is deliberately set at /<^."/2, i.e. 

 it becomes Dc when Sr is equal to Ks. This simple relationship 

 is derived from rearranged formula ( 1 ) : 



