PHOSPHATE AND SILICATE GROWTH AND UPTAKE KINETICS OF THE DIATOMS ASTERIONELLA FORMOSA AND CYCLOTELLA MENEGHINIANA IN BATCH AND SEMICONTINUOUS CULTURE1

Information on the nutrient kinetics of Asterionella formosa Hass. and Cyclotella meneghiniana Kutz. under either phosphate or silicate limitation was obtained for use in a Monod model and in a variable internal stores model of growth. Short-term batch culture growth experiments were fit to the Monod model and long-term semicontinuous culture experiments and short-term uptake experiments were fit to the variable internal stores model. Mathematical analysis indicates that the parameters of the 2 models may be expressed in terms of each other at steady state. The qualitative results of both batch and steady state culture methods agree. For limiting phosphate experiments. A. formosa is better able to grow at low PO₄-P concentrations than C. meneghiniana, as shown by its lower K for PO₄-P limited growth. The k_Q of A. formosa compared to C. meneghiniana found in long-term semicontinuous culture indicates that A. formosa is almost an order of magnitude more efficient at using internal phosphate for growth. The qualitative results under silicate-limited growth of C. meneghiniana is less than that of A. formosa. The k_Q from semicontinuous culture experiments indicates that C. meneghiniana is the more efficient at using internal silicate for growth. Nutrient uptake experiments showed more variability from a Michaelis-Menten relationship than short-term growth experiments. There were no significant differences between the 2 species in half saturation constants for either phosphate or silicate uptake. We observed a marked dependence of the coefficient of luxury consumption (R) of phosphate on the steady state growth rate. A. formosa has a higher R than C. meneghiniana.     

Estimation of metabolic flux rates in liver purine catabolism of tumour-bearing mice by computer simulation of radioactive tracer experiments

Mouse hepatocytes from healthy control mice and from Ehrlich ascites tumour-bearing mice were used for tracer-kinetic studies of purine catabolism of liver cells during different periods of tumour growth. The dynamics of the radioactive tracers were modelled mathematically by a system of differential equations. Computer simulations, i.e. direct fitting of numerical solutions of these equations to the observed time-courses of metabolites and specific radioactivites, enables one to estimate unknown kinetic parameters of a simplified model of pathways of hepatic purine catabolism in tumour-bearing mice. There occurred great differences of metabolic flux rates between control hepatocytes, hepatocytes of mice during the proliferating period of tumour growth (6th day after inoculation of the tumour) and hepatocytes of mice during the resting period of tumour growth (12th day after inoculation of the tumour). The final purine degradation of hepatocytes prepared during the proliferating period was lower in comparison with that of control hepatocytes, but it was markedly higher in hepatocytes prepared during the resting period of tumour growth. The changes in hepatocyte purine catabolism during the proliferating period of tumour growth argue for transitions which aim at the maintenance of high purine nucleotide levels in the liver itself rather than for an increased nucleoside and nucleobase supply for the tumour. This suggestion is in accordance with the increased ATP level of the liver during the proliferating phase of tumour growth. The drastic acceleration of the final steps of hepatic purine catabolism forming uric acid and allantoin during the resting period of tumour growth was predominantly due to increased flux rate from xanthosine and guanine in accordance with increased catabolism of monophosphorylated nucleotides.