Maximum Specific Growth Rate Μmax Research Articles

Photo-autotrophic growth of Thiocapsa roseopersicina on dimethyl sulfide

The purple sulfur bacterium Thiocapsa roseopersicina was examined for photo-autotrophic growth on dimethyl sulfide (DMS). The maximum specific growth rate μmax (0.068 h−1), saturation constant Ks (38 μm l−1), and yield (5.24 mg protein mmol−1 DMS) were determined in chemostat experiments. Dimethyl sulfoxide was the only product of DMS oxidation. Batch experiments revealed the simultaneous oxidation of DMS and hydrogen sulfide.

Phenotypic differences between induced and spontaneous 2?-plasmid-free segregants of Saccharomyces cerevisiae

The maximum specific growth rates (μmax) of 2 μ-plasmid-free ([cir°]) segregants of three haploid and one diploid strain of Saccharomyces cerevisiae have been determined and compared with the μmax of their 2 μ-plasmid-containing ([cir+]) progenitors. Two classes of [cir°] strains have been examined: those induced by transformation with a 2 μ-based recombinant plasmid according to the method of Dobson et al. (1980) and those isolated as spontaneous [cir°] segregants from glucose-limited continuous cultures. The μmax of the spontaneous [cir°] segregants was not found to differ significantly from that of their [cir+] parents. In all cases, however, the induced [cir°] strains had a μmax which was significantly less than that of their [cir+] counterparts. This effect was particularly marked in the case of the diploid strain where a 34% reduction in μmax was observed. The implications of these results are discussed in terms of the effect of the transformation process on host yeast cells.

Ammonia and light effect on nitrogenase activity in nitrogen-limited continuous cultures of Rhodopseudomonas capsulata. Role of glutamine synthetase

Nitrogen-limited continuous cultures of Rhodopseudomonas capsulata were used to investigate some aspects of the regulation of nitrogenase activity. The role of glutamine synthetase (GS) in this regulation was examined by measuring changes of its adenylylation state when the light intensity and the nitrogen source were varied. Maximal nitrogenase activity was observed at a dilution rate corresponding to about one third of the maximum specific growth rate (μmax), both in ammonia- and in glutamate-limited cultures. At higher dilution rates, both GS and nitrogenase were inactivated by ammonia. Determination of the kinetics of inhibition of both enzymes indicated that the degree of inactivation of nitrogenase and the adenylylation state of GS were not closely related. Increase of light intensity stimulated nitrogenase activity dramatically. Conversely, a shift-down in light intensity to a limiting value resulted in a decrease of nitrogenase activity suggesting that synthesis was inhibited. On the other hand, the adenylylation state of glutamine synthetase appeared to be unaffected by changes in light intensity, indicating that GS is probably not involved in the regulation of nitrogenase expression by light.

The impact of dilution rate and attached growth on steady-state characteristics of Pseudomonas putida

Pseudomonas putida (ATCC 111 72) was studied in a continuous culture at various dilution rates with asparagine as the sole carbon source and limiting factor. Under the experimental conditions applied, a considerable number of the cells became attached to the fermentor walls and equipment. The viable count of the attached cells was of the same magnitude as those in suspension. The following steady-state characteristics were obtained: The cell-mass (OD620 and dry weight) versus dilution rate (D) had maxima at 0.63 and 1.1 h−1. The corresponding plot of viable count had a minimum at 0.94 h−1 whereafter it reached a maximum at 1.3 h−1. Largest yield coefficient obtained was 0.44 g dry weight/g asparagine (D=1.1 h−1). The productivity of the culture increased with D up to 1.1 h−1, which is far above the D corresponding to the maximum specific growth rate (μmax) of a batch culture (0.59 h−1). The cell mass was not completly washed-out of the fermentor even at a D of 2.2 h−1. The influence of attached growth for the steady-state characteristics, and the significance of the results in relation to chemostate as an instrument for testing environmental factors, are discussed. It is suggested that the attached cells had a significantly higher (μmax) value than the suspended ones.

Growth and lactic acid production ofPediococcus pentosaceus at different gas environments, temperatures, pH values and nitrite concentrations

In order to obtain a better understanding of the behaviour ofPediococcus pentosaceus in food products as well to facilitate the designing of industrial production processes for the organism, the growth and lactic acid production ofPediococcus pentosaceus in a complex glucose medium was followed in batch cultures at different gas environments (CO2, air, N2 and static cultures without gasflow), temperatures (10–50°C), pH (4.3–7.3) and nitrite concentrations (0–700 ppm). Optimal growth was obtained in CO2 at 40°C and pH 6.3 and resulted in a maximum specific growth rate (μ max) of 1.27 h−1. In static culture at 40°C and pH 6.3 theμ max was 1.21 h−1. Theμ max was, compared with static culture, reduced in air (12%) and nitrogen (26%). At 10°C theμ max was reduced by 99% and at 50°C by 88%. The reduction at pH 4.3 and 7.3 was 65% and 57%, respectively. Nitrite did not affect theμ max at any pH but increased the lag phase at pH 4.3 by a factor of 12. The lactic acid production was linked to the growth. The total amount of lactic acid produced was the same in all the tested gases and nitrite concentrations and also within the wide temperature range (15–45°C) and pH range (5.3–7.3). Mainly L(+)-lactic acid was produced during the exponential growth phase, but after this growth declined about 30% of the L(+)-lactic acid was converted to D(−)-lactic acid. The lactic acid product yield and the cellmass yied were both affected by the temperature but not by the pH.

Growth and physiology of potassium-limited chemostat cultures of Paracoccus denitrificans

In potassium-limited chemostat cultures of Paracoccus denitrificans the maximum specific growth rate (µmax) was found to depend on the input potassium concentration: At 0.21mM µmax was 0.10–0.11 h-1; at 0.44 mM 0.15–0.16 h-1 and at 0.66 mM 0.20–0.21 h-1. The plots of the specific rates of oxygen-, succinate-and potassium consumption against μ gave straight lines. The intracellular potassium concentration was a linear function of μ and varied from 1% (0.13 M) at a μ value of 0.034 h-1 to 2.2% (0.29 M) at μ=0.26 h-1; the potassium concentration gradient and the potassium concentration in the culture fluid in the steady state were dependent on the input potassium concentration. The potassium concentration gradient varied from 8,900-1,200. At all μ values 20–25% of the total energy production was used for potassium transport. 350,100 and 30 ATP molecules were calculated to be required to maintain one potassium ion intracellular during 1 h at μ values of 0.034, 0.197 and 0.257 h-1 respectively. It is concluded that the amount of circulation of potassium is dependent on the potassium concentration gradient or on the potassium concentration in the culture in the steady state. The dependency of µmax on the input potassium concentration was explained by the assumption that at low input potassium concentrations the net uptake of potassium (influx-efflux) is not rapidly enough to maintain the high potassium gradient in the existing cells and to establish it in the newly formed cells. At high μ values and at high input potassium concentrations µmax is limited by the specific rate of oxygen consumption, which was found to be 11–12 mmol O2 g dry weight-1 h-1 at µmax for potassium-, succinate-and sulphate-limited chemostat cultures.