No abstract

Agitated bacterial tank bioleaching reactors are currently sparged with air to satisfy both oxygen and CO(2) requirements of microbial cells. Under high-sulphide loading conditions, as is the case with high-grade metal sulphide concentrates, the microbial and chemical demand for oxygen is significantly increased during the bioleaching process. Sparging with enriched oxygen gas may offer an alternative process option to increased agitation and sparged aeration, to overcome the mass transfer difficulties at elevated temperatures where thermophilic Archaea, rather than Bacteria, are used. In the case of air sparging, the DO (dissolved oxygen) concentration in tank reactors could not be increased to a point where it would become inhibitory due to the limited oxygen content of air (20.9% O(2)). The use of enriched oxygen in such reactors at large scale does, however, pose its own set of process risks. The first aim of this investigation was, therefore, to determine the effects of various DO concentrations, in both the limiting and inhibitory ranges, on the microbial activity of Sulfolobus sp. U40813, a typical thermophilic mineral-leaching archaeon. Secondly, the effect of CO(2) concentration on the rate of ferrous iron oxidation was investigated. Both the oxygen and CO(2) kinetics were examined in controlled batch cultures at 78 degrees C, using ferrous sulphate and potassium tetrathionate as energy sources. The optimal DO concentration for iron oxidation was found to be between 1.5 and 4.1 mg.l(-1). The use of elevated DO concentrations (above 4.1 mg.l(-1)) inhibited the ferrous oxidation rates. The optimal gas CO(2) concentration for ferrous iron oxidation was found to be in the range 7-17% (v/v). The iron oxidation rates were, however, severely limited at CO(2) concentrations less than 7%, indicating that the CO(2) supply was limiting in this range and inhibited the microbial growth rate.

No abstract

No abstract

The steady-state residual glucose concentrations in aerobic chemostat cultures of Saccharomyces cerevisiae ATCC 4126, grown in a complex medium, increased sharply in the respiro-fermentative region, suggesting a large increase in the apparent kS value. By contrast, strain CBS 8066 exhibited much lower steady-state residual glucose concentrations in this region. Glucose transport assays were conducted with these strains to determine the relationship between transport kinetics and sugar assimilation. With strain CBS 8066, a high-affinity glucose uptake system was evident up to a dilution rate of 0.41 h(-1), with a low-affinity uptake system and high residual glucose levels only evident at the higher dilution rates. With strain ATCC 4126, the high-affinity uptake system was present up to a dilution rate of about 0.38 h(-1), but a low-affinity uptake system was discerned already from a dilution rate of 0.27 h(-1), which coincided with the sharp increase in the residual glucose concentration. Neither of the above yeast strains had an absolute vitamin requirement for aerobic growth. Nevertheless, in the same medium supplemented with vitamins, no low-affinity uptake system was evident in cells of strain ATCC 4126 even at high dilution rates and the steady-state residual glucose concentration was much lower. The shift in the relative proportions of the high and low-affinity uptake systems of strain ATCC 4126, which might have been mediated by an inositol deficiency through its effect on the cell membrane, may offer an explanation for the unusually high steady-state residual glucose concentrations observed at dilution rates above 52% of the wash-out dilution rate.