Ethanol production by immobilized whole cells ofZymomonas mobilis in a continuous flow expanded bed bioreactor and a continuous flow stirred tank bioreactor

Continuous ethanol fermentation by immobilized whole cells ofZymomonas mobilis was investigated in an expanded bed bioreactor and in a continuous stirred tank reactor at glucose concentrations of 100, 150 and 200 g L^–1. The effect of different dilution rates on ethanol production by immobilized whole cells ofZymomonas mobilis was studied in both reactors. The maximum ethanol productivity attained was 21 g L^–1 h^–1 at a dilution rate of 0.36 h^–1 with 150 g glucose L^–1 in the continuous expanded bed bioreactor. The conversion of glucose to ethanol was independent of the glucose concentration in both reactors.     

Transpiration/biomass ratio for carrots as affected by salinity,nutrient supply and soil aeration

The influence of salinity, nutrient level and soil aeration on the transpiration coefficient, defined as amount of water transpired/unit biomass produced (transpiration/biomass ratio) of carrots was investigated under non-limiting conditions with respect to water supply.Under optimum conditions and favorable nutrient supply, the transpiration coefficient amounted to 280–310 g H₂O g^–1 storage root dry weight (RDW). The transpiration coefficient did not change significantly up to salt concentration of 16 mS cm^–1 in the soil solution under otherwise optimum conditions. Higher salt concentrations or low nutrient levels increased the transpiration coefficient to values of 390–540 g H₂O g^–1 RDW. It is suggested that the transpiration coefficient is not affected by salinity as long as toxic effects and nutrient imbalances do not occur. The transpiration coefficient was not increased by impeded soil aeration. Biomass production was more negatively influenced by adverse soil conditions (salinity, low nutrient level, impeded soil aeration) than was the transpiration coefficient.     

Influence of dilution rate on the morphology and technological properties ofLactobacillus delbrueckii subsp.bulgaricus

Lactobacillus delbrueckii subsp.bulgaricus ATCC 11842 was grown in a chemostat at 45°C and pH 5.5 using glucose as the carbon source, with the aim of optimizing biomass production. Cells were grown in a complex medium under nitrogen. At dilution rates lower than 0.18h^–1, it was difficult to keep steady-state conditions and pleomorphic forms were observed. The addition of 30mM Ca²⁺ and Mn²⁺ reverted the cells to normal shape: 30mM Mg²⁺ had no effect. Increasing the dilution rate resulted in normal morphology without the addition of any cations. Under these conditions, a maximum productivity of 1.24g dry biomass 1^–1 h^–1 was obtained. The maximum growth yield, corrected for maintenance, was 30g biomass mol^–1 glucose and the maintenance energy was 0.26g glucose g^–1 biomass h^–1. Lactate was the main fermentation product at all glucose concentrations used in the fed medium. Cells grown at high dilution rates had normal technological properties (acid production and proteolysis) when tested in milk.     

Effect of the dilution rate on the biomass yield ofBacillus thuringiensis and determination of its rate coefficients under steady-state conditions

Depending on the biomass yield on glucose and the cell morphology ofBacillus thuringiensis, three different metabolic states were observed in continuous culture. At dilution rates between 0.18 h^–1 and 0.31 h^–1 vegetative cells, sporulating bacteria and spores coexisted, while glucose and amino acids were consumed. Only vegetative cells were observed at dilution rates between 0.42 h^–1 and 0.47 h^–1 and glucose was used as the main carbon and energy source. AtD = 0.50 h^–1 the biomass yield on glucose decreases sharply. To define better the specific growth rate range in which the microorganism uses mainly glucose, a dilution rate of 0.25–0.45 h^–1 was studied. The experimental data could be adjusted to a Monod model and the following rate coefficients and growth yields were determined: maximum specific growth rate 0.54 h^–1, saturation constant 0.56 mg glucose ml^–1, biomass growth yields 0.43 g cells (g glucose)^–1, and 0.76 g cells (g oxygen)^–1, and maintenance coefficients 0.065 g glucose (g cells)^–1 h^–1 and 0.039 g oxygen (g cells)^–1 h^–1.     

Aerobic nitrate and nitrite reduction in continuous cultures of Escherichia coli E4

Nitrate and nitrite was reduced by Escherichia coli E4 in a l-lactate (5 mM) limited culture in a chemostat operated at dissolved oxygen concentrations corresponding to 90–100% air saturation. Nitrate reductase and nitrite reductase activity was regulated by the growth rate, and oxygen and nitrate concentrations. At a low growth rate (0.11 h^–1) nitrate and nitrite reductase activities of 200 nmol · mg^–1 protein · min^–1 and 250 nmol · mg^–1 protein · min^–1 were measured, respectively. At a high growth rate (0.55 h^–1) both enzyme activities were considerably lower (25 and 12 nmol mg^–1 · protein · min^–1). The steady state nitrite concentration in the chemostat was controlled by the combined action of the nitrate and nitrite reductase. Both nitrate and nitrite reductase activity were inversely proportional to the growth rate. The nitrite reductase activity decreased faster with growth rate than the nitrate reductase. The chemostat biomass concentration of E. coli E4, with ammonium either solely or combined with nitrate as a source of nitrogen, remained constant throughout all growth rates and was not affected by nitrite concentrations. Contrary to batch, E. coli E4 was able to grow in continuous cultures on nitrate as the sole source of nitrogen. When cultivated with nitrate as the sole source of nitrogen the chemostat biomass concentration is related to the activity of nitrate and nitrite reductase and hence, inversely proportional to growth rate.     

Phytoplankton growth in the Ohio, Cumberland and Tennessee Rivers, USA: inter-site differences in light and nutrient limitation

Seasonal patterns in resource limitation of phytoplankton growth were assessed monthly within three large rivers with differing extents of water regulation. The Ohio River is regulated by low dams that do not substantially modify discharge, while the Cumberland and Tennessee Rivers are impounded by a series of high dams to enhance water storage for downstream flood control. Laboratory dilution assays with light and nutrient manipulations indicated that light was the main factor limiting phytoplankton growth at irradiances below 7 E m^–2 d^–1. Light limited growth was frequent in the turbid, higher discharge of the Ohio River, but was rare in the heavily regulated Tennessee and Cumberland Rivers. When irradiance exceeded 7 E m^–2 d^–1, phytoplankton were either P-limited (Cumberland River), co-limited by P and N (Tennessee River), or Si limited (Ohio River). Site-specific differences in nutrient limitation were consistent with differences in ambient nutrient levels, with the Tennessee and Cumberland Rivers characterized by lower N and P concentrations, and the Ohio River by lower Si. Downstream nutrient depletion was evident in the Ohio River through comparison of an upstream and a downstream site, with nutrient limitation (Si) occurring more frequently downstream. Phytoplankton growth rates at ambient light and nutrient levels ranged from 0.1 to 1.5 d^–1 in the Ohio River and 0.2 to 0.6 d^–1 in the Tennessee and Cumberland Rivers. Growth rates were greatest at the onset of the summer base pool, as light intensities increased and nutrient levels were maximal. Our findings indicate that multiple factors regulate phytoplankton growth in regulated rivers and that spatial complexity may arise from differences in discharge and water aging.     

Modeling of the pyruvate production with<Emphasis Type="Italic"> Escherichia coli</Emphasis> in a fed-batch bioreactor

A family of 10 competing, unstructured models has been developed to model cell growth, substrate consumption, and product formation of the pyruvate producing strain Escherichia coli YYC202 ldhA::Kan strain used in fed-batch processes. The strain is completely blocked in its ability to convert pyruvate into acetyl-CoA or acetate (using glucose as the carbon source) resulting in an acetate auxotrophy during growth in glucose minimal medium. Parameter estimation was carried out using data from fed-batch fermentation performed at constant glucose feed rates of q_VG=10 mL h^–1. Acetate was fed according to the previously developed feeding strategy. While the model identification was realized by least-square fit, the model discrimination was based on the model selection criterion (MSC). The validation of model parameters was performed applying data from two different fed-batch experiments with glucose feed rate q_VG=20 and 30 mL h^–1, respectively. Consequently, the most suitable model was identified that reflected the pyruvate and biomass curves adequately by considering a pyruvate inhibited growth (Jerusalimsky approach) and pyruvate inhibited product formation (described by modified Luedeking–Piret/Levenspiel term).List of symbols c_A acetate concentration (g L^–1) - c_A,0 acetate concentration in the feed (g L^–1) - c_G glucose concentration (g L^–1) - c_G,0 glucose concentration in the feed (g L^–1) - c_P pyruvate concentration (g L^–1) - c_P,max critical pyruvate concentration above which reaction cannot proceed (g L^–1) - c_X biomass concentration (g L^–1) - K_I inhibition constant for pyruvate production (g L^–1) - K_I^A inhibition constant for biomass growth on acetate (g L^–1) - K_P saturation constant for pyruvate production (g L^–1) - K_P inhibition constant of Jerusalimsky (g L^–1) - K_S^A Monod growth constant for acetate (g L^–1) - K_S^G Monod growth constant for glucose (g L^–1) - m_A maintenance coefficient for growth on acetate (g g^–1 h^–1) - m_G maintenance coefficient for growth on glucose (g g^–1 h^–1) - n constant of extended Monod kinetics (Levenspiel) (–) - q_V volumetric flow rate (L h^–1) - q_VA volumetric flow rate of acetate (L h^–1) - q_VG volumetric flow rate of glucose (L h^–1) - r_A specific rate of acetate consumption (g g^–1 h^–1) - r_G specific rate of glucose consumption (g g^–1 h^–1) - r_P specific rate of pyruvate production (g g^–1 h^–1) - r_P,max maximum specific rate of pyruvate production (g g^–1 h^–1) - t time (h) - V reaction (broth) volume (L) - Y_P/G yield coefficient pyruvate from glucose (g g^–1) - Y_X/A yield coefficient biomass from acetate (g g^–1) - Y_X/A,max maximum yield coefficient biomass from acetate (g g^–1) - Y_X/G yield coefficient biomass from glucose (g g^–1) - Y_X/G,max maximum yield coefficient biomass from glucose (g g^–1) - growth associated product formation coefficient (g g^–1) - non-growth associated product formation coefficient (g g^–1 h^–1) - specific growth rate (h^–1) - _max maximum specific growth rate (h^–1)     

Renewal rate and nutrient concentration as tools to modify productivity and biochemical composition of cyclostat cultures of the marine microalga Dunaliella tertiolecta

A factorial experimental design with two nutrient concentrations (2 and 4 mmol Nl^–1 in the form of NaNO₃) and five rates of daily renewal of the cultures (10%, 20%, 30%, 40% and 50%) was carried out in cyclostat, light/dark-synchronized cultures of the marine microalga Dunaliella tertiolecta Butcher. Steady-state cellular density was a linear function inversely proportional to renewal rate. Maximal cellular productivity, 3 × 10⁹ cells1^–1 day^–1, equivalent to 0.24 g1^–1 day^–1 dry weight and 0.17 g1^–1 day^–1 organic weight, was found with renewal rates of 20%–30% and 4mmol N1^–1, but maximal protein productivity, 0.066 g1^–1 day^–1, was obtained with a renewal rate of 40% for both nutrient concentrations. The protein content ranged between 30% and 70% of the organic fraction depending on the culture conditions. Carbohydrates were the only fraction accumulating in response to nutrient stress, ranging from 57% to 10% of the organic fraction, meanwhile the lipid content was increased by increase of nutrient availability. Under non-nitrogen-limited conditions the C:N ratio stabilized around 5.2–5.3 and the protein content of the organic fraction around 70%, but the cell nitrogen quota decreased under these conditions with increasing renewal rates, owing to the lower organic content of cells obtained with high growth rates. The high capacity for changing the biochemical composition, demonstrated for D. tertiolecta in the cyclostat system, has interesting implications for the management of continuous cultures of microalgae and its applications in biotechnological processes.     

Phytoplankton primary production and nutrients in the Oosterschelde (The Netherlands) during the pre-barrier period 1980–1984

Phytoplankton primary production, nutrient concentrations and turbidity were monitored at three stations in the Oosterschelde during 1980–1984 as part of an ecosystem study.From comparisons of dissolved nutrient ratios with the nutrient requirements of phytoplankton, and of ambient nutrient concentrations with half-saturation constants for nutrient uptake by natural phytoplankton populations it was concluded that silicate was a limiting nutrient for diatoms after the spring bloom until the end of the summer. Dissolved inorganic nitrogen and phosphate were not considered to be limiting to phytoplankton growth.In general, the phytoplankton growing season started during the first fortnight of April and ended at the end of September. Column production in the whole Oosterschelde varied between 201 and 540 g C m^–2 yr^–1 and was, on average, 25% higher in the western part than in the eastern part. Basin production in the Oosterschelde varied between 120 and 466 g C m^–2 yr^–1 and was, on average, 55% higher in the western part than in the eastern part; this difference could be explained by differences in the ratio of euphotic depth to mean depth of the compartments.Estimated carbon-specific growth rates in the eastern part varied between < 0.1 and 3 d^–1 and in the western part between < 0.1 and 1 d^–1. This difference could be explained by the great differences in depth of the compartments. Carbon-specific growth rates are discussed in relation to phytoplankton loss rates. It is suggested that in the eastern part sedimentation must be an important sink for phytoplankton.Communication no. 473 of the Delta Institute for Hydrobiological Research, Yerseke, The Netherlands.     

Tolerance of wheat (<Emphasis Type="Italic">Triticum aestivum</Emphasis> L.) to high soil and solution selenium levels

The fertilisation of wheat crops with Se is a cost-effective method of enhancing the concentration of organic Se in grain, in order to increase the Se intake of animals and humans. It is important to avoid phytotoxicity due to over-application of Se. Studies of phytotoxicity of Se in wheat grown in Australia, where rainfall and grain yield are usually relatively low, have not been reported previously, and overseas studies have had varied results. This study used trials conducted in the field, glasshouse and laboratory to assess Se phytotoxicity in wheat. In field trials that used rates of up to 120 g ha^–1Se as selenate, and in pilot trials that used up to 500 g ha^–1 Se soil-applied or up to 330 g ha^–1 Se foliar-applied, with soils of low S concentrations (2–5 mg kg^–1), no Se toxicity symptoms were observed. In pot trials of four weeks duration, the critical tissue level for Se toxicity was around 325 mg kg^–1 DW, a level attained by addition to the growth medium of 2.6 mg kg^–1 Se as selenate. Solution concentrations above 10 mg L^–1 Se inhibited early root growth of wheat in laboratory studies, with greater inhibition by selenite than selenate. For selenite, Se concentrations around 70 mg L^–1 were required to inhibit germination, while for selenate germination % was unaffected by a solution concentration of 150 mg L^–1 Se. Leaf S concentration and content of wheat increased three-fold with the addition of 1 mg kg^–1 Se as selenate to the growth medium. This effect is probably due to the induction of the S deficiency response of the main sulphate transporter. This study found wheat to be more Se-tolerant than did earlier studies of tobacco, soybeans and rice. We conclude that Se phytotoxicity in wheat will not be observed at the range of Se application rates that would be used to increase grain Se for human consumption (4–200 g ha^–1 Se as selenate, which would result in soil and tissue levels well below those seen in the above studies), even when – as is common in Australia – soil S concentration and grain yield are low.     

Comparison of plant uptake and plant toxicity of various ions in wheat

The effects of varying solution concentrations of manganese (Mn), zinc (Zn), copper (Cu), boron (B), iron (Fe), gallium (Ga) and lanthanum (La) on plant chemical concentrations, plant uptake and plant toxicity were determined in wheat (Triticum aestivum L.) grown in a low ionic strength (2.7×10^–3 M solution culture). Increasing the solution concentration of Mn, Zn, Cu, B, Fe, Ga and La increased plant concentrations of that ion. Asymptotic maximum plant concentrations were reached for Zn (10 mg kg DM^–1 in the roots), Ga (2 mg kg DM^–1 in the tops and 18 mg kg DM^–1 in the roots) and La (0.4 mg kg DM^–1 in the tops and 4 mg kg DM^–1 in the roots). Plant ion concentrations were, on average, 3 times higher in the roots than the tops for Mn and Zn, 7 times for Cu, 9 times for Fe, 12 times for Ga and 15 times for La. In contrast, B concentrations were higher in the tops than the roots by, on average, 2 times. The estimated toxicity threshold (plant concentration at which a rapid decrease in yield occurred) in the tops was 0.4 mg g DM^–1 for B, 2 for Zn, 0.075 for Cu and 0.09 for La and in the roots 0.2 mg g DM^–1 for B, 5 for Zn, 0.3 for Cu and 3 for La. Plant uptake rates of the ions (as estimated by the slope of the relationship between solution ion concentrations and plant ion concentrations) was in the order B 250 mg kg DM^–1 M ^–1). Plant toxicity was estimated as the reciprocal of the plant concentration that reduced yield by 50% (change in relative yield per mg ion kg DM^–1). The plant toxicity of the ions tested was in the order Mn相似文献   

Using porewater profiles to assess nutrient availability in seagrass-vegetated carbonate sediments

We measured porewater profiles of inorganic (NH₄ ⁺, NO₃ ^–(+NO₂ ^–), PO₄ ^3– (hereafter referred to as DIP)) and organic (DON, DOP) nutrients in seagrass-vegetated sediments at two sites in a shallow bay in Bermuda within close proximity (200 m) but subject to different nutrient loading. At both sites, total dissolved and inorganic nutrient concentrations were usually 1–2 orders of magnitude higher in the sediments than in the water column, with the exception of NO₃ ^–. Organic N and P were significant components of the total dissolved nutrient pools both in the sediment porewater and in the overlying water column (up to 75% for DON and 40% for DOP), and may be important in meeting plant nutrient demands. We used two approaches to examine how well porewater nutrient concentrations reflected the relative availabilities of N and P for seagrasses: (1) a simple stoichiometric nutrient regeneration model based on the N:P ratio of decomposing organic matter and porewater NH₄ ⁺ concentrations to predict porewater DIP, and (2) fitting of the porewater profiles to estimate rates of net nutrient production (or consumption), which reflects the balance between nutrient sources and sinks in the rhizosphere. The stoichiometric model indicated that sediment porewaters were depleted in P relative to N in the low-nutrient outer bay site, and enriched in P relative to N in the higher-nutrient inner bay site. These results are consistent with the mechanism of carbonate sediments in oligotrophic tropical environments being a strong sink for dissolved inorganic P and our previous work suggesting that nutrient enrichment causes P to become disproportionately more available than N. Net nutrient production rates of porewater P at both sites and N at the inner bay site were low (typically < 2%) relative to the nutrient demands of the seagrasses. The implications of the profile interpretation are two-fold: (1) the low rates of net nutrient production indicate diffusive losses from the root zone were insignificant and that nutrient turnover rates were high, except in the P-limited outer bay where N accumulated in sediment porewaters; and (2) because standing stock nutrient concentrations often represent a small fraction of the total nutrients cycled in the sediments, they are in many cases a poor indicator of nutrient availability. Based on our estimates of losses from the root zone, decomposition, and plant uptake we have constructed a rough budget for the cycling of P and N at our two sites.     

Towards an understanding of human impact upon the hydrology of Lake Naivasha,Kenya

The growth of a strain of Moina macrocopa (Straus 1820) isolated from an experimental stabilization pond in Marrakesh, was examined at seven concentrations of algae (6.25–6.25 × 10⁵ cells ml^–1 and at 5 different temperatures (15–30 °C)). Feeding conditions influenced the growth rate as well as the maximum size that reached 1.8 mm at 25 °C and at the highest algal concentration (6.25 × 10⁵ cell ml^–1). The life span and number of moltings reached a maximum (17.4 days and 13 moltings) at average nutrient concentrations (6.25 × 10⁵ cell ml^–1). Juvenile stages varied from 1 to 3 and adult ones from 6 to 8. In the temperature interval tested, growth rate increased with temperature while longevity decreased. Temperature had less effect on maximal size than nutrient availability. Population density (but not crowding) influenced longevity and survival but had no effect on growth.     

The oxygen transfer rate as key parameter for the characterization of<Emphasis Type="Italic"> Hansenula polymorpha</Emphasis> screening cultures

Screening cultures are usually non-monitored and non-controlled due to a lack of appropriate measuring techniques. A new device for online measurement of oxygen transfer rate (OTR) in shaking-flask cultures was used for monitoring the screening of Hansenula polymorpha. A shaking frequency of 300 rpm and a filling volume of 20 ml in 250-ml flasks ensured a sufficient oxygen transfer capacity of 0.032 mol (l h)^–1 and thus a respiration not limited by oxygen. Medium buffered with 0.01 mol phosphate l^–1 (pH 6.0) resulted in pH-inhibited respiration, whereas buffering with 0.12 mol phosphate l^–1 (pH 4.1) resulted in respiration that was not inhibited by pH. The ammonium demand was balanced by establishing fixed relations between oxygen, ammonium, and glycerol consumption with 0.245±0.015 mol ammonium per mol glycerol. Plate precultures with complex glucose medium reduced the specific growth rate coefficient to 0.18 h^–1 in subsequent cultures with minimal glycerol medium. The specific growth rate coefficient increased to 0.26 h^–1 when exponentially growing precultures with minimal glycerol medium were used for inoculation. Changes in biomass, glycerol, ammonium, and pH over time were simulated on the basis of oxygen consumption.     

Poly(3-hydroxybutyrate) synthesis in fed-batch culture of Ralstonia eutropha with phosphate limitation under different glucose concentrations

Poly(3-hydroxybutyrate) (PHB) was produced by fed-batch cultures of Ralstonia eutropha with phosphate limitation under different glucose concentrations. When glucose was kept at 2.5 g l^–1, cell growth and PHB synthesis were limited due to the shortage of carbon source but a higher PHB content occurred in the cell-growth stage. This shows that a low glucose concentration is favorable for PHB accumulation in R. eutropha. PHB obtained with glucose at 9 g l^–1 is 1.6 times that obtained with 40 g l^–1. When glucose was in the range of 9 to 40 g l^–1, PHB concentration and productivity decreased significantly with the increase of glucose concentration. The highest PHB productivity was obtained with glucose at 9 g l^–1.     

Effects of oxygen partial pressure and combined nitrogen on N2-fixation (C2H2) associated withZea mays and other gramineous species

Summary The objectives of this investigation were to determine the effects of oxygen partial pressure (pO₂) and combined nitrogen (NH ₄ ⁺ ) on rates of acetylene reduction (AR) associated with roots of intact corn, sorghum, and pearl millet plants. Soil-grown plants were carefully removed from soil and incubated hydroponically with the root system enclosed in a plastic cylinder; the tops were left exposed to ambient conditions. Oxygen concentrations around the root systems were controlled by sparging the nutrient solution with known quantities of O₂ in N₂. Ammonium nitrogen was added to the nutrient solution following establishment of AR rates to determine its effect on rates of N₂-fixation (AR). Substantial AR rates (0.1–1.5 mol C₂H₄ g dry wt^–1 h^–1) were associated with roots exposed to 0–2% O₂ (v/v) (0.0–2.02 kPa) in N₂ following at 12–24 h period of exposure to the reduced oxygen tension. Root systems exposed to air failed to demonstrate AR while those exposed to 100% N₂ showed lower activity than those at reduced pO₂ values. Addition of NH ₄ ⁺ (10–20 g N ml^–1 of nutrient solution) reduced AR by 75–90% within 24 h after addition. Oxygen uptake by roots exposed to low pO₂ was substantially reduced.     

Production of xanthan by in-flow cultures of Xanthomonas campestris

The kinetics of xanthan formation in Xanthomonas campestris continuous and fed-batch fermentations was studied along with metabolic changes due to growth rate variation. A maximum growth rate within the range 0.11–0.12 h^–1 was obtained from the continuous culture data in defined medium, producing xanthan at rates up to 0.36 g l^–1 h^–1 corresponding to a maximum 67% glucose conversion at a dilution rate (D) of 0.05 h^–1. Comparatively, fed-batch cultivation was more efficient, producing maximum xanthan at 0.75 g l^–1 h^–1 and 63% glucose conversion at 0.1 h^–1. When reaching D=0.062 h^–1 in continuous cultures, a change was observed and the values of the specific rate of substrate consumption shifted, initiating an uncoupled growth region expressing a lack of balance of the catabolic and anabolic reactions. The deviation was not accompanied by a change in specific xanthan production indicating that xanthan metabolism was not affected by D. For fed-batch-grown X. campestris cells within the range D=0.03–0.1 h^–1, both metabolic parameters changed linearly with the growth rate showing a wide region coupled to growth. Outside that range, glucose accumulated and the specific xanthan production dropped, suggesting substrate inhibition. Correspondence to: J. C. Roseiro     

The effect of specific growth rates on the recovery of amino acids

Three specific growth rates, 0.23, 0.45 and 0.51 h^–1, were used to cultivate Corynebacterium glutamicum in a pH-auxostat. The specific formation rates of most amino acids increased by raising the specific growth rates. The highest specific growth rate, 0.51 h^–1, favors the production of LEU; whereas the highest production yield for ALA and GLU were at = 0.23 h^–1. A correlation among specific growth rates, glucose consumption rate, and production yields of amino acids was obtained.     

End product inhibition in methane fermentations: Effects of carbon dioxide on fermentative and acetogenic bacteria

Summary The rates of glucose utilization by fermentative bacteria and propionate and butyrate utilization by acetogenic bacteria were studied and their dependence of pCO₂ in the interval 0–1 bar was determined. A batch fermentation method was used permitting good control of fermentation parameters and rapid experiments.The rate of glucose fermentation to acids, CO₂ and H₂ was in the order of 12,000 mg glucose/l · day which was about two orders of magnitude faster than the utilization of propionic and butyric acid by acetogenic bacteria. The rate of glucose utilization was about 30% greater at low values of pCO₂ compared with 1 bar CO₂.Propionate degradation was strongly affected by pCO₂; rates were 60 mg/l · day at pCO₂=1 bar and 200 mg/l · day at pCO₂=0.2 bar. Some CO₂ was required since the rate of propionate utilization dropped rapidly below pCO₂=0.2 bar. The rate of butyric acid utilization was constant at 170 mg/l · day; slightly lower at pCO₂=1 bar.Yields of methane from glucose or acids were close to the theoretical value 50% of degraded substrate-carbon. Yields were 20–30% higher at low values of pCO₂ compared with 1 bar CO₂.The redox potential was usually between –200 and –250 mV, slowly increasing to between –150 and –200 mV during fermentation. No clear connection between rates of substrate utilization, pCO₂ and E_h was detected.     

Effect of benzoic acid on the ammonium transport system of Zygosaccharomyces bailii

Summary A study of the ammonium transport system of Zygosaccharomyces bailii was carried out using methylammonium as a non-metabolizable analogue. Benzoic acid in the growth medium decreased the capacity of the transport system from 1.46 ± 0.11 mmol.g^–1.h^–1 to 0.41±0.04 mmol.g^–1.h^–1, while the affinity for ammonium was not significatively affected. Although ammonium uptake was inhibited by benzoic acid, the ammonium transport system was still active at preservative concentrations which fully inhibited growth suggesting that inhibition of growth was not governed by the uptake of this nutrient.