Alginate molecular mass produced by <Emphasis Type="Italic">Azotobacter vinelandii</Emphasis> in response to changes of the O<Subscript>2</Subscript> transfer rate in chemostat cultures

Alginate production by Azotobacter vinelandii growing in chemostat cultures was evaluated under different O₂ transfer rates (OTR). As a result of modifying the culture’s agitation rate from 300 to 500 rpm, the OTR increased from 9 to 15.1 mmol l⁻¹ h⁻¹ and a slight variation in the alginate production (1.7–2.2 g l⁻¹) was observed. At a constant growth rate (0.1 h⁻¹), the mean molecular mass of the alginate was strongly influenced by changes in the OTR, varying from 860 to 1,690 kDa. These results support a possible relationship between alginate polymerization-depolymerization process and the O₂ uptake rate.     

Oxygen transfer rate during the production of alginate by <Emphasis Type="Italic">Azotobacter vinelandii</Emphasis> under oxygen-limited and non oxygen-limited conditions

Background  

The oxygen transfer rate (OTR) and dissolved oxygen tension (DOT) play an important role in determining alginate production and its composition; however, no systematic study has been reported about the independent influence of the OTR and DOT. In this paper, we report a study about alginate production and the evolution of the molecular mass of the polymer produced by a wild-type A. vinelandii strain ATCC 9046, in terms of the maximum oxygen transfer rate (OTR_max) in cultures where the dissolved oxygen tension (DOT) was kept constant.     

Production of alginate by Azotobacter vinelandii grown at two bioreactor scales under oxygen-limited conditions

The oxygen transfer rate (OTR) was evaluated as a scale-up criterion for alginate production in 3- and 14-L stirred fermentors. Batch cultures were performed at different agitation rates (200, 300, and 600 rpm) and airflow rates (0.25, 0.5, and 1 vvm), resulting in different maximum OTR levels (OTR_max). Although the two reactors had a similar OTR_max (19 mmol L^?1 h^?1) and produced the same alginate concentration (3.8 g L^?1), during the cell growth period the maximum molecular weight of the alginate was 1,250 kDa in the 3-L stirred fermentor and 590 kDa in 14-L stirred fermentor. The results showed for the first time the evolution of the molecular weight of alginate and OTR profiles for two different scales of stirred fermentors. There was a different maximum specific oxygen uptake rate between the two fermenters, reaching 8.3 mmol g^?1 h^?1 in 3-L bioreactor and 10.6 mmol g^?1 h^?1 in 14-L bioreactor, which could explain the different molecular weights observed. These findings open the possibility of using $ q_{{{\text{O}}_{ 2} }} $ instead of OTR_max as a scaling criterion to produce polymers with similar molecular weights during fermentation.     

Effect of the oxygen supply on pattern of growth and corrinoid and organic acid production of Propionibacterium shermanii

Propionibacterium shermanii CDB 10014 is able to grow even at high oxygen transfer rates (24.0 mmol O₂ l⁻¹ h⁻¹), in contrast to reports in the specialised literature, where all Propionibacteria are considered oxygen-sensitive microorganisms. Propionic acid is the main product in anaerobiosis. The presence of oxygen in the system leads to an inhibition of propionic acid production while acetic acid formation is enhanced. At high oxygen supply rates no propionic acid is produced and acetic acid is the main product. Lactic acid is also produced in reasonable quantities (2.7 g l⁻¹). The growth rate (μ_max) is higher in anaerobiosis (0.19 h⁻¹) than in aerobiosis (0.12–0.15 h⁻¹). The cell yield is higher in aerobiosis (0.18–0.22 g g⁻¹) than in anaerobiosis (0.14 g g⁻¹) suggesting the oxidative metabolism of glucose by Propionibacterium shermanii CDB 10014. No corrinoid production was detected at oxygen transfer rates of more than 13.6 mmol l⁻¹ h⁻¹. Received: 10 September 1997 / Received revision: 6 January 1998 / Accepted: 9 January 1998     

The Impact of Oxygen Tension on Cell Density and Metabolic Diversity of Microbial Communities in Alkane Degrading Continuous-Flow Cultures

Abstract The impact of the dissolved O₂ tension (DOT) and the dilution rate on the metabolic diversity of an autochthonous hexadecane-degrading community in continuous-flow cultures containing hexadecane-coated intertidal sediment was determined in a set of experiments. The DOT was kept constant within each culture at values of 80% (168 μmol O₂L⁻¹) or 0.4% (0.84 μmol O₂ L⁻¹). The dilution rate was increased from D= 0.012 h⁻¹ to D= 0.06 h⁻¹. To determine the culture activity, we analyzed the hexadecane degradation rate, the protein production rate, and the oxygen consumption rate. The cell concentration of different metabolic groups was determined by colony forming units (CFU), and by most probable number (MPN). The metabolic diversity was determined by the substrate utilization spectrum in Biolog GN microtiter plates. The substrate utilization pattern of the cultures decreased considerably as D increased. This effect was more pronounced at 0.4% of DOT than at 80% of DOT. The MPN and CFU revealed that as D increased, only minor changes occurred in the community structure. The hexadecane degradation rate, the protein production rate, and the oxygen consumption rate increased parallel to D independently of the DOT. This means that the biocenosis at 0.4% of DOT was different from the biocenosis at 80% of DOT, although the metabolic activity of the cultures was unaffected by a 200-factor difference in the oxygen tension and revealed a considerable buffer capacity with respect to changes in DOT. Received: 23 May 1998; Accepted: 24 August 1998     

Ethanol production from seaweed extract

Extracts from Laminaria hyperborea could possibly be fermented to ethanol commercially. In particular, seaweed harvested in the autumn contains high levels of easily extractable laminaran and mannitol. Four microorganisms were tested to carry out this fermentation, one bacterium and three yeasts. Only Pichia angophorae was able to utilise both laminaran and mannitol for ethanol production, and its substrate preferences were investigated in batch and continuous cultures. Laminaran and mannitol were consumed simultaneously, but with different relative rates. In batch fermentations, mannitol was the preferred substrate. Its share of the total laminaran and mannitol consumption rate increased with oxygen transfer rate (OTR) and pH. In continuous fermentations, laminaran was the preferred substrate at low OTR, whereas at higher OTR, laminaran and mannitol were consumed at similar rates. Optimisation of ethanol yield required a low OTR, and the best yield of 0.43 g ethanol (g substrate)⁻¹ was achieved in batch culture at pH 4.5 and 5.8 mmol O₂ l⁻¹ h⁻¹. However, industrial production of ethanol from seaweed would require an optimisation of the extraction process to yield a higher ethanol concentration. Journal of Industrial Microbiology & Biotechnology (2000) 25, 249–254. Received 25 February 2000/ Accepted in revised form 05 August 2000     

Composition and Activity of Marine Alkane-Degrading Bacterial Communities in the Transition from Suboxic to Anoxic Conditions

The impact of the oxygen supply rate (OSR) on the metabolic activity and on the composition of hexadecane-degrading bacterial communities in a quasi-anoxic milieu (nominal DOT=0%) was studied in continuous cultures containing intertidal sediment. The dilution rate was kept constant at 0.035 h⁻¹. The OSR was stepwise reduced from 3.5 mmol O₂L⁻¹ h⁻¹ to 0.06 mmol O₂L⁻¹ h⁻¹. Activity was determined by analyzing the respiration quotient (RQ) and the rates of hexadecane degradation (Q_Hex), of hexadecane mineralization, and of protein production (PPR). The community composition and size were investigated by fluorescence in situ hybridization (FISH), by dilution plating (colony forming units or CFU), and by most probable number (MPN). The culture showed an aerobic hexadecane metabolism down to an OSR of 0.35 mmol O₂L⁻¹ h⁻¹. Below this OSR, anaerobic metabolism was initiated. The relationship among the RQ, PPR, Q_Hex, and the OSR can be approximated by hyperbola (Michaelis-Menten kinetics). We suggest that the metabolic adaptation of the culture to low OSRs is due to regulation of protein expression and enzyme activity. Reducing the OSR resulted in minor but significant changes in the concentration of different physiological and phylogenetic groups. This means that, in addition to protein expression and activity regulation, the adaptation of the population to low OSRs is due to changes in the community composition.     

Improvement in Production and Quality of Gellan Gum by Sphingomonas paucimobilis Under High Dissolved Oxygen Tension Levels

The effect of agitation rate and dissolved oxygen tension (DOT) on growth and gellan production by Sphingomonas paucimobilis was studied. Higher cell growth of 5.4 g l⁻¹ was␣obtained at 700 rpm but maximum gellan (15 g l⁻¹) was produced at 500 rpm. DOT levels above 20% had no effect on cell growth but gellan yield was increased to 23 g l⁻¹ with increase in DOT level to 100%. Higher DOT levels improved the viscosity and molecular weight of the polymer with change in acetate and glycerate content of the polymer.     

Oxygen uptake, heart rate and blood lactate concentration during a normal training session of an aerobic dance class

The aim of this research was to investigate the physiological responses and, in particular, the participation of lactic acid anaerobic metabolism in aerobic dance, which is claimed to be pure aerobic exercise. In contrast to previous studies, that have put subjects in very unfamiliar situations, the parameters were monitored in the familiar context of gymnasium, practice routine and habitual instructor. A group of 30 skilled fairly well-trained women performed their usual routine,␣a combination of the two styles: low (LI) and high impact (HI), and were continuously monitored for heart rate (HR) and every 8 min for blood lactate concentration ([La⁻]_b). Of the group, 15 were tested to determine their maximal aerobic power (V˙O_2max) using a cycleergometer. They were also monitored during the routine for oxygen uptake (V˙O₂) by a light telemetric apparatus. The oxygen pulses of the routine and of the corresponding exercise intensity in the incremental test were not statistically different. The mean values in the exercise session were: peak HR 92.8 (SD 7.8)% of the subject's maximal theoretical value, peak V˙O₂ 99.5 (SD 12.4)% of V˙O_2max, maximal [La⁻]_b 6.1 (SD 1.7) mmol · l^−l, and mean 4.8 (SD 1.3) mmol · l^−l. Repeated measures ANOVA found statistically significant differences between the increasing [La⁻]_b values (P < 0.001). In particular, the difference between the [La⁻]_b values at the end of the mainly LI phase and those of the LI-HI combination phase, and the difference between the samples during the combination LI-HI phase were both statistically significant (both P= 0.002 and P= 0.002). The similar oxygen pulses confirmed the validity of the present experiment design and the reliability of HR monitoring in this activity. The HR, V˙O₂ and, above all, the increase of [La⁻]_b to quite high values, showing a non steady state, demonstrated the high metabolic demand made by this activity that involved lactic acid metabolism at a much higher level than expected. Accepted: 23 September 1997     

Influence of agitation and aeration on growth and antibiotic production by <Emphasis Type="Italic">Xenorhabdus nematophila</Emphasis>

The effect of agitation and aeration on the growth and antibiotic production by Xenorhabdus nematophila YL001 grown in batch cultures were investigated. Efficiency of aeration and agitation was evaluated through the oxygen mass transfer coefficient (K _L a). With increase in K _L a, the biomass and antibiotic activity increased. Activity units of antibiotic and dry cell weight were increased to 232 U ml⁻¹ and 19.58 g l⁻¹, respectively, productivity in cell and antibiotic was up more than 30% when K _L a increased from 115.9 h⁻¹ to 185.7 h⁻¹. During the exponential growth phase, DO concentration was zero, the oxygen supply was not sufficient. So, based on process analysis, a three-stage oxygen supply control strategy was used to improved the DO concentration above 30% by controlling the agitation speed and aeration rate. The dry cell weight and activity units of antibiotic were further increased to 24.22 g l⁻¹ and 249 U ml⁻¹, and were improved by 24.0% and 7.0%, compared with fermentation at a constant agitation speed and a constant aeration rate (300 rev min⁻¹, 2.5 l min⁻¹).     

Purification and properties of the cell-associated beta-xylosidase from Aureobasidium

β-Xylosidase was extracted from Aureobasidium sp. ATCC 20524 and purified to homogeneity. The molecular mass was estimated at 411 kDa. The enzyme contained 15.3% (w/w) carbohydrate. The optimum pH and temperature were pH 3.5 and 80°C, respectively. The enzyme was stable at pH 3.5–9 after 3 h and at 80°C after 15 min. The Michaelis constant (K _m) and maximum velocity (V _max) toward p-nitrophenyl-β-D-xyloside were 2.0 mmol l⁻¹ and 0.94 mmol min⁻¹ mg⁻¹ protein, respectively. The enzyme was inhibited strongly by mercury, lead, and copper ions. Journal of Industrial Microbiology & Biotechnology (2001) 26, 276–279. Received 02 August 2000/ Accepted in revised form 15 December 2000     

Performance assessment of malolactic fermenting bacteria Oenococcus oeni and Lactobacillus brevis in continuous culture

The growth performance of malolactic fermenting bacteria Oenococcus oeni NCIMB 11648 and Lactobacillus brevis X₂ was assessed in continuous culture. O. oeni grew at a dilution rate range of 0.007 to 0.052 h⁻¹ in a mixture of 5:6 (g l⁻¹) of glucose/fructose at an optimal pH of 4.5, and L. brevis X₂ grew at 0.010 to 0.089 h⁻¹ in 10 g l⁻¹ glucose at an optimal pH of 5.5 in a simple and safe medium. The cell dry weight, substrate uptake and product formation were monitored, as well as growth kinetics, yield parameters and fermentation balances were also evaluated under pH control conditions. A comparison of growth characteristics of two strains was made, and this showed significantly different performance. O. oeni has lower maximum specific growth rate (μ_max=0.073 h⁻¹), lower maximum cell productivity (Q _x ^max=17.6 mg cell l⁻¹ h⁻¹), lower maximum biomass yield (Y _x/s ^max=7.93 g cell mol⁻¹ sugar) and higher maintenance coefficient (m _s=0.45 mmol⁻¹ sugar g⁻¹ cell h⁻¹) as compared with L. brevis X₂ (μ_max=0.110 h⁻¹; Q _x ^max=93.2 g⁻¹ cell mol⁻¹ glucose; Y _x/s ^max=22.3 g cell mol⁻¹ glucose; m _s=0.21 mmol⁻¹ glucose g⁻¹ cell h⁻¹). These data suggest a possible more productive strategy for their combined use in maturation of cider and wine.     

Kinetic comparisons of mesophilic and thermophilic aerobic biomass

Kinetic parameters describing growth and decay of mesophilic (30°C) and thermophilic (55°C) aerobic biomass were determined in continuous and batch experiments by using oxygen uptake rate measurements. Biomass was cultivated on a single soluble substrate (acetate) in a mineral medium. The intrinsic maximum growth rate (μ _max) at 55°C was 0.71±0.09 h⁻¹, which is 1.5 times higher than the μ _max at 30°C (0.48±0.11 h⁻¹). The biomass decay rates increased from 0.004 h⁻¹ at 30°C to 0.017 h⁻¹ at 55°C. Monod constants were very low for both types of biomass: 9±2 mg chemical oxygen demand (COD) l⁻¹at 30°C and 3±2 mg COD l⁻¹at 55°C. Theoretical biomass yields were similar at 30 and 55°C: 0.5 g biomass COD (g acetate COD)⁻¹. The observed biomass yields decreased under both temperature conditions as a function of the cell residence time. Under thermophilic conditions, this effect was more pronounced due to the higher decay rates, resulting in lower biomass production at 55°C compared to 30°C. Electronic Publication     

Production of lipase by high cell density fed-batch culture of Candida cylindracea

Candida cylindracea NRRL Y-17506 was grown to produce extracellular lipase from oleic acid as a carbon source. Through flask cultures, it was found that the optimum initial oleic acid concentration for cell growth was 20 g l⁻¹. However, high initial concentrations of oleic acid up to 50 g l⁻¹ were not inhibitory. The highest extracellular lipase activity obtained in flask culture was 3.0 U ml⁻¹ after 48 h with 5 g l⁻¹ of initial oleic acid concentration. Fed-batch cultures (intermittent and stepwise feeding) were carried out to improve cell concentration and lipase activity. For the intermittent feeding fed-batch culture, the final cell concentration was 52 g l⁻¹ and the extracellular lipase activity was 6.3 U ml⁻¹ at 138.5 h. Stepwise feeding fed-batch cultures were carried out to simulate an exponential feeding and to investigate the effects of specific growth rate (0.02, 0.04 and 0.08 h⁻¹) on cell growth and lipase production. The highest final cell concentration obtained was 90 g l⁻¹ when the set point of specific growth rate (μ_set) was 0.02 h⁻¹. High specific growth rate (0.04 and 0.08 h⁻¹) decreased extracellular lipase production in the later part of fed-batch cultures due to build-up of the oleic acid oversupplied. The highest extracellular lipase activity was 23.7 U ml⁻¹ when μ_set was 0.02 h⁻¹, while the highest lipase productivity was 0.31 U ml⁻¹ h⁻¹ at μ_set of 0.08 h⁻¹.     

Production of ascorbic acid glucoside by alginate-entrapped mycelia of <Emphasis Type="Italic">Aspergillus niger</Emphasis>

The mycelia of Aspergillus niger, cultivated in a medium containing 45 g l⁻¹ maltose, 66 g l⁻¹ yeast extract, and 5 g l⁻¹ K₂HPO₄ at 30°C and 200 rpm, were used as a biocatalyst in the glucosylation of ascorbic acid. Free mycelia from 3-day-old culture, when used in a 6-h reaction with maltose as the acyl donor, gave 16.07 g l⁻¹ ascorbic acid glucoside corresponding to a volumetric productivity of 2.68 g l⁻¹ h⁻¹ and a conversion of 67%. Mycelia from 3-day-old cultures were entrapped in calcium alginate beads and used as a catalyst in the glucosylation of ascorbic acid. An ascorbic acid-to-maltose molar ratio of 1:9 was found to be optimum, and the conversion reached 75% after 12 h. The concentration of ascorbic acid glucoside produced at this molar ratio was 17.95 g l⁻¹, and the productivity was 1.5 g l⁻¹ h⁻¹. The biocatalyst was repeatedly used in a fixed bed bioreactor for the synthesis of ascorbic acid glucoside and approximately 17 g l⁻¹ of ascorbic acid glucoside corresponding to a volumetric productivity of 1.42 g l⁻¹ h⁻¹ was produced in each use. The conversion was retained at 70% in each use. The entrapped mycelia also exhibited exceptionally high reusability and storage stability. The product was purified to 85% by anion exchange and gel permeation chromatography with a final yield of 75%.     

Fermentative production of superoxide dismutase with <Emphasis Type="Italic">Kluyveromyces marxianus</Emphasis>

This work sought to develop a fermentative process for the microbial production of superoxide dismutase (SOD), to overcome extraction from animal tissues. Twenty-eight wild-type yeast strains were screened for SOD productivity. Kluyveromyces marxianus L3 showed the highest SOD activity (62 U mg⁻¹) and was used for process development. Oxidative stress conditions and parameters affecting oxygen transfer rate were exploited to improve production. The effects of dilution rate (0.067 vs 0.2 h⁻¹), aeration pressure (0.3 vs 1.2 bar) and H₂O₂ (0 vs 50 mM) were studied during chemostat experiments. Low dilution rate, high pressure and H₂O₂ resulted in an increase in CuZn–SOD up to 475 U mg⁻¹. When a regulation of oxygen saturation was applied during batch cultures, CuZn–SOD was progressively higher at 60, 80 and 90% dissolved oxygen tension (DOT) (250, 330 and 630 U mg⁻¹, respectively). Furthermore, the highest growth rate and biomass yield were achieved at 90% DOT, this being therefore the best DOT condition for high overall productivity. Growth and productivity on different carbon sources were compared. Specific activity was higher on glycerol than on lactose or glucose (496, 454 and 341 U mg⁻¹, respectively). The highest biomass yield was achieved on lactose. It may be therefore the best substrate for SOD production.     

Alginate production and <Emphasis Type="Italic">alg8</Emphasis> gene expression by <Emphasis Type="Italic">Azotobacter vinelandii</Emphasis> in continuous cultures

Alginates are polysaccharides that are used as thickening agents, stabilizers, and emulsifiers in various industries. These biopolymers are produced by fermentation with a limited understanding of the processes occurring at the cellular level. The objective of this study was to evaluate the effects of agitation rate and inlet sucrose concentrations (ISC) on alginate production and the expression of the genes encoding for alginate-lyases (algL) and the catalytic subunit of the alginate polymerase complex (alg8) in chemostat cultures of Azotobacter vinelandii ATCC 9046. Increased alginate production (2.4 g l⁻¹) and a higher specific alginate production rate (0.1 g g⁻¹ h⁻¹) were obtained at an ISC of 15 g l⁻¹. Carbon recovery of about 100% was obtained at an ISC of 10 g l⁻¹, whereas it was close to 50% at higher ISCs, suggesting that cells growing at lower sucrose feed rates utilize the carbon source more efficiently. In each of the steady states evaluated, an increase in algL gene expression was not related to a decrease in alginate molecular weight, whereas an increase in the molecular weight of alginate was linked to higher alg8 gene expression, demonstrating a relationship between the alg8 gene and alginate polymerization in A. vinelandii for the first time. The results obtained provide a possible explanation for changes observed in the molecular weight of alginate synthesized and this knowledge can be used to build a recombinant strain able to overexpress alg8 in order to produce alginates with higher molecular weights.     

A physiological and enzymatic study of Debaryomyces hansenii growth on xylose- and oxygen-limited chemostats

The effect of changing growth rate and oxygen transfer rate (OTR) on Debaryomyces hansenii physiology was studied using xylose-limited and oxygen-limited chemostat cultures, respectively, and complemented with enzymatic assays. Under xylose-limited chemostat (oxygen-excess), neither ethanol nor xylitol was produced over the entire range of dilution rate (D). The maximal volumetric biomass productivity was 2.5 g l^–1 h^–1 at D =0.25 h^–1 and cell yield was constant at all values of D. The respiratory rates and xylose consumption rate increased linearly with growth rate but, above 0.17 h^–1, oxygen consumption rate had a steeper increase compared to carbon dioxide production rate. Enzymatic analysis of xylose metabolism suggests that internal fluxes are redirected as a function of growth rate. For values of D up to 0.17 h^–1, the xylose reductase (XR) titre is lower than the xylitol dehydrogenase (XDH) titre, whereas above 0.17 h^–1 XR activity is about twice that of XDH and the NADPH-producing enzymes sharply increase their titres indicating an internal metabolic flux shift to meet higher NADPH metabolic requirements. Moreover, the enzymes around the pyruvate node also exhibited different patterns if D was above or below 0.17 h^–1. Under oxygen-limited chemostat (xylose-excess) the metabolism changed drastically and, due to oxidative phosphorylation limitation, cell yield decreased to 0.16 g g^–1 for an OTR of 1.4 mmol l^–1 h^–1 and xylitol became the major extracellular product along with minor amounts of glycerol. The enzymatic analysis revealed that isocitrate dehydrogenase is not regulated by oxygen, whereas XR, XDH and the NADPH-producing enzymes changed their levels according to oxygen availability. Electronic Publication     

Rapid assessment of oxygen transfer impact for Corynebacterium glutamicum

Oxygen supply is crucial in industrial application of microbial systems, such as Corynebacterium glutamicum, but oxygen transfer is often neglected in early strain characterizations, typically done under aerobic conditions. In this work, a new procedure for oxygen transfer screening is presented, assessing the impact of maximum oxygen transfer conditions (OTR_max) within microtiter plate-based cultivation for enhanced throughput. Oxygen-dependent growth and productivity were characterized for C. glutamicum ATCC13032 and C. glutamicum DM1933 (lysine producer). Biomass and lysine product yield are affected at OTR_max below 14 mmol L^?1 h^?1 in a standardized batch process, but not by further increase of OTR_max above this threshold value indicating a reasonable tradeoff between power input and oxygen transfer capacity OTR_max. The described oxygen transfer screening allows comparative determination of metabolic robustness against oxygen transfer limitation and serves identification of potential problems or opportunities later created during scale-up.     

Oxygen requirements for d-xylose fermentation to ethanol and polyols by Pachysolen tannophilus

The oxygen requirements for ethanol production from d-xylose (10 or 20 g l^?1) by Pachysolen tannophilus have been determined by controlling the availability of oxygen to shake flasks. Under anaerobic conditions no ethanol was produced whereas under aerobic conditions mainly biomass was formed. Semi-anaerobic conditions resulted in maximum ethanol production. By varying the stirring speed of a fermenter and supplying air to the liquid surface at various rates, the oxygen transfer rate (OTR) was controlled under semi-anaerobic conditions. By increasing the OTR from 0.05 to 16.04 mmol l^?1 h^?1, the ethanol yield coefficient decreased from 0.28 to 0.18 while the cell yield coefficient increased from 0.14 to 0.22. The accumulation of polyols decreased from 0.88 to 0.56 g l^?1 with increasing OTR. At OTRs between 0.09 and 1.18 mmol l^?1 h^?1, specific ethanol productivity attained a maximum value of 0.07 h^?1 and decreased with either increasing or decreasing OTR. The results indicate that the OTR must be carefully controlled for efficient ethanol production from d-xylose by P. tannophilus.