Butyric fermentation: metabolic behaviour and production performance of Clostridium tyrobutyricum in a continuous culture with cell recycle

Summary Two physiological characteristics of butyric fermentation, inhibition by the acids produced, butyrate and acetate, and dependence on the growth rate of the distribution of these acids, prompted a study of butyrate production in a continuous fermentation system with cell recycle by microfiltration. The influence of the main operating parameters, glucose input (feed concentration and dilution rate) and bleed dilution rate on production of acids and biomass was studied. The performance of the system greatly exceeded the results obtained in batch and simple continuous fermentations as a high productivity for butyrate (9.5 g l^–1 h^–1) was achieved whilst retaining a satisfactory concentration of butyrate (29.7 g l^–1) and low acetate production (0.6 g l^–1) at a cell biomass concentration of 35 g l^–1. Cell growth rate was found to be a critical parameter for performance stability as oscillations in metabolic activity due to inhibition by acids were observed at bleed dilution rates below 0.016 h^–1.Offprint requests to: J. P. Vandecasteele     

Effect of salinity on mycorrhizal onion and tomato in soil with and without additional phosphate

Summary Vesicular-arbuscular mycorrhizal fungi (VAM) are known to increase plant growth in saline soils. Previous studies, however, have not distinguished whether this growth response is due to enhanced P uptake or a direct mechanism of increased plant salt tolerance by VAM. In a glasshouse experiment onions (Allium cepa L.) were grown in sterilized, low-P sandy loam soil amended with 0, 0.8, 1.6 mmol P kg^–1 soil with and without mycorrhizal inoculum. Pots were irrigated with saline waters having conductivities of 1.0, 2.8, 4.3, and 5.9 dS m^–1. Onion colonized withGlomus deserticola (Trappe, Bloss, and Menge) increased growth from 394% to 100% over non-inoculated control plants when soil P was low ( 0.2 mmol kg^–1 NaHCO₃-extractable P) at soil saturation extract salinities from 1.1 dS m^–1 to 8.8 dS m^–1. When 0.8 and 1.6 mM P was added no dry weight differences due to VAM were observed, however, K and P concentrations were higher in VAM plants in saline treatments.Glomus fasciculatum (Gerdeman and Trappe) andGlomus mosseae (Nicol. and Gerd.) isolates increased growth of VAM tomato 44% to 193% in non-sterilized, saline soil (10 dS m^–1 saturation extract) despite having little effect on growth in less saline conditions when soil P was low. Higher tomato water potentials, along with improved K nutrition by VAM in onion, indicate mechanisms other than increased P nutrition may be important for VAM plants growing under saline stress. These effects appear to be secondary to the effects of VAM on P uptake.     

The role of Gloeotrichia echinulata in the transfer of phosphorus from sediments to water in Lake Erken

The abundance of Gloeotrichia echinulata colonies in the sediments of Lake Erken and their phosphorus content were investigated to determine the contribution of Gloeotrichia colonies to total sediment phosphorus. Moreover, the potential size of the algal inoculum and the migration to the water during summer were estimated.The surplus phosphorus content of the resting colonies in the sediment was about 45% of total phosphorus, which maximized at 8.5 µg P (mg dw)^–1 or 81 ng P colony^–1. The C:P ratio (by weight) in the early colonies appearing in the lake water was 50:1, while the ratio stabilized at 150 during the major migration period. The internal supply of surplus phosphorus was used during the pelagic growth of the colonies.The internal phosphorus loading to the epilimnion of Lake Erken due to Gloeotrichia migration could, from the measurements of the increase in particulate epilimnetic phosphorus, be estimated at 40 mg P m ^–2 or 2.5 mg P m^–2 d^–1 in late July and early August. Determination of the number of colonies in the sediment before and during the migration verified this value to be a conservative estimate of the internal phosphorus loading due to Gloeotrichia migration to the epilimnion in Lake Erken.The sediment P content calculated from the P concentration in early epilimnion colonies resulted in a value of 35 µg P (g dw)^–1 as a maximum. This corresponds to only 3% of the total phosphorus content in Lake Erken sediment.     

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     

Axial rod growth and age estimation of the sea pen,Halipteris willemoesi Kölliker

Halipteris willemoesi is a large octocoral commonly found in the Bering Sea. It is a member of a ubiquitous group of benthic cnidarians called sea pens (Octocorallia: Pennatulacea). Sea pens have a skeletal structure, the axial rod, that in cross section exhibits growth rings. Pairs of adjacent rings, or ring couplets, were assumed to be annuli and were used to estimate the age and growth of H. willemoesi. Twelve axial rods, extracted from H. willemoesi collected in the Bering Sea, were selected to represent small (25–29 cm total length), medium (97–130 cm TL) and large (152–167 cm TL) colonies. Each rod resembled a tapered dowel; the thickest part (0.90–6.75 mm in diameter) was at about 5–10% of total length from the base tip, the distal part was more gradually tapered than was the base. The number of ring couplets increased with rod size indicating their utility in estimating age and growth. Estimated age among rods was based on couplet counts at the thickest part of each rod; the average estimated age (±SE) was 7.1 ±0.7, 19.3 ±0.5, and 44.3 ±2.0 yr for small, medium and large-size rods, respectively. Based on these estimated ages, average growth rate in total length was 3.9 ± 0.2, 6.1 ± 0.3, and 3.6 ± 0.1 cm yr^–1 for small, medium, and large-size colonies. The average annual increase in maximum rod diameter among all colonies was 0.145 ± 0.003 SE mm yr^–1; therefore, age prediction from maximum rod diameter was calculated (estimated age (yr) = 7.0 * (maximum rod diameter, mm) –0.2; R ² = 0.99). At maximum diameter, the average couplet width was relatively constant among the three colony sizes (0.072 ± 0.05 mm). X-ray diffraction and electron microprobe analyses revealed that the inorganic portion of the rod is composed of a high-magnesium calcite. Radiometric validation of these age and growth rate estimates was attempted, but high amounts of exogenous ²¹⁰Pb precluded using the disequilibria of ²¹⁰Pb:²²⁶Ra. Instead, ²¹⁰Pb activities were measured in a series of cores extracted along the axial rod. These activities ranged from 0.691 ± 0.036 (SE) to 2.76 ± 0.13 dpm g^–1, but there was no pattern of decay along the length of the rod; therefore, the growth rates and corresponding ages could not be validated. Based on estimated age from ring couplet counts, growth in total rod length is slow at first, fastest at medium size, and slows toward maximum size, with an estimated longevity approaching 50 yr.     

The effect of mercury on the growth rate of Fragilaria crotonensis kitton and Asterionella formosa Hass.

The effects of two mercury salts Hg(NO₃)₂ and HgCl₂ on the growth rate ofFragilaria crotonensis Kitton andAsterionella formosa Hass. were investigated.Fragilaria crotonensis showed total inhibition at 0.1 ppm and a 4 day increase in lag phase along with a 2–4 fold reduction in growth rate at 0.05 ppm.Asterionella formosa showed a gradual increase in lag phase and reduction in growth rate with increasing concentrations of mercury up to 0.25 ppm with total inhibition at 0.5 ppm. Mercury salts in cultures of both species with soil extract additives were significantly less toxic than in totally defined systems. Cultures ofA. formosa deviated from the typical 8–16 celled stellate colony at sublethal concentrations to form large cylindrical stacks composed of 25 to 30 colonies.This study was supported in part by funds provided by the Navajo and Kaiparowits project participants.This study was supported in part by funds provided by the Navajo and Kaiparowits project participants.     

Anaerobiosis and low specific growth rates enhance hemolysin BL production by <Emphasis Type="Italic">Bacillus cereus</Emphasis> F4430/73

Bacillus cereus F4430/73 produced the highest levels of hemolysin BL (HBL) when grown under anaerobiosis in MOD medium. Anaerobic cells grown in a chemostat at low specific growth rate (0.1–0.2 h^–1) expressed up to sevenfold more HBL than did cells held at a faster growth rate. At 0.2 h^–1, the presence of 90 mM glucose resulted in inhibition of HBL production. Glucose was found to repress HBL induction at the mRNA level, indicating the potential involvement of catabolite repression in the regulation of HBL. Based on these data, it is suggested that growth rate could be an effector of catabolite regulation of HBL.     

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.     

Light-temperature interactions on the growth of Antarctic diatoms

Summary The combined effect of various temperatures and light intensities on the growth of seven species of antarctic diatoms in culture has been studied. With the exception of Chaetoceros deflandrei whose thermal tolerance is fairly good, these obligatory psychrophils cannot survive in temperatures above 6° to 9° C. Their mean growth rate is relatively low, between 0.24 div d^–1 for Corethron criophilum and 0.63 div d^–1 for C. deflandrei. Regardless of light intensity, growth rate increased with the temperature to reach a maximum between 3° and 5° C. The highest rates were obtained between 115 and 220 mol m^–2 s^–1 with 0.38 div d^–1 for C. criophilum, 0.56 div d^–1 for Synedra sp. and between 0.71 and 0.88 div d^–1 for the other 5 species. A reduction in light intensity from 220 to 46 mol m^–2 s^–1 slowed growth by nearly 50%. These results suggest that the combined effect of temperature and light is one of the factors involved in the limitation of antarctic phytoplankton growth. The low temperatures of the environment do not permit rapid growth, which, even under optimal light conditions remains low. In addition, in the euphotic layer, the overall light energy available for algae is considerably reduced due to turbulence, a factor which exacerbates the reduced growth rate.     

Effect of pH and acetic acid on growth and 2,3-butanediol production of Enterobacter aerogenes in continuous culture

Summary The effect of pH and acetic acid on growth and 2,3-butanediol production of Enterobacter aerogenes from glucose was investigated in a microaerobic continuous culture. At a dilution rate of 0.20 h^–1 and a fixed oxygen uptake rate (OUR) of 31.5 mmol l^–1 h^–1 the biomass concentration increased with pH ranging from 5.0 to 7.0, while the specific ATP requirement of the cells decreased. In the pH range 5.5–6.5 the product concentration (butanediol + acetoin) was maximal and nearly constant. However, the specific production continuously declined with increasing pH. Experiments with addition of acetic acid showed that the various effects of pH are due to inhibition of the by-product acetic acid on cell growth. The strength of the acetic and inhibition depended only on the concentration of its undissociated form [HAc]. The biomass concentration and the specific OUR were also only functions of [HAc], irrespective of the pH. Although the specific ATP requirement (q ATP) strongly depended on the pH, [HAc] at constant pH. Offprint requests to: W.-D. Deckwer     

Development of a fermentation process for production of an alginate G-lyase from Klebsiella pneumoniae

A high-density-cell fermentation process for production of an exracellular alginat lyase from Klebseilla pneumoniae on a defined medium has been developed. The process employs a strategy using two carbon sources. One low-molecular-mass, low-viscosity carbon source (sucrose) with high water solubililty is used as the main carbons source for growth, while the high-molecular-mass and viscoous alginate in low concentration is used as an inducer for enzyme synthesis. The repression of algiante lyase production by sucrose and the growth inhibition that we observed at increased levels of ammonia were circumvented by a computer-assisted fed-batch addition of the carbon sources (succrose and alginate) and by supplying nitrogen source as ammonia in the pH control. No enzyme production was observed when dissolved oxygen limited growth at an oxygen uptake rate of 40%–50% of the maximum uptake rate. An optimal composition of the feeding solution (12.5 g alginate and 587.5 g sucrose 1^–1) was found both for the maximum final concentration of enzyme (1330 U 1^–1) and for the maximum volumetric rate of enzyme production (67 U 1^–1 h^–1). The enzyme production dependes of the growth rate in the linear growth phase, giving a maximum enzyme concentration at the highest growth rate tested. The final enzyme concentration shows a fiveflod increase compare with previously reproted daata where alginate was used as a carbon source. In addition, the ratio of alginate lyase by a factor of apporximately 15. A doubling in extracellular specific activity of the enzyme was observed, a property of significant interest, especially for purification of the enzyme. On the othr hand, the final dry cell weight concentration of the bacteria also increased by a factor of 15–20 thus giving a relatively lower specific productivity of 0.4 U (g cell dry weight)^–1 h^–1.     

Effects of aflatoxin B1 on in vitro cultures ofNicotiana tabacum var. Samsun

Calli ofNicotiana tabacum (tobacco) were treated with two dose ranges of aflatoxin B₁ (0.1–2.0 µg ml^–1 - low does; 5–25 µg ml^–1 aflatoxin B₁). The ability of calli to recover following 3 weeks of toxin exposure was also investigated. The I₅₀ (50% inhibition) value for fresh mass accumulation was approximately 2 µg ml^–1 AFB₁. Fresh mass accumulation was significantly lower than the control value from 0.5 µg ml^–1 AFB₁. Following 3 weeks growth without a toxin source, the growth of calli up to and including 10 µg ml^–1 AFB₁, was significantly greater than control calli, indicating reversibility of the toxic effects. With increasing toxin concentration, chlorophyll content of callus was inhibited from 0.5 µg ml^–1. Transfer to a toxin-free medium resulted in a degree of recovery (up to 0.5 µg ml^–1). In the dose range 5–25 µg ml^–1, the levels of chlorophyll were drastically reduced, with no recovery following AFB₁ removal. Electron microscopy revealed a disruption of chloroplast structure as an early deteriorative event in AFB₁ exposure of callus cells. Protein levels were less sensitive, with inhibition manifested only in the high dose range. Shoot development occurred at all concentrations, but was significantly inhibited from 5 µg ml^–1 AFB₁. Recovery following toxin removal was minimal at these higher AFB₁ concentrations. The number of necrotic calli increased progressively from 5 µg ml^–1 as toxin levels increased.     

The impact of NO inf3 sup− loading on the freshwater macrophyte Littorella uniflora: N utilization strategy in a slow-growing species from oligotrophic habitats

The decline and disappearance of Littorella uniflora from oligotrophic waters which have become eutrophic has been associated with shading or reduced CO₂ supply. However NO _inf3 ^sup– concentrations can reach very high levels (100–2000 mmol m^–3 compared with <1–3 in oligotrophic habitats). To investigate the impact of NO _inf3 ^sup– loading alone, plants were grown under three NO _inf3 ^sup– regimes (very low, near-natural and high). The interactive effects of NO _inf3 ^sup– and photon flux density (low and high regimes) on N assimilation and accumulation, CO₂ concentrating mechanisms, C₃ photosynthesis and growth were also examined. The results were unexpected. Increased NO _inf3 ^sup– supply had very little effect on photosynthetic capacity, crassulacean acid metabolism (CAM) or lacunal CO₂ concentrations ([CO₂]_i), although there was considerable plasticity with respect to light regime. In contrast, increased NO _inf3 ^sup– supply resulted in a marked accumulation of NO _inf3 ^sup– , free amino acids and soluble protein in shoots and roots (up to 25 mol m^–3, 30 mol m^–3 and 9 mg g^–1 fresh weight respectively in roots), while fresh weight and relative growth rate were reduced. Total N content even under the very low NO _inf3 ^sup– regime (1.6–2.3%) was mid-range for aquatic and terrestrial species (and 3.1–4.3% under the high NO _inf3 ^sup– regime). These findings, together with field data, suggest that L. uniflora is not growth limited by low NO _inf3 ^sup– supply in natural oligotophic habitats, due not to an efficient photosynthetic nitrogen use but to a slow growth rate, a low N requirement and to the use of storage to avoid N stress. However the increased NO _inf3 ^sup– concentrations in eutrophic environments seem likely have detrimental effects on the long-term survival of L. uniflora, possibly as a consequence of N accumulation.     

Response on scallop culture to enhanced nutrient supply by experimental fertilisation of a landlocked bay

The effect of an enhanced nutrient supply to coastal waters of a landlocked bay, Hopavågen in Central Norway, on the phytoplankton production and biomass, and on growth of scallops (Pecten maximus) was studied in 1997–1999. Nitrogen, silicon and phosphorous (N:Si:P = 16:8:1, atomic) were added daily between May and October in 1998 at a level of 0.4 mg P m^–3 day^–1. The concentration of nutrient addition was doubled in 1999 during the same period. High addition of nutrients (1999) resulted in a significantly higher phytoplankton biomass in the summer period, expressed as chlorophyll a content, than without nutrient (1997) and low nutrient (1998). The respective mean chlorophyll a levels were 2.4 in 1999, 1.6 in 1998 and 1.2 g l^–1 in 1997. The mean primary production during the summers generally increased with the addition of nutrients from an average level of 320 mg carbon m^–2 day^–1 in 1997 to 1200 mg carbon m^–2 day^–1 in 1999. Scallops placed at 10 m depth in Hopavågen showed an increase in growth rate of the outer scallop shell in the period July–September from 0.16% day^–1 in 1997 to 0.53% day^–1 in 1998. Scallops grown in an unfertilised control station in the fjord outside Hopavågen had a significantly lower growth rate than those grown in the fertilised water of Hopavågen. The results showed decreased growth rate with increasing shell sizes. However, for all size groups studied a higher growth rate of the scallops was observed when nutrients were added to the bay. The tissue dry weight content of scallops grown in Hopavågen was 2–4 times higher than in the control scallops.     

Effect of natural zooplankton feeding on the tissue and skeletal growth of the scleractinian coral<Emphasis Type="Italic"> Stylophora pistillata</Emphasis>

Laboratory experiments were designed to estimate the ingestion rates of the scleractinian coral Stylophora pistillata under varying prey concentrations and feeding regimes and to assess the effect of feeding on the tissue and skeletal growth. Six sets of corals were incubated under two light (80 and 300 µmol photons m^–2 s^–1) and three feeding levels (none, fed twice, and fed six times per week) using freshly collected zooplankton. Results showed that the number of prey ingested was proportional to prey density, and no saturation of feeding capability was reached. Capture rates varied between 0.5 and 8 prey items 200 polyp^–1 h^–1. Corals starved for several days ingested more plankton than did fed corals. Fed colonies exhibited significantly higher levels of protein, chlorophyll a, and chlorophyll c₂ per unit surface area than starved colonies. Feeding had a strong effect on tissue growth, increasing it by two to eight times. Calcification rates were also 30% higher in fed than in starved corals. Even moderate levels of feeding enhanced both tissue and skeletal growth, although the processes involved in this enhancement remain to be determined.     

Potential of Thiobacillus ferrooxidans for waste gas purification. Part 1. Kinetics of continuous ferrous iron oxidation

Kinetic data of ferrous iron oxidation by Thionacillus ferrooxidans were determined. The aim was to remove H₂S (<0.5 ppm) from waste gas by a process proposed earlier. Kinetic data necessary for industrial scale-up were investigated in a chemostat airlift reactor (dilution rate 0.02–0.12 h^–1; pH 1.3). Due to the low pH, ferric iron precipitation and wall growth could be avoided. The maximum ferrous iron oxidation rate of submersed bacteria was 0.77 g 1^–1 h^–1, the maximum specific growth rate about 0.12 h^–1 and the yield coefficient was found to be 0.007 g g^–1 Fe²⁺. The specific O₂ demand of an exponentially growing, ironoxidizing batch culture was 1.33 mg O₂ mg^–1 biomass h^–1. The results indicate that a pH of 1.3 has no negative influence on the kinetics of iron oxidation and growth. Correspondence to: W. Schäfer-Treffenfeldt     

The effect of light on the growth rate of intertidal Acropora pulchra (Brook) from Phuket,Thailand, lat. 8°N

Series of apices of Acropora pulchra from an intertidal reef at Phuket, Thailand, were grown at different depths in the sea, and the length growth was monitored at 12–24 h intervals with laser diffraction. The growth rates of actively growing apices were normally distributed and showed a high variability, with an average coefficient of variation of 58%. There was a highly significant difference in average growth rate between adjacent colonies. A significant linear relationship was found between irradiance and length growth, with a saturating level at 300–400 Wm^-2. At 1000 Wm^-2 length growth was significantly reduced. Under normal daylight conditions, day and night growth rates were equal.     

Status and structure of the griffon vulture (Gyps fulvus) population in Crete

Griffon vulture (Gyps fulvus) population surveys were conducted during 1996–2002 in the island of Crete (Greece) to document population status and structure. Fieldwork was carried out during the breeding period when birds could be monitored in their colonies. Total population size was estimated at 379 individuals (range = 341–417) with adult birds comprising 63%. The breeding population was estimated at 141 pairs, which were distributed on an average in 23 colonies per year (range = 16–30) while the mean number of breeding pairs that laid eggs was 98 (range= 64–126). Crete thus supports the largest insular population of the species in the world and hosts 70–80% of the breeding population of the species in Greece. Population density was estimated at 6.9 individuals/100 km², 2.6 breeding pairs/100 km² and 1.8 nesting pairs/100 km². The average home range of an occupied colony (i.e., breeding group) was estimated at ca. 204 km² producing a theoretical foraging range of 8 km radius around the breeding cliff. No trends in the total number of individuals and breeding pairs appeared to exist, although significant differences in population size of individual colonies occurred between the years. The majority of the population was concentrated in small-sized colonies, which showed a low occupancy rate. The number of abandoned sites and the colonization of new ones could represent a shift of breeding pairs to alternative colonies provoked by local food abundance and conspesific attraction.     

n-3 Polyunsaturated fatty acid productivity of the marine microalgaIsochrysis galbana. Growth conditions and phenotypic selection

Two set of isolates were obtained in an isolation/selection programme to select eicosapentaenoic acid (EPA) and/or docosahexaenoic acid (DHA) rich strains ofIsochrysis galbana. EPA content was improved up to an average of 5.3% (d.wt) for the second set of isolates. On the other hand, with the aeration rate, pH and irradiance kept at low levels, the growth rate was slow and EPA synthesis was enhanced, but productivity increased when growth rates were maximum. A model relating steady-state dilution rates in chemostat cultures ofI. galbana to internal average irradiance is proposed. The greatest productivities were obtained between 0.0295 h^–1 and 0.0355 h^–1 with 300 mg m^–3 h^–1 for EPA and 130 mg m^–3 h^–1 for DHA.     

Relationship between substrate inhibition and maintenance energy ofChlamydomonas reinhardtii in heterotrophic culture

The relationship between substrate inhibition and maintenance energy ofChlamydomonas reinhardtii grown heterotrophically on acetate was investigated. At low acetate concentrations (<0.4 g l^–1), where no inhibition of cell growth was observed, the cell growth yield and specific growth rate could be represented by the Pirt model, 1/Y=1/Y _g +m/ with a constant value of maintenance energy coefficient m. However, at high acetate concentrations (>0.4 g l^–1), inhibition of cell growth occurred, in which m became variable and dependent on the acetate concentration. A simple mathematical model was proposed to predict the actual maintenance energy coefficient m in the inhibited cultures and experimentally validated.Author for correspondence