Microbial sensor system which usesMethylomonas sp. for the determination of methane

Summary 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.     

Kinetics of styrene biodegradation by <Emphasis Type="Italic">Pseudomonas</Emphasis> sp. E-93486

The research into kinetics of styrene biodegradation by bacterial strain Pseudomonas sp. E-93486 coming from VTT Culture Collection (Finland) was presented in this work. Microbial growth tests in the presence of styrene as the sole carbon and energy source were performed both in batch and continuous cultures. Batch experiments were conducted for initial concentration of styrene in the liquid phase changed in the range of 5–90 g m⁻³. The Haldane model was found to be the best to fit the kinetic data, and the estimated constants of the equation were: μ _m = 0.1188 h⁻¹, K _S = 5.984 mg l⁻¹, and K _i = 156.6 mg l⁻¹. The yield coefficient mean value Y_\textxs^\textapp Y_{\text{xs}}^{\text{app}} for the batch culture was 0.72 g_{dry cells weight} (g_substrate)⁻¹. The experiments conducted in a chemostat at various dilution rates (D = 0.035–0.1 h⁻¹) made it possible to determine the value of the coefficient for maintenance metabolism m _d = 0.0165 h⁻¹ and the maximum yield coefficient value Y_\textxs^\textM = 0.913 Y_{\text{xs}}^{\text{M}} = 0.913 . Chemostat experiments confirmed the high value of yield coefficient Y_\textxs^\textapp Y_{\text{xs}}^{\text{app}} observed in the batch culture. The conducted experiments showed high activity of the examined strain in the styrene biodegradation process and a relatively low sensitivity to inhibition of its growth at higher concentrations of styrene in the solution. Such exceptional features of Pseudomonas sp. E-93486 make this bacterial strain the perfect candidate for technical applications.     

Immobilization of Kluyveromyces marxianus cells containing inulinase activity in open pore gelatin matrix: 2. Application for high fructose syrup production

Batch and continuous production of high fructose syrup from Jerusalem artichoke tubers has been studied using yeast cells immobilized in open pore gelatin matrix. In a batch reactor, the hydrolysis was 93% ($\text{d}\text{-fructose/}\text{d}\text{-glucose}\text{= 90/10}$) and 42 mg d-fructose per ml was produced from the artichoke tuber extract by immobilized cells in 3 h. The same immobilized cells were recycled and used repeatedly for 10 batch cycles starting with fresh juice at the beginning of each cycle. It was found that immobilized cells were extremely stable and the percent hydrolysis was almost constant for all 10 batch cycles. In a continuous reactor using an immobilized cell concentration of 65.7 g (dry wt) l^?1 of total working bioreactor volume, the percent hydrolysis was found to remain constant at ～100% at dilution rates <1.26 h^?1, but beyond that it decreased. Volumetric productivity attained its maximum value at D = 2.08 h^?1 and was found to be 100 g l^?1 h^?1. This was achieved at a feed sugar conversion of 80%. At 90% conversion and D = 1.66 h^?1, the productivity was found to be 90 g l^?1 h^?1. Continuous operation of the immobilized cell bioreactor at a constant dilution rate of 1.65 h^?1 for 240 h resulted in only 2% loss of original activity.     

Molecular Analysis of Surfactant-Driven Microbial Population Shifts in Hydrocarbon-Contaminated Soil

We analyzed the impact of surfactant addition on hydrocarbon mineralization kinetics and the associated population shifts of hydrocarbon-degrading microorganisms in soil. A mixture of radiolabeled hexadecane and phenanthrene was added to batch soil vessels. Witconol SN70 (a nonionic, alcohol ethoxylate) was added in concentrations that bracketed the critical micelle concentration (CMC) in soil (CMC′) (determined to be 13 mg g⁻¹). Addition of the surfactant at a concentration below the CMC′ (2 mg g⁻¹) did not affect the mineralization rates of either hydrocarbon. However, when surfactant was added at a concentration approaching the CMC′ (10 mg g⁻¹), hexadecane mineralization was delayed and phenanthrene mineralization was completely inhibited. Addition of surfactant at concentrations above the CMC′ (40 mg g⁻¹) completely inhibited mineralization of both phenanthrene and hexadecane. Denaturing gradient gel electrophoresis of 16S rRNA gene segments showed that hydrocarbon amendment stimulated Rhodococcus and Nocardia populations that were displaced by Pseudomonas and Alcaligenes populations at elevated surfactant levels. Parallel cultivation studies revealed that the Rhodococcus population can utilize hexadecane and that the Pseudomonas and Alcaligenes populations can utilize both Witconol SN70 and hexadecane for growth. The results suggest that surfactant applications necessary to achieve the CMC alter the microbial populations responsible for hydrocarbon mineralization.     

Strong seasonal patterns of DOC release by a temperate seaweed community: Implications for the coastal ocean carbon cycle

Release of dissolved organic carbon (DOC) by seaweed underpins the microbial food web and is crucial for the coastal ocean carbon cycle. However, we know relatively little of seasonal DOC release patterns in temperate regions of the southern hemisphere. Strong seasonal changes in inorganic nitrogen availability, irradiance, and temperature regulate the growth of seaweeds on temperate reefs and influence DOC release. We seasonally surveyed and sampled seaweed at Coal Point, Tasmania, over 1 year. Dominant species with or without carbon dioxide (CO₂) concentrating mechanisms (CCMs) were collected for laboratory experiments to determine seasonal rates of DOC release. During spring and summer, substantial DOC release (10.06–33.54 μmol C · g DW⁻¹ · h⁻¹) was observed for all species, between 3 and 27 times greater than during autumn and winter. Our results suggest that inorganic carbon (C_i) uptake strategy does not regulate DOC release. Seasonal patterns of DOC release were likely a result of photosynthetic overflow during periods of high gross photosynthesis indicated by variations in tissue C:N ratios. For each season, we calculated a reef-scale net DOC release for seaweed at Coal Point of 7.84–12.9 g C · m⁻² · d⁻¹ in spring and summer, which was ~16 times greater than in autumn and winter (0.2–1.0 g C · m⁻² · d⁻¹). Phyllospora comosa, which dominated the biomass, contributed the most DOC to the coastal ocean, up to ~14 times more than Ecklonia radiata and the understory assemblage combined. Reef-scale DOC release was driven by seasonal changes in seaweed physiology rather than seaweed biomass.     

Ethanol production from cane juice by Zymomonas mobilis

Summary Zymomonas mobilis Z 7 fermented 100 to 200 g.l^{- 1} sucrose in cane juice to ethanol without addition of cofactors or mineral salts in 1 ltr laboratory and 100 ltr pilot plant fermenters. Ethanol yields (E_yield) were from 60 to 88% with fermentation times of 20 to 29 h at 35 °C.Nomenclature V_s max g.1^-1 .h^-1 maximum sucrose hydrolysis rate - V_g max g.1^-1 .h^-1 maximum glucose uptake rate - V_fmax g.1^-1 .h^-1 maximum fructose uptake rate - V_e max g.1^-1 .h^-1 maximum ethanol production rate - S_h g.1^-1 sucrose hydrolyzed at t_ferm - G_u g.1^-1 glucose utilized at t_ferm - F_u g.1^-1 fructose utilized at t_ferm - E_max g.1^-1 ethanol produced at t_ferm - G_i g.1^-1 initial free glucose (before sucrose hydrolysis) - E_yield g.g^-1 ethanol produced divided by the theoretical ethanol yield from sucrose hydrolyzed - t_ferm h fermentation time to ethanol max     

Characterization of microbial community and kinetics for spent sulfidic caustic applied autotrophic denitrification

Spent sulfidic caustic was applied to sulfur utilizing autotrophic denitrification as the simultaneous source of electron donor and alkalinity. The two experiment set-up of upflow anoxic hybrid growth reactor (UAHGR) and upflow anoxic suspended growth reactor (UASGR) was adopted and nitrate removals were similar in both reactors. Approximately 90% of the initial nitrate was denitrified at nitrate loading rate of 0.15∼0.40 kgNO₃ ⁻/m³·d. The experimental stoichiometric ratio of sulfate production to nitrate removal was ranged from 1.5 to 2.1 mgSO₄ ²⁻/mgNO₃ ⁻. During the operation period, denaturing gradient gel electrophoresis (DGGE) analysis of polymerase chain reaction (PCR)-amplified 16S rDNA fragments for the sludge sample of both reactors showed the change of microbial communities. Thiobacillus denitrificans-like microorganism occupied 28.5% (18 clones) of the 63 clones by cloning the PCR products from the sludge sample of UAHGR. Acidovorax avenae, which can reduce nitrate to nitrogen gas while oxidizing phenol (heterotrophic denitrifier), was also found in 7 clones (11.1%). Although an organic carbon source was not added to the medium, a microorganism (Kaistella koreensis) capable of oxidizing organic compounds was found in 7 clones (11.1%). Therefore, the microbial community of spent sulfidic caustic applied autotrophic denitrification process well corresponds to the substrate components of spent sulfidic caustic. Through the batch cultivation of microorganisms in UAHGR, the microbial kinetic coefficients of spent sulfidic caustic applied autotrophic denitrification were estimated to be μ _max = 0.097 h⁻¹, k _d = 0.0021 h⁻¹, K _s = 200 mgNO₃ ⁻/L, and Y = 0.31 mgMLVSS/mgNO₃ ⁻.     

Anaerobic degradation of phenol in batch and continuous cultures by a denitrifying bacterial consortium

Summary The anaerobic degradation of phenol under denitrifying conditions by a bacterial consortium was studied both in batch and continuous cultures. Anaerobic degradation was dependent on NO_f3 ^p– and concentrations up to 4 mm phenol were degraded within 2–5 days. During continuous growth in a fermenter, steady states could be maintained at eight dilution rates (D) corresponding to residence times between 12.5 and 50 h. Culture wash-out occurred at D=0.084 h^–1. The kinetic parameters obtained for anaerobic degradation of phenol under denitrifying conditions by the consortium were: maximam specific growth rate = 0.091 h^–1; saturation constant = 4.91 mg phenol/l; true growth yield = 0.57 mg dry wt/mg phenol; maintenance coefficient = 0.013 mg phenol/mg dry wt per hour. The Haldane model inhibition constant was estimated from batch culture data giving a value of 101 mg/l. The requirement of CO₂ for the anaerobic degradation of phenol with NO_f3 ^p– indicates that phenol carboxylation to 4-hydroxybenzoate was the first step of phenol degradation by this culture. 4-Hydroxybenzoate, proposed as an intermediate of phenol carboxylation under these conditions, was detected only in continuous cultures at very low growth rates (D=0.02 h^–1), but was never detected as a free intermediary metabolite either in batch or in continuous cultures. Correspondence to: N. Khoury     

Anaerobic degradation of p-cresol in batch and continuous cultures by a denitrifying bacterial consortium

Summary The anaerobic degradation of p-cresol under denitrifying conditions by a bacterial consortium was studied in batch and continuous cultures. Concentrations up to 3 mm were degraded within 5–6 days with 4-hydroxybenzyl alcohol, 4-hydroxybenzaldehyde and 4-hydroxybenzoate as intermediates. Steady states could be maintained at only one dilution rate, D=0.04 h^–1. A further increase in the dilution rate to 0.0 8 h^–1 resulted in culture wash-out. An estimation of the Saturation constant was made (<1 mg/l), taking the maximum specific growth rate as 0.045 h^–1, thus yielding a value of 0.125 mg p-cresol/l. Correspondence to: N. Khoury     

Porewater Stoichiometry of Terminal Metabolic Products, Sulfate, and Dissolved Organic Carbon and Nitrogen in Estuarine Intertidal Creek-bank Sediments

Porewater equilibration samplers were used to obtain porewater inventories of inorganic nutrients (NH₄⁺, NO_x, PO₄³⁻), dissolved organic carbon (DOC) and nitrogen (DON), sulfate (SO₄²⁻), dissolved inorganic carbon (DIC), hydrogen sulfide (H₂S), chloride (Cl⁻), methane (CH₄) and reduced iron (Fe²⁺) in intertidal creek-bank sediments at eight sites in three estuarine systems over a range of salinities and seasons. Sulfate reduction (SR) rates and sediment particulate organic carbon (POC) and nitrogen (PON) were also determined at several of the sites. Four sites in the Okatee River estuary in South Carolina, two sites on Sapelo Island, Georgia and one site in White Oak Creek, Georgia appeared to be relatively pristine. The eighth site in Umbrella Creek, Georgia was directly adjacent to a small residential development employing septic systems to handle household waste. The large data set (>700 porewater profiles) offers an opportunity to assess system-scale patterns of porewater biogeochemical dynamics with an emphasis on DOC and DON distributions. SO₄²⁻ depletion (SO₄²⁻)_Dep was used as a proxy for SR, and (SO₄²⁻)_Dep patterns agreed with measured (³⁵S) patterns of SR. There were significant system-scale correlations between the inorganic products of terminal metabolism (DIC, NH₄⁺ and PO₄³⁻) and (SO₄²⁻)_Dep, and SR appeared to be the dominant terminal carbon oxidation pathway in these sediments. Porewater inventories of DIC and (SO₄²⁻)_Dep indicate a 2:1 stoichiometry across sites, and the C:N ratio of the organic matter undergoing mineralization was between 7.5 and 10. The data suggest that septic-derived dissolved organic matter with a C:N ratio below 6 fueled microbial metabolism and SR at a site with development in the upland. Seasonality was observed in the porewater inventories, but temperature alone did not adequately describe the patterns of (SO₄²⁻)_Dep, terminal metabolic products (DIC, NH₄⁺, PO₄³⁻), DOC and DON, and SR observed in this study. It appears that production and consumption of labile DOC are tightly coupled in these sediments, and that bulk DOC is likely a recalcitrant pool. Preferential hydrolysis of PON relative to POC when overall organic matter mineralization rates were high appears to drive the observed patterns in POC:PON, DOC:DON and DIC:DIN ratios. These data, along with the weak seasonal patterns of SR and organic and inorganic porewater inventories, suggest that the rate of hydrolysis limits organic matter mineralization in these intertidal creek-bank sediments.     

Removal of n-hexane by Fusarium solani with a gas-phase biofilter

A gas-phase biofilter inoculated with the fungus Fusarium solani, isolated from a consortium grown on hexane vapors, was used to degrade this compound. The biofilter, packed with perlite and operated with an empty bed residence time of 60 s, was supplied with hexane concentrations between 0.5 g m⁻³ and 11 g m⁻³. Biofilter performance was evaluated over 100 days of operation. Several strategies for supplying the nutritive mineral medium were assayed to maintain favorable conditions for the fungal growth and activity. The Fusarium system was able to sustain an average elimination capacity of 90 g m⁻³ _reactor h⁻¹ with a maximum of 130 g m⁻³ _reactor h⁻¹ . The mass transfer limitations due to high biomass development in the biofilter were confirmed in batch experiments. Bacterial contamination was observed, but experiments in the biofilter and in batch reactors using selective inhibitors and controlled pH confirmed the predominant role of the fungus. Results indicate that fungal biofilters can be an effective alternative to conventional abatement technologies for treating hydrophobic compounds.

     

Effect of endogenous proteins on growth and antibody productivity in hybridoma batch cultures

It has been shown that some B-cell hybridomas secrete autocrine factorsin vitro which can influence cell metabolic processes. Rather than screen specifically for suspected cytokines, that may or may not affect our cell line, we have examined the lumped effects of intracellular and secreted factors on cell proliferation and monoclonal productivity in hybridoma batch cultures. Firstly, supplements of total soluble intracellular proteins combined with other intracellular metabolites were found to both decrease the specific growth rate and increase the antibody production rate at higher concentrations in batch culture. This is an important consideration in high cell density cultures, such as perfusion systems, where a reduction of growth by the presence of intracellular factors may be compensated by an increase in MAb production. In addition, flow cytometry data revealed that the average cell cycle G₁ phase fraction was unaffected by the variation in the maximum specific growth rates during the exponential growth phase, caused by the addition of intracellular factors; this suggests that higher MAb productivity at lower growth rates are not a result of cell arrest in the G₁ phase. Secondly, secreted extracellular proteins larger than 10,000 Daltons, which were concentrated from spent culture supernatant, were shown to have no significant effect on growth and specific MAb productivity when supplemented to batch culture at levels twice that encountered late in normal batch culture. This indicates that endogenous secreted cytokines, if at all present, do not play a major autocrine role for this cell line.Abbreviations FBS fetal bovine serum - MAb monoclonal antibody - MWCO molecular weight cut off - SDS-PAGE sodium dodecyl-sulphate-polyacrylamide gel electrophoresis - k _d exponential phase death rate, h^–1 - q _MAb exponential phase specific monoclonal antibody productivity, pg/(cell·h) - t time, h - X _d dead cell density, cells/mL - X _v viable cell density, cells/mL - specific growth rate, h^–1 - _{max app} apparent maximum specific growth rate, h^–1 - _max maximum specific growth rate, h^–1 _max = _{max app + Kd}      

Comparison of relative rates of BTEX biodegradation using respirometry

Biodegradation of BTEX by a microbial consortium isolated from a closed municipal landfill was studied using respirometric techniques. The kinetics of biodegradation were estimated from experimental oxygen uptake data using a nonlinear parameter estimation technique. All of the six compounds were rapidly degraded by the microbial culture and no substrate inhibition was observed at the concentration levels examined (200 mg L⁻¹ as COD). Microbial growth and contaminant degradation were adequately described by the Monod equation. Considerable differences were observed in the rates of BTEX biodegradation as seen from the estimates of the kinetic parameters. A three-fold variation was seen in the values of the maximum specific growth rate, μ_max. The highest value of μ_max was 0.389 h⁻¹ for p-xylene while o-xylene was characterized by a μ_max value of 0.14 h⁻¹, the lowest observed in this study. The half saturation coefficient, K _s, and the yield coefficient, Y, varied between 1.288–4.681 mg L⁻¹ and 0.272–0.645 mg mg⁻¹, respectively. Benzene and o-xylene exhibited higher resistance to biodegradation while toluene and p-xylene were rapidly degraded. Ethylbenzene and m-xylene were degraded at intermediate rates. In biodegradation experiments with a multiple substrate matrix, substrate depletion was slower than in single substrate experiments, suggesting an inhibitory nature of substrate interaction. Received 15 February 1998/ Accepted in revised form 5 July 1998     

Quantitative physiology of Pichia pastoris during glucose‐limited high‐cell density fed‐batch cultivation for recombinant protein production

Pichia pastoris has become one of the major microorganisms for the production of proteins in recent years. This development was mainly driven by the readily available genetic tools and the ease of high‐cell density cultivations using methanol (or methanol/glycerol mixtures) as inducer and carbon source. To overcome the observed limitations of methanol use such as high heat development, cell lysis, and explosion hazard, we here revisited the possibility to produce proteins with P. pastoris using glucose as sole carbon source. Using a recombinant P. pastoris strain in glucose limited fed‐batch cultivations, very high‐cell densities were reached (more than 200 g_CDW L^?1) resulting in a recombinant protein titer of about 6.5 g L^?1. To investigate the impact of recombinant protein production and high‐cell density fermentation on the metabolism of P. pastoris, we used ¹³C‐tracer‐based metabolic flux analysis in batch and fed‐batch experiments. At a controlled growth rate of 0.12 h^?1 in fed‐batch experiments an increased TCA cycle flux of 1.1 mmol g^?1 h^?1 compared to 0.7 mmol g^?1 h^?1 for the recombinant and reference strains, respectively, suggest a limited but significant flux rerouting of carbon and energy resources. This change in flux is most likely causal to protein synthesis. In summary, the results highlight the potential of glucose as carbon and energy source, enabling high biomass concentrations and protein titers. The insights into the operation of metabolism during recombinant protein production might guide strain design and fermentation development. Biotechnol. Bioeng. 2010;107: 357–368. © 2010 Wiley Periodicals, Inc.     

Fundamental aspects of waste sewage sludge treatment: Microbial solids biodegradation in an aerobic thermophilic semi-continuous system

The solubilization and biodegradation of whole microbial cells by an aerobic thermophilic microbial population was investigated over a 72 h period. Various parameters were followed including total suspended solids reduction, changes in the dissolved organic carbon, protein and carbohydrate concentrations, and carboxylic acid production and utilisation. From the rates of removal of the various fractions a simple model for the biodegradation processes is proposed and verified with respect to acetic acid production and utilization, and total suspended solids removal. The process is initiated by enzymic degradation of the substrate microbe cell walls followed by growth on the released soluble substrates at low dissolved oxygen concentration with concommitant carboxylic acid production. Subsequent utilization of the unbranched, lower molecular weight carboxylic acids allows additional energy supply following exhaustion of the easily utilisable soluble substrate from microbial cell hydrolysis.List of Symbols Y _Xp/Xs kg/kg yield process microbes on substrate yeast cells - Y _Xp/Ac kg/kg yield process microbes on acetate - Y _Ac/Ss kg/kg yield acetate produced by process microbes growing on substrate yeast cells - Y _Ss/Xs kg/kg yield soluble substrate from lysis of yeast cells - Y _Ss/Xp kg/kg yield soluble substrate from lysis of process microbes - Y _P/Xs kg/kg yield particulates from lysis of yeast cells - Y _P/Xp kg/kg yield particulates from lysis of process microbes - _{max (Ss)} h^–1 maximum specific growth rate constant for growth of process microbes on soluble substrate - _{max (Ac)} h^–1 maximum specific growth rate constant for growth of process microbes on acetate - Ks _Ss kg/m³ saturation coefficient for growth of process microbes on soluble substrate - Ks _Ac kg/m³ saturation coefficient for growth of process microbes on acetate - K _d h^–1 death/lysis rate constant for process microbes - K _i kg/m³ inhibition constant for growth of process microbes on acetate - K _L h^–1 lysis rate constant for whole yeast cells - K _h h^–1 hydrolysis rate constant for particulate biomass     

Batch and fed-batch fermentation of optically pure D (-) lactic acid from Kodo millet (Paspalum scrobiculatum) bran residue hydrolysate: growth and inhibition kinetic modeling

Abstract

A low-cost Kodo millet bran residue was utilized as feedstock for the production of D (?) lactic acid (DLA) using Lactobacillus delbrueckii NBRC3202 under anaerobic condition. Data culled from a series of batch fermentation processes with different initial Kodo millet bran residue hydrolysate (KMBRH) and DLA concentrations were used for kinetic model development. Both simulated and experimental data were in good agreement for cell growth, KMBRH utilization, and DLA formation. The values of kinetic constants specific growth rate, (μ_m = 0.17?h^?1); growth (α_P = 0.96?g.g^?1) and non-growth (β_P = 1.19?g.g^?1.h^?1) associated constant for DLA production and the maximum specific KMBRH utilization rate, (q_{G, max} = 1.18?g.g^?1.h^?1) were in good agreement with the literature reports. Kinetic analysis elucidated that L. delbrueckii growth was predominantly influenced by KMBRH limitation and highly sensitive to DLA inhibition. Fed-batch fermentation studies demonstrated the existence of substrate and product inhibition paving the scope for process intensification.     

Sedimentation in Boreal Lakes—The Role of Flocculation of Allochthonous Dissolved Organic Matter in the Water Column

We quantified sedimentation of organic carbon in 12 Swedish small boreal lakes (<0.48 km²), which ranged in dissolved organic carbon (DOC) from 4.4 to 21.4 mg C l⁻¹. Stable isotope analysis suggests that most of the settling organic matter is of allochthonous origin. Annual sedimentation of allochthonous matter per m² lake area was correlated to DOC concentration in the water (R ² = 0.41), and the relationship was improved when sedimentation data were normalized to water depth (R ² = 0.58). The net efflux of C as CO₂ from the water to the atmosphere was likewise correlated to DOC concentration (R ² = 0.52). The losses of organic carbon from the water column via mineralization to CO₂ and via sedimentation were approximately of equal importance throughout the year. Our results imply that DOC is a precursor of the settling matter, resulting in an important pathway in the carbon cycle of boreal lakes. Thus, flocculation of DOC of terrestrial origin and subsequent sedimentation could lead to carbon sequestration by burial in lake sediments.     

Growth kinetics and Pho84 phosphate transporter activity of <Emphasis Type="Italic">Saccharomyces cerevisiae</Emphasis> under phosphate-limited conditions

The effect of phosphate (P _i ) concentration on the growth behavior of Saccharomyces cerevisiae strain CEN.PK113-5D in phosphate-limited batch and chemostat cultures was studied. The range of dilution rates used in the present study was 0.08–0.45 h⁻¹. The batch growth of yeast cells followed Monod relationship, but growth of the cells in phosphate-limited chemostat showed change in growth kinetics with increasing dilution rates. The difference in growth kinetics of the yeast cells in phosphate-limited chemostat for dilution rates below and above approximately 0.2 h⁻¹ has been discussed in terms of the batch growth kinetic data and the change in the metabolic activity of the yeast cells. Immunological detection of a C-terminally myc epitope-tagged Pho84 fusion protein indicated derepressive expression of the Pho84 high-affinity P _i transporter in the entire range of dilution rates employed in this study. Phosphate transport activity mediated by Pho84 transporter was highest at very low dilution rates, i.e. 0.08–0.1 h⁻¹, corresponding to conditions in which the amount of synthesized Pho84 was at its maximum.     

Grape skins as a natural support for yeast immobilization

Grape skins were used to immobilize Saccharomyces cerevisiae. In repeated batch fermentations of grape by immobilized and free cells, the maximum specific rate of alcohol production on glucose decreased from 7.98 h^–1 at 25 °C to 0.7 h^–1 at 5 °C. The rate was approximately twice as high as that on fructose. The rates for free cells were very low. The maximum alcohol yield (0.45 g g^–1) was obtained at 5 °C when the immobilized biocatalyst was used.