Range Of Specific Growth Rates Research Articles

Proteome allocations change linearly with the specific growth rate of Saccharomyces cerevisiae under glucose limitation

Saccharomyces cerevisiae is a widely used cell factory; therefore, it is important to understand how it organizes key functional parts when cultured under different conditions. Here, we perform a multiomics analysis of S. cerevisiae by culturing the strain with a wide range of specific growth rates using glucose as the sole limiting nutrient. Under these different conditions, we measure the absolute transcriptome, the absolute proteome, the phosphoproteome, and the metabolome. Most functional protein groups show a linear dependence on the specific growth rate. Proteins engaged in translation show a perfect linear increase with the specific growth rate, while glycolysis and chaperone proteins show a linear decrease under respiratory conditions. Glycolytic enzymes and chaperones, however, show decreased phosphorylation with increasing specific growth rates; at the same time, an overall increased flux through these pathways is observed. Further analysis show that even though mRNA levels do not correlate with protein levels for all individual genes, the transcriptome level of functional groups correlates very well with its corresponding proteome. Finally, using enzyme-constrained genome-scale modeling, we find that enzyme usage plays an important role in controlling flux in amino acid biosynthesis.

B12 production by marine microbial communities and Dinoroseobacter shibae continuous cultures under different growth and respiration rates

AME Aquatic Microbial Ecology Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials AME 83:251-262 (2019) - DOI: https://doi.org/10.3354/ame01921 B12 production by marine microbial communities and Dinoroseobacter shibae continuous cultures under different growth and respiration rates Josué Villegas-Mendoza1,*, Ramón Cajal-Medrano2, Helmut Maske2 1UABC, Facultad de Ciencias Marinas, Carretera Tijuana-Ensenada km 106, Ensenada, Baja California CP 22860, Mexico 2CICESE, Carretera Tijuana-Ensenada No. 3918, Ensenada, Baja California CP 22860, Mexico *Corresponding author: jvillegas18@uabc.edu.mx ABSTRACT: In situ dissolved B12 concentration in marine ecosystems is controlled by the balance between rates of release of B12 by prokaryotes, uptake by prokaryotes and eukaryotes, and abiotic degradation. We used chemostats at a range of specific growth rates (μ, d-1; 0.1 to 1) with natural communities of prokaryotes and monospecific cultures of a B12 producer, Dinoroseobacter shibae. We measured the dissolved B12 concentration produced in the culture (B12-d), the B12 in the particulate fraction (B12-p), cell concentration, respiration rate, particulate organic carbon and nitrogen (POC, PON), and the 16S amplicon composition. Total dissolved B12 concentrations (0.92 to 4.90 pmol l-1) were comparable to those found in the surface ocean. B12-p concentration was 6 to 35 times higher than B12-d. B12-d, B12-p, and community composition showed no relation to μ for either natural populations or D. shibae. The chemostats allowed calculation of the rates of production: B12-d (0.34 ± 0.28 pmol l-1 d-1) and B12-p (5.65 ± 2.34 pmol l-1 d-1), and the B12 cell quota (900 to 3300 molecules cell-1). In multispecies and D. shibae cultures, B12 production rates per cell increased with respiration rates (volumetric or per cell), and with rates of cellular organic carbon and nitrogen production. Rates increased with μ, but not the concentrations of B12-d or of B12-p. To understand the physiological and ecological dynamics of B12, concentrations alone are insufficient since they do not provide rates, which are important in understanding the dynamics between producers and consumers. KEY WORDS: Marine bacteria · Intracellular and dissolved B12 · Continuous cultures Full text in pdf format Supplement 1Supplement 2 PreviousNextCite this article as: Villegas-Mendoza J, Cajal-Medrano R, Maske H (2019) B12 production by marine microbial communities and Dinoroseobacter shibae continuous cultures under different growth and respiration rates. Aquat Microb Ecol 83:251-262. https://doi.org/10.3354/ame01921 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in AME Vol. 83, No. 3. Online publication date: October 30, 2019 Print ISSN: 0948-3055; Online ISSN: 1616-1564 Copyright © 2019 Inter-Research.

Characterizing the effect of expression of an acetyl-CoA synthetase insensitive to acetylation on co-utilization of glucose and acetate in batch and continuous cultures of E. coli W

BackgroundDue to its high stress tolerance and low acetate secretion, Escherichia coli W is reported to be a good production host for several metabolites and recombinant proteins. However, simultaneous co-utilization of glucose and other substrates such as acetate remains a challenge. The activity of acetyl-CoA-synthetase, one of the key enzymes involved in acetate assimilation is tightly regulated on a transcriptional and post-translational level. The aim of this study was to engineer E. coli W for overexpression of an acetylation insensitive acetyl-CoA-synthetase and to characterize this strain in batch and continuous cultures using glucose, acetate and during co-utilization of both substrates.ResultsEscherichia coli W engineered to overexpress an acetylation-insensitive acetyl-CoA synthetase showed a 2.7-fold increase in acetate uptake in a batch process containing glucose and high concentrations of acetate compared to a control strain, indicating more efficient co-consumption of glucose and acetate. When acetate was used as the carbon source, batch duration could significantly be decreased in the overexpression strain, possibly due to alleviation of acetate toxicity. Chemostat cultivations with different dilution rates using glucose revealed only minor differences between the overexpression and control strain. Accelerostat cultivations using dilution rates between 0.20 and 0.70 h−1 indicated that E. coli W is naturally capable of efficiently co-utilizing glucose and acetate over a broad range of specific growth rates. Expression of acetyl-CoA synthetase resulted in acetate and glucose accumulation at lower dilution rates compared to the control strain. This observation can possibly be attributed to a higher ratio between acs and pta-ackA in the overexpression strain as revealed by gene expression analysis. This would result in enhanced energy dissipation caused by an imbalance in the Pta-AckA-Acs cycle. Furthermore, yjcH and actP, genes co-transcribed with acetyl-CoA synthetase showed significant down-regulation at elevated dilution rates.ConclusionsEscherichia coli W expressing an acetylation-insensitive acetyl-CoA synthetase was shown to be a promising candidate for mixed feed processes using glucose and acetate. Comparison between batch and continuous cultures revealed distinct differences in glucose-acetate co-utilization behavior, requiring additional investigations such as multi-omics analysis and further engineering towards even more efficient co-utilization strains of E. coli W.

The acetylation degree of alginates in Azotobacter vinelandii ATCC9046 is determined by dissolved oxygen and specific growth rate: studies in glucose-limited chemostat cultivations

Alginates are polysaccharides that may be used as viscosifiers and gel or film-forming agents with a great diversity of applications. The alginates produced by bacteria such as Azotobacter vinelandii are acetylated. The presence of acetyl groups in this type of alginate increases its solubility, viscosity, and swelling capability. The aim of this study was to evaluate, in glucose-limited chemostat cultivations of A. vinelandii ATCC9046, the influence of dissolved oxygen tension (DO) and specific growth rate (μ) on the degree of acetylation of alginates produced by this bacterium. In glucose-limited chemostat cultivations, the degree of alginate acetylation was evaluated under two conditions of DO (1 and 9 %) and for a range of specific growth rates (0.02-0.15 h⁻¹). In addition, the alginate yields and PHB production were evaluated. High DO in the culture resulted in a high degree of alginate acetylation, reaching a maximum acetylation degree of 6.88 % at 9 % DO. In contrast, the increment of μ had a negative effect on the production and acetylation of the polymer. It was found that at high DO (9 %) and low μ, there was a reduction of the respiration rate, and the PHB accumulation was negligible, suggesting that the flux of acetyl-CoA (the acetyl donor) was diverted to alginate acetylation.

Qualitative analysis of the stability of a continuous vermicomposting system

A mathematical model was established to describe ecological relationships in a continuous vermicomposting system. The distributions of organic matter, microbes and earthworms on non-dimensional specific growth rates were simulated. The range of specific growth rates were visualized utilizing three-dimensional reconstruction technology. The stability of a vermicomposting system was not influenced by the initial concentrations of microbes and earthworms, only their species. The coordinates of the stable point depended on the dilution rate and initial amount of organic matter. The method described could be help for establishing a stable continuous vermicomposting system.

Growth of juvenile Atlantic cod Gadus morhua in land-based recirculation systems: : Effects of feeding regime, photoperiod and diet

The combined effect of feeding regime and photoperiod on the growth of juvenile Atlantic cod Gadus morhua in land-based recirculating aquaculture systems (RAS) was examined using three different commercial diets. Fish of 8–10 g were reared in 1 m 3 tanks at an initial density of 10 kg m − 3 for 78 d. Three RAS units were used to simultaneously test three feed/photoperiod regimes that might be encountered in the wild or aquaculture; LightDark (LD) 24:0, LD12:11 (+ 1 h crepuscular periods) and LD6:6 (+ 12 h crepuscular periods). Feed was administered during the light period every 30 min for a 3 min feeding duration. In each RAS unit three diets (A, B and C) were tested, which were broadly similar in composition but from different manufacturers. Water exchange rate averaged 10–19% in the three recirculation systems, and key water quality parameters such as NH 4 + and CO 2 remained at low effect concentrations (< 0.4 and < 3 mg L − 1 , respectively). Final stocking densities were 45–60 kg m − 3 . There was a significant influence of both feed/photoperiod regime and diet on specific growth rate (SGR). Fish receiving the LD12:11 and LD6:6 regimes and Diet A grew best (SGR 2.59 and 2.54% d − 1 respectively). Fish fed Diet B and C also grew best under the LD12:11 and LD6:6 feed/photoperiod regimes (SGR range of 2.41–2.46% d − 1 ). Conversely, fish kept in the LD24:0 feed/photoperiod regime grew relatively slowly irrespective of diet type (SGR range of 2.26–2.32% d − 1 ). The feed conversion performance of the feed/photoperiod regimes and diets followed the same pattern.

Autonomous oscillations in Saccharomyces cerevisiae during batch cultures on trehalose

We report that autonomous oscillations, which usually happen in aerobic glucose-limited continuous cultures of yeast at low dilution rate, were also observed in trehalose discontinuous cultures of Saccharomyces cerevisiae. This unexpected oscillatory behaviour was therefore examined using fast Fourier transformation of online gas measurements. This robust mathematical analysis underlined the existence of two types of oscillation. The first was found to be linked to the cell cycle because (a) the periodicity corresponded to a fraction of the generation time and (b) the oscillations were accompanied by a transient increase in the budding index, mobilization of storage carbohydrates, and fermentative activity. Moreover, these oscillations occurred in a range of specific growth rates between 0.04 and 0.15 h(-1). All these criteria were consistent with the cell-cycle-related metabolic oscillations observed in the same range of growth rates in glucose-limited continuous cultures. The second type were short-period respiratory oscillations, independent of the specific growth rate. Both types of oscillation were found to take place consecutively and/or simultaneously during batch culture on trehalose. In addition, mobilization of intracellular trehalose emerged as a key parameter for the sustainability of these autonomous oscillations as they were no longer observed in a mutant defective in neutral trehalase activity. We propose that batch culture on trehalose may be an excellent device for further investigation of the molecular mechanisms that underlie autonomous oscillations in yeast.

Nursery rearing of the Asian catfish, Clarias macrocephalus (Günther), at different stocking densities in cages suspended in tanks and ponds

Growth and survival of hatchery-bred Asian catfish, Clarias macrocephalus (Günther), fry reared at different stocking densities in net cages suspended in tanks and ponds were measured. The stocking densities used were 285, 571 and 1143 fry m−3 in tanks and 114, 228 and 457 fry m−3 in ponds. Fish were fed a formulated diet throughout the 28-day rearing period. Generally, fish reared in cages in ponds grew faster, with a specific growth rate (SGR) range of 10.3–14.6% day−1, than those in cages suspended in tanks (SGR range 9–11.3% day−1). This could be attributed to the presence of natural zooplankton (copepods and cladocerans) in the pond throughout the culture period, which served as additional food sources for catfish juveniles. In both scenarios, the fish reared at lower densities had significantly higher SGR than fish reared at higher densities. In the pond, the SGR of fish held at 228 and 457 m−3 were similar to each other but were significantly lower than those of fish held at 114 m−3. The zooplankton in ponds consisted mostly of copepods and cladocerans, in contrast to tanks, in which rotifers were more predominant. Per cent survival ranged from 85% to 89% in tanks and from 78% to 87% in ponds and did not differ significantly among stocking densities and between rearing systems. In conclusion, catfish nursery in cages suspended in tanks and ponds is density dependent. Catfish fry reared at 285 m−3 in tanks and at 114 m−3 in ponds had significantly faster growth rates than fish reared at higher densities. However, the desired fingerling size of 3–4 cm total length for stocking in grow-out culture can still be attained at stocking densities of 457 m−3 in nursery pond and 571 m−3 in tanks.

Protein secretion in phosphate-limited cultures of Bacillus subtilis 168.

The secretion of proteins from Bacillus subtilis was studied under physiologically well-defined conditions in continuous cultures at a range of specific growth rates. The kinetics of secretion was analysed by using pulse-chase and immunoprecipitation techniques that allowed both processing and release to be monitored. Growth conditions were selected that were known to lead to significant changes in the anionic polymer composition of the cell wall. Under magnesium limitation only low levels of native proteins were released into the growth medium. In contrast, much higher amounts of released protein were observed under phosphate limitation. Although synthesis of native secretory proteins appeared to be highly regulated, only minor changes in the secretion of heterologous proteins were detected. Comparable kinetics of protein release of cells grown under different conditions indicated similar cell wall permeabilities. The large changes in the amounts of released proteins were not reflected in the production of chaperones and components required fro protein secretion. The data suggest that the capacity of the secretion machinery is not a major limiting step in the export of native secretory proteins.

SILICEOUS SCALE PRODUCTION IN CHRYSOPHYTE AND SYNUROPHYTE ALGAE. I. EFFECTS OF SILICA‐LIMITED GROWTH ON CELL SILICA CONTENT, SCALE MORPHOLOGY, AND THE CONSTRUCTION OF THE SCALE LAYER OFSYNURA PETERSENII1

ABSTRACTThe cells of synurophyte flagellates (algal class Synurophyceae, formerly included in the Chrysophyceae) are enclosed within a regularly imbricate layer of ornamented siliceous scales. Scale morphology is of critical taxonomic importance within this group of algae, and the scales are valuable indicator microfossils in paleolimnological studies. The data presented here demonstrate that scale morphology and the integrity of the scale layer can exhibit extreme variability in culture as a function of the cellular quota of silica under silica‐limited growth. Silica‐limited, steady‐state populations of the colonial flagellateSynura peterseniiKorsh. were maintained over a range of specific growth rates (μ= 0.11–0.69 days−1) and silica cell quotas (Qsi= 0.13–2.40 pmoles Si · cell1). Scale morphology and the organization of the scale layer became increasingly aberrant as silica stress increased. Under severe stress, scale deposition was completely suppressed so that cells appeared scale‐free. This depression of scale deposition was reversible; populations of silica‐starved, scale‐free cells rapidly regenerated new scale layers when placed in batch culture and spiked with dissolved silica. During recovery from silica stress, cell division was repressed for 24 h while mean cell silica quota increased 25‐fold. The first new scales appeared within 2 h after the silica addition, and development of the new scale layer proceeded in an approximately synchronous manner, residting in normal scale layers on virtually all cells after 48 h of recovery in Sirich medium. Silica content of silica‐repleteSynuracells is comparable to freshwater diatoms of siynilar size, butSynurahas much greater potential quota variability than diatoms and no apparent threshold silica requirement. Silica‐limited growth kinetics and competition between diatoms andSynurafor silica are discussed.The results suggest that morphological variability of siliceous scales in natural populations of synurophyte flagellates may result from silica stress and that the experimental approach developed here has great potential value as a means for circumscribing ecotypic variation in scale morphology. Results also demonstrate that scale production can be uncoupled from cell division, suggesting that cell cycle regulation of silica biomineralization in the Synurophyceae may be fundamentally different from that of diatoms (algal class Bacillariophyceae). This experimental system has application in the future study of the intracellular membrane systems and the regulatory processes involved in silica biomineralization.

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.

Foreign protein expression from S phase specific promoters in continuous cultures of recombinant CHO cells

Foreign protein expression from the commonly used SV40 promoter has been found to be primarily during the S-phase of the cell cycle. Simple mathematical models with this cell cycle phase dependent expression of foreign protein suggest that the specific production rate will be proportional to the cell growth rate, which is particularly disadvantageous in high cell density fed-batch or perfusion bioreactors. In this study we investigate this predicted relationship between the production rate and growth rate by culturing recombinant CHO cells in a continuous suspension bioreactor. One CHO cell line, GS-26, has been stably transfected with the plasmid pSVgal, which contains the E. coli lac Z gene under the control of the SV40 promoter. This GS-26 cell line was grown in suspension cultures over a range of specific growth rates in batch and continuous modes. The intracellular β-galactosidase activity was assayed using a standard spectrophotometric method after breaking the cells open and releasing the enzyme. A strong growth associated relationship is found between the intracellular β-galactosidase content and the specific growth rate in batch and continuous cultures, as predicted.

Effects of cell size and specific growth rate on stable carbon isotope discrimination by two species of marine diatom

Effects of cell size and/or specific growth rate were studied in 2 species of marine diatom, the large-celled Ditylum brightwellii and the smaller Chaetoceros calcitrans. Cells were grown as lightlimlted continuous cultures to produce a wide range of specific growth rates from 0.12 d' in D. brightwellii to 1.01 d-' in C. calcitrans. Carbon isotope discrimination (A) values, relative to source 8':'C of dissolved inorganic carbon (DIC), showed no relationship to specific growth rate within species. When examined interspecifically there was some evidence that growth rate or cell size affected the '/C ratios of the diatoms. At each photon flux density (PFD) used for growth, the specific growth rate of C. calcitrans was at least twice that of D. brightwehi. Values of A were greater in D. brightwellii at PFDs of 5. 20 and 40 pm01 photon m-2 S-'. These data are in agreement with a hypothesis stating that fastergrowing diatoms should be enriched in I3C. However, at the highest growth irradiance of 60 pm01 m-' S-', A values were higher in C. calcitrans than in D. brightwellii. Source 6'.'(: values varied between individual cultures and demonstrated the ~mportance of directly measuring the 6' of DIC. The value of physiological data in fully interpreting the stable carbon isotope ratios of diatoms is also discussed.

Culture instability of auxotrophic amino acid producers

The long-term dynamic characteristics of the L-lysine producer Corynebcterium glutamicum in continuous culture were studied over a range of specific growth rates. The double-auxotroph parent strain was found to be susceptible to a back mutation, or reversion, which negated the regulatory bypass that allows this strain to accumulate L-lysine in culture but also gives rise to a L-threonine auxotrophic requirement. Consequently, the revertant cells no longer over-produced L-lysine, nor were they limited in their growth by the low levels of L-threonine in the medium. All continuous culture experiments were eventually taken over by these revertants. The instability of the culture was found to be primarily due to the growth rate differential between the two competing populations, the (productive) parent auxotrophs and the (nonproductive) revertants. A deterministic mathematical model of the culture dynamics, incoroporating two limiting-substrate balances, satisfactorily described the takeover profiles. A linear stability analysis of the model equations identified that although long-term culture demise is inevitable, the dimensionless ratio of the two limiting substrates controls the rate of takeover by nonproductive cells. The analysis further demonstrated the importance of proper medium design in delaying the onset of takeover in cultures of this double-auxotroph strain. The theoretical medium design criterion was then confirmed experimentally by the stabilization of a fed-batch culture against revertant takeover for an extended fermentation time.

Kinetic study of hybridoma cell growth in continuous culture: II. Behavior of producers and comparison to nonproducers

A hybridoma cell line, AFP-27-P, was cultivated in continuous culture under glucose-limited conditions. The viable cell concentration, dead-cell concentration, and cell volume all varied with the dilution rate. A model previously developed for a nonproducing clone of the same cell line, AFP-27-NP, was extended to describe the behavior of the cells. The relationship between the specific growth rate and glucose concentration is described by a function similar to the Monod model. A threshold glucose concentration and a minimum specific growth rate are incorporated; the model is meaningful only at glucose concentration and a minimum specific growth rate are incorporated; the model is meaningful only at glucose concentrations and specific growth rates above these levels. The relationship between the death rate and the glucose concentration is described by an inverted Monod-type function. Furthermore, the yield coefficient based on glucose is constant in the lower range of specific growth rates and changes to a new constant value in the upper range of specific growth rates. No maintenance term for glucose consumption is used; in the plot of specific glucose consumption rate vs. specific growth rate, the line intercepts the specific growth rate at a value close to the minimum growth rate. The productivity of antibody as a function of the specific growth rate is described by a mixed type model with a noon-growth-associated term and a negative-growth-associated term. The values for the model parameters were determined from regression analysis of the steady state data.

Environmental factors influencing photosynthetic efficiency of the micro red alga Porphyridium cruentum (Agardh) Nägeli in light‐limited cultures

summarySteady state growth of the red alga Porphyridium cruentum was obtained in light‐limited chemostat cultures at different temperatures, pH and salinities over a range of specific growth rates. Geometric design of the reactors was found to have no effect on the yield and photosynthetic efficiency (PE) of the biomass. Under optimal growth conditions, that is at 23 °C, pH 8 and NaCl concentration of 0.42 M, the maximum specific growth rate was 0.065h−l which corresponded to a generation time of 10.7 h. With a given light input the biomass output rate was found to be constant over the range of specific growth rates studied. The values of the maximum growth yield (YG) and maximum PE of P. Cruentum were in the range of 0.0064–0.0074 g kj−1 and 13.4–15.5% respectively, when growth was optimal. This PE value corresponds to a quantum demand of 15 per O2 molecule evolved. Different temperatures, pH and salinities were demonstrated to cause a 1.5 to 2‐fold variation in the maximum PE. The maintenance energy was found to be virtually zero under all conditions.

Kinetic study of hybridoma cell growth in continuous culture. I. A model for non‐producing cells

The kinetic behavior of a nonproducing hybridoma clone AFP-27-NP was investigated in continuous culture under glucose-limited conditions. A total of more than 21, 000 h of cultures were operated at dilution rates ranging from 0.01 to 0.06 h(-1). The viable cell concentrations, dead cell concentrations, and cell volumes all varied with the dilution rate. A steady-state model was developed based on the biomass concentration and the glucose concentration. The specific growth rate as a function of glucose concentration is described by a model similar to the Monod model with a threshold glucose concentration and a minimum specific growth rate incorporated; the model is meaningful only at glucose concentrations and specific growth rates above these levels. A death rate is included in the model which is described by an inverted Monod-type function of glucose concentration. The yield coefficient based on glucose is constant in the lower range of specific growth rates and changes to a new constant value in the upper region of specific growth rates. No maintenance term for glucose consumption was needed; in the plot of specific glucose consumption rate vs. specific growth rate, the line intercepted the specific growth rate axis at a value close to the minimum growth rate. The values for the model parameters were determined from regression analysis of the steady-state data. The model predictions and experimental results fit very well.

Methods for determination of growth-rate-dependent changes in hybridoma volume, shape and surface structure during continuous recycle.

Hybridoma volume and surface membrane structure were found to vary as a function of specific growth rate using a method of cell recycle with continuous medium perfusion to vary growth rate. Mean hybridoma volume determined at constant osmolality by both electronic particle counting and scanning electron microscopic (SEM) methods indicated that rapidly growing cells are significantly larger than very slowly growing cells. We have previously determined that during both rapid and slow growth over a range of L-glutamine provision rates (Gln PR) that specific monoclonal antibody (MoAb) secretion rate was not changed. In this study a constant MoAb secretion rate per unit of membrane area was found which may indicate that changing membrane area is not a rate-determining factor in MoAb secretion. SEM methods were of limited use for accurate determination of cell volume due to cell shrinkage and large coefficients of variations. In spite of this limitation, SEM stereology methods were useful in confirming that cells remained spherical over a wide range of specific growth rates and that hybridoma cells were not circular. Sequential SEM observations also revealed that surface membrane structure of the 9.2.27 murine hybridoma investigated was correlated with growth rate. Under conditions of very slow growth, hybridoma surface microvilli density appeared to be significantly reduced.

Bacterial response to labile dissolved organic matter increases associated with marine discontinuities

Selected oligotrophic or low-nutrient (LN) bacteria were used in laboratory studies to measure the amounts of available or labile dissolved organic carbon (DOC) in waters collected from various locations and depths in the central and northeastern Pacific Ocean. Two parameters, bacterial specific growth rates and maximum cell densities, were examined as indicators of DOC availability in unsupplemented seawater samples. Maximum growth rates of LN bacteria were 0.1–0.3 h −1 regardless of sample location. Growth rates were slightly depressed due to anthropogenic inputs such as chlorine or metals. The range of specific growth rates measured for the test bacteria is considerably less (approximately 3-fold) than that for maximum cell density (approximately three orders of magnitude). Highest levels of labile DOC (as determined by maximum cell densities attained) were observed in waters collected from interfaces or discontinuities such as surface films, thermocline and oxygen minima waters, chlorophyll maxima and ammonium regeneration zones, benthic boundary layers, sediment/water interface and sediment trap supernatants. We estimate labile DOC pools of < 1% to approximately 45% of the total measured DOC.

Bacterial response to labile dissolved organic matter increases associated with marine discontinuities

Selected oligotrophic or low-nutrient (LN) bacteria were used in laboratory studies to measure the amounts of available or labile dissolved organic carbon (DOC) in waters collected from various locations and depths in the central and northeastern Pacific Ocean. Two parameters, bacterial specific growth rates and maximum cell densities, were examined as indicators of DOC availability in unsupplemented seawater samples. Maximum growth rates of LN bacteria were 0.1–0.3 h−1 regardless of sample location. Growth rates were slightly depressed due to anthropogenic inputs such as chlorine or metals. The range of specific growth rates measured for the test bacteria is considerably less (approximately 3-fold) than that for maximum cell density (approximately three orders of magnitude). Highest levels of labile DOC (as determined by maximum cell densities attained) were observed in waters collected from interfaces or discontinuities such as surface films, thermocline and oxygen minima waters, chlorophyll maxima and ammonium regeneration zones, benthic boundary layers, sediment/water interface and sediment trap supernatants. We estimate labile DOC pools of < 1% to approximately 45% of the total measured DOC.