Glucose Respiration Research Articles

Phosphorus acquisition by Chlamydomonas acidophila under autotrophic and osmo-mixotrophic growth conditions

Chlamydomonas acidophila Negoro is a green algal species abundant in acidic waters where inorganic phosphorus (P(i)) and carbon (CO(2)) are considered the most important growth-limiting nutrients for the phytoplankton. This paper describes the P(i) uptake and growth kinetics under varying carbon supply by cultivating the alga autotrophically, with and without CO(2) aeration, and osmo-mixotrophically with glucose under low P(i) conditions at pH 2.7. The low minimum cellular phosphorus quota (Q(0); ranging from 0.6 to 1.1 mmol P mol(-1) C) suggested P(i)-limiting conditions under all different modes of carbon supply, and was lowest under CO(2)-aerated conditions. The threshold P(i) concentration for growth did not vary from zero, suggesting no detectable metabolic costs. Maximum P(i)-uptake rates (V(max)) were a better indication of P(i) limitation when compared with the affinity constant for P(i) uptake (K(m)), as V(max) was only high under P(i)-limited conditions whereas K(m) was low under both P(i)-limited and P(i)-replete conditions. Osmo-mixotrophic growth conditions did not result in decreased extracellular phosphatase activity, but often resulted in physiological characteristics comparable with CO(2)-aerated cells, suggesting intracellular CO(2) production by glucose respiration. In addition, at low CO(2) and in autotrophic conditions, C. acidophila had a higher Q(0), lower dissolved organic carbon concentration, lower maximum P(i)-uptake rates, and lower phosphatase activity, suggesting that growth was co-limited by CO(2) and P(i). Furthermore, cells may respond physiologically to both nutrient limitations simultaneously.

Seasonal dynamics of pelagic and benthic (littoral and profundal) bacterial abundances and activities in a deep prealpine lake (L. Constance)

In order to obtain a first insight into the characteristics of pelagic, littoral and profundal bacterial communities and their potential role for the degradation of organic matter in the deep prealpine Lake Constance, we compared the seasonal dynamics of bacterial abundances and activities during the annual cycle in the epilim- netic water of a central station as well as in surface layers of sediments at a central pro- fundal and a littoral site. For this purpose, bacterial abundances (DAPI counts), rates of leucine incorporation, respiration of 14 C-labeled substrates (glucose, phenol) and ectoenzymatic activities were measured biweekly-monthly at the different sampling sites. Bacterial densities in lake sediments generally exceeded those of epilimnetic waters by at least two orders of magnitude with higher values in profundal sediments. A seasonal pattern was well expressed for bacterioplankton, less pronounced for ben- thic littoral bacteria and hardly visible for profundal bacteria. Compared to pelagic bacteria, benthic bacteria exhibited lower specific rates of leucine incorporation, espe- cially during the warmer periods. For profundal benthic communities specific rates of glucose respiration and of enzyme activities were permanently low with little seasonal fluctuation. In contrast, littoral benthic communities showed the highest values for these specific metabolic activities among the three sites tested. Additionally, the capac- ity of metabolism of phenol (used here as a representative of more recalcitrant aro- matic substances) was by far highest respired for littoral communities, and almost lacking for pelagic communities. Evidence from the literature suggests that the qualita- tively and quantitatively enhanced level of degradation capacities observed for littoral bacteriobenthos may be predominantly a result of the combined effect of increased supply of additional organic matter (e. g. allochthonous matter and macrophytes), an enriched bacterial gene reservoir in sediments, and frequent resuspension. Littoral ben- thic communities showed the highest specific metabolic activities exceeding those of bacterioplankton and profundal bacteriobenthos by an order of magnitude. As a whole,

Activity and stability of a complex bacterial soil community under simulated Martian conditions

A simulation experiment with a complex bacterial soil community in a Mars simulation chamber was performed to determine the effect of Martian conditions on community activity, stability and survival. At three different depths in the soil core short-term effects of Martian conditions with and without ultraviolet (UV) exposure corresponding to 8 Martian Sol were compared. Community metabolic activities and functional diversity, measured as glucose respiration and versatility in substrate utilization, respectively, decreased after UV exposure, whereas they remained unaffected by Martian conditions without UV exposure. In contrast, the numbers of culturable bacteria and the genetic diversity were unaffected by the simulated Martian conditions both with and without UV exposure. The genetic diversity of the soil community and of the colonies grown on agar plates were evaluated by denaturant gradient gel electrophoresis (DGGE) on DNA extracts. Desiccation of the soil prior to experimentation affected the functional diversity by decreasing the versatility in substrate utilization. The natural dominance of endospores and Gram-positive bacteria in the investigated Mars-analogue soil may explain the limited effect of the Mars incubations on the survival and community structure. Our results suggest that UV radiation and desiccation are major selecting factors on bacterial functional diversity in terrestrial bacterial communities incubated under simulated Martian conditions. Furthermore, these results suggest that forward contamination of Mars is a matter of great concern in future space missions.

Modeling threshold phenomena, metabolic pathways switches and signals in chemostat-cultivated cells: the Crabtree effect in Saccharomyces cerevisiae

The long-term Crabtree effect in Saccharomyces cerevisiae cultivated in aerobic chemostat at steady state has been studied for three different substrate concentrations in the feed of the bioreactor (data: J. Gen. Microbiol., 129 (1983) 653). We have shown that a model using two ways of transport/metabolization ( T/ M) of hyperbolic form, with high and low affinity for the substrate, allowed to represent correctly the main characteristics of the phenomenon. The model is based on an explicit form of the T/ M kinetics when the bioreactor is considered as a polyphasic dispersed system (PDS). Mass balances analysis also allows to quantify the critical dilution rate value (threshold), D c , of the transition between respiratory and respirofermentative mode, for which ethanol is produced. A good approximation for the threshold is D c = V S 0 Y X c , S where Y X c , S is the average yield coefficient before transition and V S 0, the maximum specific rate of high affinity T/ M pathway. The theoretical value is 0.3 h −1, and is equal to the experimental value. We thus show in a quantitative way that the transition depends both on culture conditions (global characteristic of the system) and on strain properties (intrinsic characteristic of the microorganism as well). Using two different methods to calculate the residual substrate has carried out the comparison between the simulations end the experimental data. This allowed showing that the latter is not well represented by Monod's model and has confirmed that the affinity for the substrate varies according to the biomass. We have then shown how to calculate the most important specific rates (or metabolic flux) related to biomass, ethanol, oxygen, hydrogen, respiratory and fermentative CO 2 and H 2O within the cellular phase. It has appeared that the oxygen uptake rate directly depends on high-affinity T/ M pathway. This let us think that the regulation of the Crabtree effect in S. cerevisiae depends on the saturation of some glucose metabolization and transport pathways rather than on saturation of the respiratory chains. The specific rates analysis has also allowed us to show, at least in this case, that the metabolization rate (biosynthesis+fueling) had its maximum value on the whole dilution rates interval; metabolites excretion (ethanol and fermentative CO 2) only intervenes to drain a “surplus” glucose flux. As a consequence, the transport capacity must be higher than the one of metabolization. Maximization of the metabolization specific rate could then be used as an optimization criterion in the stoichiometric calculation of metabolic flux (and not the specific growth rate maximization because growth is limited in a chemostat ( μ= D)). We have also shown that the mass balances based on the T/ M processes are in agreement with molar and elementary balances of the general stoichiometric equation for glucose respiration and fermentation under aerobic conditions. Thanks to the specific rates calculating the stoichiometric coefficients has done this. The total mass balance difference does not exceed 4%, which is compatible with the experimental carbon balance. Finally, we have emphasized that the ratio of biosynthesis flux and metabolization flux is constant before and after transition. This observation could be applied as soon as the free substrate concentration in the cellular phase is low. The paper succinctly describes the former theoretical results on which the model is built and sufficiently explains the algorithm for straightforward implementation.

Respiration activity of ectomycorrhizas from Cenococcum geophilum and Lactarius sp. in relation to soil water potential in five beech forests

Forest trees are involved in root symbioses with hundreds of species of ectomycorrhizal fungi which constitute functional guilds able to improve the water and mineral nutrition of host trees. In temperate ecosystems, water shortage is a main factor limiting tree vitality. To assess how soil water conditions affected the physiological state of beech (Fagus silvatica L.) ectomycorrhizal roots, we monitored glucose respiration of two ectomycorrhizal types (Lactarius sp. and Cenococcum geophilum) during two complete growing seasons. Five stands of contrasting soil conditions were chosen in north-eastern France. The top soil horizons were equipped with micropsychrometers for measuring water potential and temperature. Glucose respiration on individual ectomycorrhizas was measured in vitro by trapping [14C]-CO2 from radiolabelled glucose. For soil water potential <-0.2 MPa, the potential respiration activity of C. geophilumectomycorrhizas was significantly less altered than that of Lactariussp. ectomycorrhizas, indicating that C. geophilumis more likely than Lactariussp. to maintain the physiological integrity of beech roots facing drought stress.

Bacterial utilization of glucose in the water column from eutrophic to oligotrophic pelagic areas in the eastern North Atlantic Ocean

Abstract Vertical profiles of glucose utilization rates were compared at three sampling stations in the eastern part of the tropical North Atlantic Ocean. The investigation area was along 20–21 °N and the three sampling sites, characterised by differences in their primary productivity, were located at 18 °W, 21 °W and 31 °W. In the superficial waters, maximum (Vmax) glucose utilization (respiration plus incorporation) depended on the nutritional load being 20-fold higher in the eutrophic, compared to the oligotrophic zone. Due to these variations, natural turnover times for this labile compound were approximately 1 day in the eutrophic area, and up to 435 days in the oligotrophic area. Bacterial activity showed a steep decline immediately below the mixed layer in the mesotrophic and eutrophic areas and below the deep chlorophyll maximum in the oligotrophic area. Discrepancies between microbial activities in the three areas decreased with increasing depth: at depths below 250 m potential utilization rates of glucose were similar whatever the nutrient richness of the photic layer. Nevertheless, the distribution of microbial activities through the whole water column depended greatly on the productivity of superficial waters. In nutrient-rich areas 73% of glucose utilization activity was realized in the productive upper layer, whereas only 4% was metabolized at depths below 250 m. Conversely, in the oligotrophic area, more than 40% of the glucose utilized in the whole water column was processed in the intermediate and deep-water masses. Integration of Vmax values for the whole water column, suggested potential carbon fluxes due to bacterial utilization of glucose of 6 and 34 mg C m−2 d−1 in the oligotrophic and eutrophic areas, respectively. The fate of the metabolised carbon depended on the nutrient availability. In the mixed-water layer the glucose respiration percentage (%R) increased from 30% in nutrient-rich areas to 60% under oligotrophic conditions, moreover %R increased with depth. This infers that at lower nutritional loads, a greater proportion of highly labile compounds is used for energetic purposes, and therefore return to the inorganic carbon pool, but with very low turnover rates.

Effects of pH on a high conductance Ca-dependent chloride channel: a patch-clamp study in Ascaris suum.

Plasma membrane vesicles prepared from the bag region of the somatic muscle cells of the parasitic nematode Ascaris suum contain high conductance, voltage sensitive, Ca-dependent chloride channels, suggested to be involved in the excretion of carboxylic acids produced by the anaerobic respiration of glucose (Valkanov, Martin & Dixon, 1994). The effect of altered pH on this channel was investigated using the patch-clamp technique and isolated inside-out membrane patches. Changes in pH had little effect on channel conductances and only a small effect on reversal potentials. Under control conditions (symmetrical pH 7.2) the channel had the highest probability of opening at approximately -35 mV (the resting membrane potential of the cell). At positive membrane potentials the probability of opening decreased. The Boltzmann equation was used to describe the relationship between membrane potential and probability of channel opening, and to calculate the effective gating charge. Reduction of external pH produced an increase in the probability of channel opening at hyperpolarized membrane potentials. An increase in internal pH caused a voltage-independent increase in the probability of channel opening and made the effective gating charge less negative. The effect of reducing internal pH was marked: the channel then opened most frequently at positive membrane potentials and the probability of opening at -35 mV was greatly reduced. The decrease in internal pH changed the polarity of the effective gating charge. A simple model was constructed to describe the effects of pH on channel gating.

Concentrations and uptake of neutral monosaccharides along 14°W in the equatorial Pacific: Contribution of glucose to heterotrophic bacterial activity and the DOM flux

We examined concentrations and uptake of dissolved neutral monosaccharides (DNMS) in order to determine the contribution of DNMS to heterotrophic bacterial production and to the flux of dissolved organic matler (DOM) in the equatorial Pacific. DNMS concentrations were greater during El Niño‐affected months of February–April 1992 than during August–October 1992; in contrast, glucose turnover was the opposite— turnover was faster in August–October than in February–April. The variation in sugar concentrations and turnover probably resulted from El Niño‐induced changes in primary production; as El Niño waned primary production increased, which appeared to stimulate bacterial activity, especially glucose turnover, that in turn forced down DNMS concentrations. In all months, however, DNMS concentrations were low, especially compared with total dissolved organic carbon concentrations (&lt;1%). Glucose was the dominant neutral monosaccharide and alone supported 15–47% of bacterial production. Other monosaccharides apparently did not support much bacterial growth; concentrations of other sugars were low, as probably was turnover. Respiration of glucose (30–60% of uptake) and mannose (60–90%) was relatively high, suggesting that DNMS supported a large fraction of bacterial respiration as well as biomass production. These results point to the importance of DNMS and glucose in particular in supporting bacterial growth and in contributing to the flux of labile DOM.

Characterization of mutations that overcome the toxic effect of glucose on phosphoglucose isomerase less strains of Saccharomyces cerevisiae.

Glucose inhibits growth of yeast phosphoglucose isomerase mutants in permissive media. Mutants insensitive to this effect were isolated by selection on media containing 2% fructose + 2% glucose. A nuclear, monogenic, recessive mutation named rgl was responsible for this phenotype. The mutants isolated belonged to two complementation groups and have been termed rgl1 and rgl2. When the double mutants were grown on fructose, fermentation of fructose or glucose was similar to that of the parental pgi strain but was not measurable when grown on fructose+glucose. Under these conditions, respiration of glucose and to a lesser extent of fructose was enhanced. The double mutants pgi rgl did not grow on fructose+glucose in the presence of antimycin A or ethidium bromide and their cytochrome oxidase was no longer sensitive to glucose repression. The results are interpreted as an indication that in the double mutants the glucose may be channeled through the pentose phosphate pathway to respiration.

Effects of arsenic on survival and metabolism of Crangon crangon

Metabolic changes and toxic effects in Crangon crangon (collected during summer 1989 near Helsingor, Denmark and at Kulhuse in the Isefjord) were observed during exposure to different arsenate concentrations. Survival, respiration, and concentrations of hemolymph glucose, muscle glycogen andadenine nucleotides were measured. C. crangon was very tolerant of arsenate; survival was only affected at arsenate concentrations >25 ppm. Small individuals (0.2 to 0.29 g wet weight) were less tolerant than medium-sized and large individuals (0.5 to 0.59 g and 0.8 to 0.89 g wet weight), e.g. at 50 ppm the LT50 for small individuals was 180 h compared to 343 h and 360 h in middle-sized and large individuals. The relation between size and toxicity is probably due to the change in surface/volume ratio. Under clean conditions the respiration of C. crangon showed a clear relation between routine MO2 (O2 in mlg-1 h-1) and wet weight (r2=0.89; P<0.001). During shortterm exposure to arsenate (10 and 50 ppm), the respiratory rates (MO2) seemed to decrease in a size-dependent manner. The concentrations of blood glucose and muscle glycogen did not change in response to arsenate exposure. The concentrations of adenylate phosphates were high and constant throughout the experiments. C. crangon had maximal adenylate energy charge (AEC) values of ca. 0.8. No decrease in AEC due to the exposure to arsenate was observed.

MICROBIAL BIOMASS MEASUREMENT IN ACIDIC SOIL

The chloroform fumigation-incubation method for estimating soil microbial biomass is applicable to most soils; but, problems are associated with its use with extremely acidic soils and with soils amended with carbonaceous substrates. To evaluate the basis for these difficulties, this project was commenced with the objectives of (a) evaluating the variation in the fungal respiration in fumigated and native acidic forest soil and of (b) assessing the effect of chloroform fumigation on microbial respiration and invertase activity in this soil amended with glucose or corn stover. The soil samples were collected from the O-horizon (pH 3.5) and A-horizon (pH 3.5) of a Lakehurst soil series (a mesic coated Haplaquodic quartzipsamment). Cycloheximide and streptomycin inhibition of glucose respiration indicated that in native O-horizon soil, the fungal to bacterial activity ratio was approximately 82:12. During the incubation period following fumigation, fungal activity ranged from 92 to 99% of total glucose respiration in fumigated O-horizon soil samples and 83 to 90% in the nonfumigated soils. Comparable values for the A-horizon soils were 85 to 90% in the fumigated soils and 33 to 84% in the native samples. Return of glucose metabolism to pre-fumigation rates occurred within 2 days after fumigation in the chloroform treated O-horizon soils whereas 10 days were required in the A-horizon soils. Chloroform fumigation of soils receiving corn stover or glucose caused a greater production of CO2 and more invertase activity due to the amendment than was detected in comparable nonfumigated soil samples. These data suggest that differences in basic microbial population activity ratios between chloroform-fumigated acidic soils and unfumigated control soil samples contribute to the difficulties of applying the chloroform fumigation-incubation method to extremely acidic soils and that the method is likely invalid for use with extremely acidic soils receiving inputs of carbonaceous compounds.

Planktonic bacterial biomass and seasonal pattern of the heterotrophic uptake and respiration of glucose and amino acids in the shallow sandpit lake of Cr�teil (Paris Suburb, France)

Biomass and activity of planktonic bacteria were investigated during a one year study in a shallow sandpit lake. The shallowness of the lake helped keep the water column homogeneous regarding bacterioplankton. Small free-living bacteria (0.03 µm3 cell−1) dominated the populations throughout the period studied. Bacterial abundances varied from 1 to 11 × 106 cells ml−1. Kinetic parameters (V max, K + S and T) were determined with 14C labelled compounds (glucose and amino acids mixture). V max values were high and averaged 0.056 and 0.050 µgCl−1 h−1 for glucose and amino acids respectively. Maximal V max values were observed in summer at the highest temperatures, but also in early spring. T values were much greater in winter. K + S values were significantly higher for amino acids (3 µg Cl−1) than for glucose (1 µg Cl−1). A low percentage of mineralization (about 25% for both tracers) could be the expression of the high growth efficiency expected when bacteria are growing at the expense of low molecular weight compounds as phytoplankton exudates.

Aerobic biodegradation potential of subsurface microorganisms from a jet fuel-contaminated aquifer

In 1975, a leak of 83,000 gallons (314,189 liters) of jet fuel (JP-4) contaminated a shallow water-table aquifer near North Charleston, S.C. Laboratory experiments were conducted with contaminated sediments to assess the aerobic biodegradation potential of the in situ microbial community. Sediments were incubated with 14C-labeled organic compounds, and the evolution of 14CO2 was measured over time. Gas chromatographic analyses were used to monitor CO2 production and O2 consumption under aerobic conditions. Results indicated that the microbes from contaminated sediments remained active despite the potentially toxic effects of JP-4. 14CO2 was measured from [14C]glucose respiration in unamended and nitrate-amended samples after 1 day of incubation. Total [14C]glucose metabolism was greater in 1 mM nitrate-amended than in unamended samples because of increased cellular incorporation of 14C label. [14C]benzene and [14C]toluene were not significantly respired after 3 months of incubation. With the addition of 1 mM NO3, CO2 production measured by gas chromatographic analysis increased linearly during 2 months of incubation at a rate of 0.099 mumol g-1 (dry weight) day-1 while oxygen concentration decreased at a rate of 0.124 mumol g-1 (dry weight) day-1. With no added nitrate, CO2 production was not different from that in metabolically inhibited control vials. From the examination of selected components of JP-4, the n-alkane hexane appeared to be degraded as opposed to the branched alkanes of similar molecular weight. The results suggest that the in situ microbial community is active despite the JP-4 jet fuel contamination and that biodegradation may be compound specific.(ABSTRACT TRUNCATED AT 250 WORDS)

Methodology for studies of carbon dynamics between phytoplankton primary production and heterotrophic bacterial secondary production

The SCENTO-System was used to study the carbon dynamics between phytoplankton primary production and heterotrophic bacterial secondary production. Most of the methods used nowadays in situ for limnological synecology studies were applied. Primary production measurement showed an increasing tendency with increasing content of chlorophylla. It provided a true photosynthetic rate lying within the range of eutrophic lakes. Net EOC released from the algae ranged from 8.5 to 27.5 μg C l−1(6h)−1. Accompanying the algal products the number of bacteria increased from 1.475 ×109 to 8.074×109 cells l−1. The bacterial mean cell volume was small, between 0.0315 and 0.0548μm3. Bacterial carbon production from direct growth estimates was compared with independent calculations of bacterial growth from EOC uptake and3H-thymidine incorporation. Direct estimates were 2.97–10.0 μg Cl−1 (24h)−1 with the exception of a zero-growth on the third day. EOC uptake was 123.5–191.0 μg Cl−1 (6h)−1. That calculated from3H-thymidine incorporation was 0.2–0.5 μg Cl−1 (6h)−1.14C-glucose dark uptake ran parallel to the increasing bacterial biomass. The respiration of glucose was 6.5% (avg.) of the gross uptake. Since the system operated without grazing pressure, a real carbon flow from primary production to bacterial secondary production could be observed.

Evidence of structural and functional adaptation in epilithon exposed to zinc

Artificial streams were used to study the effect of chronic zinc dosing (0.00,0.05 or 1.00 mg Zn 1- ') on epilithic communities during summer and early fall, 1984 and 1985. Following zinc addition, epilithon was periodically measured for '4C-glucose and 14C-glutamate respiration, ash-free dry weight (AFDW), protein, carbohydrate, chlorophyll a (Chl a), pheophytin a (pheo a), and colony forming units (CFU) on zinc amended and unamended media. Transient peaks in glucose respiration rates mg- ' protein and AFDW occurred within 5 to 10 days in streams dosed with 1.0 mg Zn 1- ' and coincided with decreases in AFDW, protein, carbohydrate, Chl a, and pheo a. Respiration of glutamate did not show a significant response to dosing (a = 0.05). Epilithic plate counts demonstrated more total and zinc-tolerant CFU in dosed than in control epilithon after 20 days. After 30 days, greater biomass and lower protein: carbohydrate ratios were evident in epilithon dosed with 1.0 mg Zn 1- ' compared to unamended treatments. Peaks in glucose respiration suggested the presence of zinc-tolerant heterotrophs transiently capable of increased respiration of glucose but not glutamate. Concurrently, minimal but measurable biomass inferred the persistence of zinc-tolerant microorganisms in zinc-dosed epilithon. By the end of the studies, results indicated that metal-tolerant populations replaced metal-sensitive organisms under conditions of elevated zinc concentration. These zinc adapted communities had lower potential value as a food source to grazers (low protein: carbohydrate ratio) and were no longer distinctive with respect to glucose respiration.

Assessment of the contributions of glycolysis and the pentose phosphate pathway to glucose respiration in ectomycorrhizas and non-mycorrhizal roots of spruce (Picea abies L. Karsten)

Assessment of the contributions of glycolysis and the pentose phosphate pathway to glucose respiration in ectomycorrhizas and non-mycorrhizal roots of spruce (Picea abies L. Karsten)

Torphological and physiological changes in ectomycorrhizas of spruce [Picea excelsa (Lam.) Link] associated with ageing

summaryEctomycorrhizas of four‐year‐old Picea excelsa (Lam.) Link seedlings, grown in nursery soil, are described. The main ectomycorrhiza type, occurring on 55–80% of the root apices, exhibited a normal sheath. The associated symbiotic fungus probably belonged to the Hebeloma genus. Behind these extomycorrhizal root lips, older ectomycorrhizas were observed and the apical ones presumably originated from those. One‐ and two‐year‐old ectomycorrhizas lacked sheaths, but a normal Hartig net could be observed in transverse sections. The metabolic‐activity of these older ectomycorrhizas, as measured by radiorespirometry of [14C]glucose, was reduced compared to the younger symbiotic organs. This glucose respiration was, however, still larger than the activity of non‐mycorrhizal exploratory roots. The mechanism of sheath loss and its significance as an ageing process are discussed.

Changes in epilithic communities due to individual and combined treatments of zinc and snail grazing in stream mesocosms.

Effects of 0.5 mg/liter zinc (Zn) and snail grazing (400 snails/m2) on density of dominant algal and protozoan taxa, epilithic glucose respiration, and ash-free dry weight (AFDW) were examined using established (12-day colonization) periphyton communities in flow-through stream mesocosms with four treatments (Zn, snails, Zn and snails, control) for 30 days. Grazing and Zn similarly reduced the abundance of 5 of 10 dominant algal taxa and AFDW during the first 10 days of treatment. Abundance of these taxa and AFDW in grazed (ambient Zn) treatments approached control levels after 10 days as the effect due to snails decreased. Decreasing temperatures may have reduced snail activity. Snails, Zn, and the combination of these treatments contributed to higher rates of glucose respiration per unit AFDW. Protozoan species abundance was reduced to less than half by Zn but was unaffected by snails. Although Zn and snails individually altered structural and functional aspects of this microbial community, the effects when both treatments were combined could not always be inferred from the individual effects. Testing individual and combined variables that affect periphyton with a corresponding assessment of population dynamics, biomass, and community functional attributes will enhance understanding of the overall effects of pollutants on periphyton communities.

Measurement of Glucose Utilization by Pseudomonas fluorescens That Are Free-Living and That Are Attached to Surfaces

The assimilation and respiration of glucose by attached and free-living Pseudomonas fluorescens were compared. The attachment surfaces were polyvinylidene fluoride, polyethylene, and glass. Specific uptake of [C]glucose was determined after bacterial biomass was measured by (i) microscopic counts or (ii) prelabeling of cells by providing [H]leucine as substrate, followed by dual-labeling scintillation counting. The glucose concentration was 1.4, 3.5, 5.5, 7.6, or 9.7 muM. Glucose assimilation by cells which became detached from the surfaces during incubation with glucose was also measured after the detached cells were collected by filtration. The composition of the substratum had no effect on the amount of glucose assimilated by attached cells. Glucose assimilation by attached cells exceeded that by free-living cells by a factor of between 2 and 5 or more, and respiration of glucose by surface-associated cells was greater than that by free-living bacteria. Glucose assimilation by detached cells was greater than that by attached bacteria. Measurements of biomass by microscopic counts gave more consistent results that those obtained with dual-labeling, but in general, results obtained by both methods were corroborative.

Redox balances in the metabolism of sugars by yeasts

The central role of the redox couples NAD+/NADH and NADP+/NADPH in the metabolism of sugars by yeasts is discussed in relation to energy metabolism and product formation. Besides their physical compartmentation in cytosol and mitochondria, the two coenzyme systems are separated by chemical compartmentation as a consequence of the absence of transhydrogenase activity. This has considerable consequences for the redox balances of both coenzyme systems and hence for sugar metabolism in yeasts. As examples, the competition between respiration and fermentation of glucose, the Crabtree effect, the Custers effect, adaptation to anaerobiosis, the activities of the hexose monophosphate pathway, and the fermentation of xylose in yeast are discussed.