In situ determinations of bacterial selectivity and filtration rates by five cladoceran zooplankters in a hypertrophic subtropical reservoir

Cladoceran in situ feeding rates on natural bacteria labelledwith [methyl-³H] were studied in parallel with feeding ratedeterminations on ¹⁴C-labelled Chlorella in a hypertrophic subtropicalreservoir (Lake Hartbeespoort) through spring and summer (1986/87).Community filtration rates (CFR5) on bacteria and algae weresimilar, but selection for Chlorella (relative to natural bacteria)increased from midsummer in association with declining bacterialdensity and increasing dominance of ‘inedible’ componentsof the natural phytoplankton. Species-specific filtration rates(SSFRs) were determined for Daphnia pulexllongispina, Ceriodaphniareticulata, Diaphanosoma excisum, Bosmina longirostris and Moinamicrura during their respective seasonal occurrence in the studyperiod. SSFRs on algae and bacteria increased with body length(L, mm) in all species apart from Bosmina. Species-specificdifferences in absolute feeding rate (FR, ml animal^–1day^–1), the slope of the FR-L relationship and bacteriaselectivity were evident. The feeding rate of all cladoceranson bacteria is described by the power equation FR 5.231L^1.42FR values on bacteria relative to FR values on algae averaged     

Feeding of planktonic rotifers on ciliates: a method using natural ciliate assemblages labelled with fluorescent microparticles

A method was developed to allow direct measurements of predationexerted by metazooplankton on ciliates. The method relied onthe use of ciliates labelled with fluorescent microparticles(FMP). Optimal labelling conditions were determined with ciliatesfrom cultures (Tetrahymena pyriformis) and with natural ciliateassemblages sampled in a river. Labelled T. pyriformis wereused as tracer food to determine gut passage time (GPT) andingestion rates of the rotifer Brachionus calyciflorus in thelaboratory. Predation of metazooplankton from the lowland riverMeuse (Belgium) was determined by labelling natural assemblagesof ciliates and using them as tracer food for metazooplankterssampled in the river. Optimal labels of ciliates, i.e. sharpdistribution of FMP in cells, were obtained with short incubations(10 min) and low FMP concentrations (1 x 10⁵ mL^–1). GPTvaried between 30 and 45 min for B. calyciflorus and from 25up to >35 min for rotifers from the river. The ingestionrate of B. calyciflorus fed with T. pyriformis was 3.3 ±0.6 ciliate rot^–1 h^–1, i.e. 1.4 ± 0.3 ngCrot^–1 h^–1. Metazooplankton species for which theingestion of ciliates could be measured were the rotifers Keratellacochlearis, Euchlanis dilatata and Synchaeta spp. Ingestionrates measured ranged from 0.4 to 12.5 ngC rot^–1 h^–1.The method proposed proved to be useful in estimating the predationof microplankton on ciliates in semi- in situ conditions; infurther developments, labelled natural assemblages of ciliatescould be used for in situ incubations with the Haney chamber.     

A comparison of net and gross rates of oxygen production as a function of light intensity in some natural plankton populations and in a Synechococcus culture

In vitrorates of gross and net oxygen production were measuredas a function of light intensity in some plankton communitiescollected from Bedford Basin, Nova Scotia, and in a monoclonalculture of Synechococcus. The rate of gross oxygen productionwas measured by a technique in which the stable oxygen isotope,¹⁸O, serves as a photosynthetic tracer Net oxygen productionwas measured by automated Winkler technique. The rate of communityrespiration in the light was then determined by the differencebetween gross and net rates of oxygen production. In the naturalpopulations examined, neither gross nor net oxygen productionrates were significantly inhibited at the highest light intensitymeasured (500–800 µE m^–2 s^–1) In a samplein which the dark respiration rate was small relative to themaximal rate of production [P_max;sensu Platt et al (1980) JMar. Res., 38, 687–701] the rates of ‘light’respiration were 3 times greater. In two other communities,with high rates of dark respiration relative to P_maxthe ratesof ‘light’ respiration were closer to rates of darkrespiration. In the Synechococcus clone, both gross and netoxygen production rates were inhibited at high light intensities.Rates of ‘light’ respiration were found to varyas a function of light intensity. The greatest rates of respirationwere measured in samples incubated at light intensities thatwere just saturating (100 µE m^–2 s^–1). Therates of ¹⁴C production were also measured as a function oflight intensity The photosynthetic quotients, based on ¹⁴C productionrates and gross oxygen production rates, average 1 9     

Estradiol-17beta stimulates proliferation of mouse embryonic stem cells: involvement of MAPKs and CDKs as well as protooncogenes

Although the importance of estradiol-17 (E₂) in many physiological processes has been reported, to date no researchers have investigated the effects of E₂ on embryonic stem (ES) cell proliferation. Therefore, in the present study, we have examined the effect of E₂ on the DNA synthesis of murine ES (ES-E14TG2a) cells and its related signaling pathways. The results of this study show that E₂ (10^–9 M) significantly increased [³H]thymidine incorporation at >4 h and that E₂ (>10^–12 M) induced an increase of [³H]thymidine incorporation after 8-h incubation. Moreover, E₂ (>10^–12 M) also increased 5'-bromo-2'-deoxyuridine (BrdU) incorporation and cell number. Indeed, E₂ stimulated estrogen receptor (ER)- and - protein levels and increased mRNA expression levels of protooncogenes (c-fos, c-jun, and c-myc). Tamoxifen (antiestrogen) completely inhibited E₂-induced increases in [³H]thymidine incorporation. In addition, estradiol-6-O-carboxymethyl oxime-BSA (E₂-BSA; 10^–9 M) increased [³H]thymidine incorporation at >1 h, and E₂-BSA (>10^–12 M) increased [³H]thymidine incorporation after 1-h incubation. E₂-BSA-induced increase in BrdU incorporation also occurred in a dose-dependent manner. Tamoxifen had no effect on E₂-BSA-induced increase of [³H]thymidine incorporation. Also, E₂ and E₂-BSA displayed maximal phosphorylation of p44/42 MAPKs at 10 and 5 min, respectively. E₂ increased cyclins D1 and E as well as cyclin-dependent kinase (CDK)2 and CDK4. In contrast, E₂ decreased the levels of p21^cip1 and p27^kip1 (CDK-inhibitory proteins). Increases of these cell cycle regulators were blocked by 10^–5 M PD-98059 (MEK inhibitor). Moreover, E₂-induced increase of [³H]thymidine incorporation was inhibited by PD-98059 or butyrolactone I (CDK2 inhibitor). In conclusion, estradiol-17 stimulates the proliferation of murine ES cells, and this action is mediated by MAPKs, CDKs, or protooncogenes. cyclin-dependent kinase; mitogen-activated protein kinase     

Corrigendum

Light response curves for (•) gross ¹⁶O₂ evolution, and() CO² uptake in 210 mmol mol^–1 O₂ with 900–1000µbar CO₂ or () in air by leaves of Hirschfeldia incana.The difference between (•) and () or () was quantitativelyequivalent to the measured ¹⁸O₂ uptake. The areas under thecurves are labelled to identify regions of assimilatory andnon-assimilatory electron flow redrawn from data of Canvin etal. (1980). It should be noted that the data and the labelling of the figureaxes are correct as printed.     

Seasonal fluctuations in the biomass and metabolic activity of bacterioplankton and phytoplankton in a well-mixed estuary: the Ems-Dollard (Wadden Sea)

The biomass and metabolic activity of bacterioplankton weremeasured over 1 year in the Ems Dollard Estuary, a part of theDutch-German Wadden Sea. Very productive phyzoplankton blooms,composed mainly of diatoms and the haptophycean alga Phaeocystispouchetii, are a feature of the estuarine section studied. Theproduction of bacterial populations, as measured using the [³H]thymidinemethod broadly followed the phytoplankton density during bloomsin spring and late summer. There was no indication of a disproportionateincrease in bacterial production during the bloom or declineof particular algal species. The rate at which ¹⁴C-labelledglucose, glutamate and leucine were incorporated by bacterialpopulations, measured as a metabolic potential, varied seasonally,but did not precisely follow the rate of [³H]thymidine incorporation.The utilization of the absorbed ¹⁴C substrates for respirationor for cellular synthesis was constant over a prolonged periodof bacterial growth; apparent yields of 0.7, 0.4 and 0.8 weremeasured for glucose, glutamate and leucine, respectively. Inthe colder season most of the absorbed substrate was respired.The production by pelagic bacteria was 60 gC m^–2 y^–1,a value that amounted to 12% of the pelagic primary production.A preliminary experiment with a mixed culture of the abundantlyoccurring diatom species Thalassiosira excentrica and a marinespirillum, indicated that part of the algal exudates were rapidlyconverted by the bacteria, but another part resisted degradation.The incomplete bacterial degradation of algal exudates togetherwith the short residence time of the estuarine phytoplanktonmay contribute to the apparently incomplete mineralisation ofthe primary production in the water.     

Trophic links in the lowland River Meuse (Belgium): assessing the role of bacteria and protozoans in planktonic food webs

Trophic interactions within the plankton of the lowland RiverMeuse (Belgium) were measured in spring and summer 2001. Consumptionof bacteria by protozoa was measured by monitoring the disappearanceof ³H-thymidine-labelled bacteria. Metazooplankton bacterivorywas assessed using 0.5-µm fluorescent microparticles (FMPs),and predation of metazooplankton on ciliates was measured usingnatural ciliate assemblages labelled with FMPs as tracer food.Grazing of metazooplankton on flagellates was determined throughin situ incubations with manipulated metazooplankton densities.Protozooplankton bacterivory varied between 6.08 and 53.90 mgC m^–3 day^–1 (i.e. from 0.12 to 0.86 g C^–1bacteria g C^–1 protozoa day^–1). Metazooplankton,essentially rotifers, grazing on bacteria was negligible comparedwith grazing by protozoa (1000 times lower). Predation of rotiferson heterotrophic flagellates (HFs) was generally low (on average1.77 mg C m^–3 day^–1, i.e. 0.084 g C^–1 flagellatesg C^–1 rotifers day^–1), the higher contribution ofHF in the diet of rotifers being observed when Keratella cochleariswas the dominant metazooplankter. Predation of rotifers on ciliateswas low in spring samples (0.56 mg C m^–3 day^–1,i.e. 0.014 g C^–1 ciliates g C^–1 rotifers day^–1)in contrast to measurements performed in July (8.72 mg C m^–3day^–1, i.e. 0.242 g C^–1 ciliates g C^–1 rotifersday^–1). The proportion of protozoa in the diet of rotiferswas low compared with that of phytoplankton (<30% of totalcarbon ingestion) except when phytoplankton biomass decreasedbelow the incipient limiting level (ILL) of the main metazooplantonicspecies. In such conditions, protozoa (mainly ciliates) constituted50% of total rotifer diet. These results give evidence thatmicrobial organisms play a significant role within the planktonicfood web of a eutrophic lowland river, ciliates providing analternative food for metazooplankton when phytoplankton becomesscarce.     

Effect of light on the metabolism of thymidine in the long-day duckweed, Lemna gibba G3

Metabolism patterns of exogenous thymidine as disclosed by ECTEOLAcellulose column chromatography, were examined with a long-dayduckweed, Lemna gibba G3, under dark and light conditions. Whenthymidine-6-³H was applied, the pattern of thymidine metabolismin the light was not very different from that in the dark. However,when thymidine (methyl-³H) was used, incorporation of radioactivityinto two major ( and ß) and one minor components ofboth B and D fractions separated by column chromatography, wasstrikingly stimulated by the light, through photosynthetic activity.Component a of fraction B was tentatively concluded to be ß-ureidoisobutyricacid by paper chromatography. As the radioactivity from thymidine-6-³Hwas hardly recovered in the a component of fraction D, the partof the thymidine molecule incorporated into this component wasnot the pyrimidine ring, but a methyl residue. (Received March 29, 1973; )     

Investigations of the {delta}13C Pattern in Leaves of Fagus sylvatica L.

The natural abundance of ¹³C in different parts of beech (Fagussylvatica L.) leaves was analysed. Values for leaf ribs wereconsistently higher than those for intercostal tissue. Similardifferences occur between petiole and stem, with petiole beingless negative. The pattern of results is the same, independentof the position in the tree. However, the absolute values differby up to 6%. from the bottom to the top of the tree. Valuesof ¹³C are in the range of – 29 to – 32%. for thelower leaf strata; while values between – 24 and –26%. have been measured for the top of the tree. Absolute ¹³Cvalues of the whole tissue and cellulose differ by about 2%.,but relative ¹³C trends are almost identical. However, ¹³C trendsare not identical for different leaf parts. A comparison ofcellulose and whole tissue ¹³C data makes it unlikely that the¹³C variations are primarily due to different compositions ofchemical compounds. No fractionation seems to exist betweenleaf and wood cellulose. Tissue from different areas of a leafrevealed identical carbon isotope compositions. Key words: Carbon isotope ratio, Fagus sylvatica L., beech leaves     

Potassium Transport Across the Membranes of Chara: I. THE RELATIONSHIP BETWEEN RADIOACTIVE TRACER INFLUX AND ELECTRICAL CONDUCTANCE

Smith, J. R., Smith, F. A. and Walker, N. A. 1987. Potassiumtransport across the membrane of Chara. I. The relationshipbetween radioactive tracer influx and electrical conductance.—J.exp. Bot. 38:731–751. The ⁴²K influx () and the electrical conductance (G_m) were measured simultaneously for the ‘membrane’of internodal cells of Chara australis as a function of theexternal [KCl] (K?. In bathing solutions of pH = 5?0, progressively increased from 20?5to 430?60 nmol m^–2 s^–1 and G_m increased from 0?36?0?02to 3?8?0?8 S m^–2 when K? was increased from 0?1 to 10mol m^–3. The resting membrane potential difference (p.d.)was approximately -135 mV for low K? and approached the expectedNernst equilibrium p.d. for K⁺ ions when K? > 1?0 mol m^–3.Measurements of ³⁶Cl influx suggested that the ⁴²K influx waspredominantly electrogenic. The equivalent Goldman permeabilityto K⁺ ions (P_k) was approximately 20–30 nm s^–1 anddid not vary significantly with increasing K?. The equivalentconductance attributable to the electrogenic transport of K⁺ ions was calculated from assuming passive, independent diffusionof K⁺ ions and the ratio was found to be typically close to one. It was also found that themagnitudes of and G_m measuredsimultaneously for each individual cell were also well correlatedfor K? 1?0 mol m^–3, and that the slope of the line ofbest fit was close to one. For each K? it was found that theconductance not attributable to K⁺ translocation and presumablyassociated primarily with the transport of protons or theirequivalents was typically 0?2–0?5 Sm^–2. For K? >1?0 mol m^–3 the results indicated that the transport ofK⁺ ions was essentially independent, i.e. there was no evidencefor flux interactions. The results also indicated that the equivalentconductance derived from the measured ⁴²K influx could usefullyindicate the fraction of the electrical conductance attributableto the translocation of K⁺ ions. Key words: Potassium, conductance, influx     

Estimation of ammonium regeneration efficiencies associated with bacterivory in pelagic food webs via a 15N tracer method

Phagotrophic protists are major components of pelagic food webs,both as consumers of bacterial and phytoplankton cells, andas regenerators of inorganic nutrients. In this study, we estimatedthe efficiency of ammonium regeneration by protists feedingon bacteria within natural plank-tonic assemblages, using a¹⁵N tracer method, in which the excretion of ¹⁵N-labeled ammoniumdue to grazing on ¹⁵N pre-labeled bacteria was followed overtime. We tested this approach in experiments based on the additionof heat-killed ¹⁵N-labeled bacteria to laboratory cultures andto samples of coastal seawater. During two experiments, variationin abundance of bacterivores and bacterioplankton resulted innon-constant grazing rates. Deterministic computer models thatused abundance of bacteria and protists as variables were developedto estimate best-fit values of grazing mortality (g, h^–1)and of ammonium regeneration efficiency (R_E, fraction of theinitial ¹⁵N label in added bacteria which is released as ammonium).Estimated ammonium R_E were 0.30–0.35 for one trophic linksystems with both a monospecific culture and a mixed speciesassemblage of bacterivorous flagellates. R_E was higher for multi-trophicstep food webs: 0.60 for 5 µm pre-screened coastal seawaterand 0.90 for whole coastal seawater.     

Nitrate Transport Characteristics in Hordeum vulgare L. Seedlings using Three Different Tracer Techniques

Deane-Drummond, C. E. and Thayer, J.R. 1986. Nitrate transportcharacteristics in Hordeum vulgare L. seedlings using threedifferent tracer techniques.—J. exp. Bot. 37: 429–439. and have been used to investigate various properties of nitrate uptake and translocation intoHordeum vulgare L seedlings. Short term / influx into seed lings grown in CaSO⁴ was stimulated by after a lag of 2 h. The apparent kinetics of shortterm / influx over the concentration range 0?0–0?7mol m fitted Michaelis-Menten equations The apparent V_max didnot change when seedlings were used that had been pretreatedin 10 or 100 mmol m^–3 and V^max=3.77 and 3?56µmol g^–1 fr. wt. h^–1respectively. The apparent Michaelis constants were also similarand K_m=0?47 and 0?45 mol m^–3 respectively. Longer term pulse chase experiments with the heavy isotope ¹⁵Nhave shown that feeding roots with resulted in the preferential appearance of ¹⁵N labelled aminoacids in the xylem sap. Pulse chase experiments with the radioisotope¹³N have shown that feeding shoots with resulted in a radial pattern of distnbution of labelin the leaf veins, which can be detected using autoradiography. The limitations and advantages of all three techniques are comparedby reference to other known experimental data. Key words: ³⁶Chlorate, ¹³nitrate, ¹⁵nitrate, Hordeum vulgare     

Rates of microbial degradation of dissolved organic carbon from phytoplankton cultures

Dissolved organic carbon (DOC) decay was measured for samplesfrom cultures of the diatoms Chaetoceros gracilis and Phaeodactylumtricornutum, the flagellate Isochrysis galbana, the dinoflagellateAlexandrium tamarense, and a natural algal assemblage from theNorthwest Arm, Nova Scotia, Canada, by a high-temperature catalyticoxidation (HTCO) method. Decay rate constants were determinedusing first-order reaction kinetics in the multi-G model ofBerner (In Early Diagenesis, a Theoretical Approach, PrincetonUniversity Press, 1980). Decay rates as high as 0.37 day^–1wereobtained, which demonstrated that DOC released by phytoplanktonmight be highly labile to bacterial utilization and could bedegraded significantly within hours. Decay rates for most speciestested followed much the same pattern, with K₀₁ values around0.3–0.4, K₀₂ values around 0.03, and K₀₃ and K₀₄ valuesaround 10^–3 day^–1. DOC released by the senescentcells of A.tamarense was found to be essentially bacteria resistant,in contrast to that of the other species tested. The decay ofDOC was directly temperature dependent over the 10–20°Crange. Six methods for DOC preservation were tested. Acidificationwith HCl and refrigerated storage was demonstrated to be themost convenient and practical method. This method can be usedfor both short- and long-term preservation of DOC samples containinghighly labile organic compounds.     

Measurement of Yield Threshold and Cell Wal Extensibility of Intact Wheat Roots under Different Ionic, Osmotic and Temperature Treatments

Yield stress threshold (Y) and volumetric extensibility () arethe rheological properties that appear to control root growth.In this study they were measured in wheat roots by means ofparallel measurement of the growth rate (r) of intact wheatroots and of the turgor pressures (P) of individual cells withinthe expansion zone. Growth and turgor pressure were manipulatedby immersion in graded osmoticum (mannitol) solutions. Turgorwas measured with a pressure probe and growth rate by visualobservation. The influence of various growth conditions on Yand was investigated; (a) At 27 °C.In 0.5 mol m^–3 CaCl₂ r, P, Y and were20.7±4.6 µm min^–1, 0.77±0.05 MPa,0.07±0.03 MPa and 26±1.9 µm min^–1MPa^–1 (expressed as increase in length), respectively.Following 24 h growth in 10 mol m^–3 KC1 these parametersbecame 12.3±3.5 µm min^–1, 0.72±0.04MPa, 0.13±0.01 MPa and 21±0.7 µm min^–1MPa^–1. After 24 h osmotic adjustment in 150 mol m^–3mannitol/0.5 mol m^–3 CaCl₂ r= 19.6±4.2 µmmin^–1, P = 0.68±0.05 MPa and Y and were 0.07±0.04MPa and 30±0.2 µm min^–1 MPa^–01, respectively.After 24 h growth in 350 mol m^–3 mannitol/0.5 mol m^–3CaCl₂ r= 13.3±4.1 µm min^–1, P= 0.58±0.07MPa, Y=0.12±0.01 MPa and ø 32±0.2 tim min^–1MPa^–1. During osmotic adjustment in 200 mol m^–3mannitol/0.5 mol m^–3 CaCl₂, with or without KCl, the recoveryof growth rate corresponded to turgor pressure recovery (t1/2approximately 3 h). (b) At 15 °C. Lowered temperature dramatically influencedthe growth parameters which became r= 8.3±2.8 um min^–1,P=0.78 MPa, r=<0.2 MPa and =15±0.1 µm min^–1MPa^–1. Therefore, Y and are influenced by 10 mol m^–3 K⁺ ionsand low temperature. In each case the effective pressure forgrowth (P-Y) was large indicating that small fluctuations ofsoil water potential will not stop root elongation. Key words: Yield threshold, cell wall extensibility, wheat root growth, temperature, turgor pressur     

Bacterial dynamics and distribution during a spring diatom bloom in the Hudson River plume, USA

Bacterial abundance and [³H]thymidine incorporation rates wereused to characterize bacterial distributions and dynamics duringa spring diatom bloom in the coastal plume of the Hudson Riverduring March, 1981. Bacterial abundance did not decline significantlyaway from the plume or across the continental shelf. However,a pronounced gradient was observed for [³H]thymidine incorporation. Following a northeast gale, a bloom of Skeletonema costatumdeveloped in response to coastal upwelling. As the plume becamesaltier, warmer and larger, bacterial abundance averaged 1.3x 10⁹ cells 1^–1. Bacterial incorporation of [³H]thymidineinto cold 5% TCA insoluble materials, bacterial production andspecific growth rates averaged 80–120 pmol 1^–1 d^–1,1.4–2.0 x 10⁹ cells 1^–1 and 1.2–1.5 d^–1.respectively. The mean density of bacteria in the plume didnot change even though growth rates were higher than expectedlosses from sinking, mixing and export. The abundance and production levels of attached bacteria inthe plume were significantly higher than they were in coastalwater outside the plume. In contrast, free bacterial levelswere similar in all regions of the shelf and plume. Bacterialparameters in the plume were not correlated with phytoplanktonpigment concentrations, while significant positive correlationswere found in shelf waters >33 salinity. Thus bacteria werecoupled to phytoplankton in shelf waters but not in the plumewhere allochthonous dissolved matter was probably supportingthe bacteria. ^*Lamont-Doherty Geological Observatory Contribution No. 3459 ²Present address: Department of Microbiology, University ofGeorgia, Athens, GA 30602, USA.     

Membrane cholesterol extraction decreases Na+ transport in A6 renal epithelia

In this study, we have investigated the dependence of Na⁺ transport regulation on membrane cholesterol content in A6 renal epithelia. We continuously monitored short-circuit current (I_sc), transepithelial conductance (G_T), and transepithelial capacitance (C_T) to evaluate the effects of cholesterol extraction from the apical and basolateral membranes in steady-state conditions and during activation with hyposmotic shock, oxytocin, and adenosine. Cholesterol extraction was achieved by perfusing the epithelia with methyl--cyclodextrin (mCD) for 1 h. In steady-state conditions, apical membrane cholesterol extraction did not significantly affect the electrophysiological parameters; in contrast, marked reductions were observed during basolateral mCD treatment. However, apical mCD application hampered the responses of I_sc and G_T to hypotonicity, oxytocin, and adenosine. Analysis of the blocker-induced fluctuation in I_sc demonstrated that apical mCD treatment decreased the epithelial Na⁺ channel (ENaC) open probability (P_o) in the steady state as well as after activation of Na⁺ transport by adenosine, whereas the density of conducting channels was not significantly changed as confirmed by C_T measurements. Na⁺ transport activation by hypotonicity was abolished during basolateral mCD treatment as a result of reduced Na⁺/K⁺ pump activity. On the basis of the findings in this study, we conclude that basolateral membrane cholesterol extraction reduces Na⁺/K⁺ pump activity, whereas the reduced cholesterol content of the apical membranes affects the activation of Na⁺ transport by reducing ENaC P_o. epithelial Na⁺ channel; Na⁺-K⁺-ATPase activity; short-circuit current; methyl--cyclodextrin; channel open probability     

H2O2 increases sheep tracheal blood flow, permeability, and vascular response to luminal capsaicin

Wells, U. M., S. Duneclift, and J. G. Widdicombe.H₂O₂increases sheep tracheal blood flow, permeability, and vascular response to luminal capsaicin. J. Appl.Physiol. 82(2): 621-631, 1997.Exogenous hydrogenperoxide(H₂O₂)causes airway epithelial damage in vitro. We have studied the effectsof luminalH₂O₂in the sheep trachea in vivo on tracheal permeability tolow-molecular-weight hydrophilic (technetium-99m-labeleddiethylenetriamine pentaacetic acid;^99mTc-DTPA) and lipophilic([¹⁴C]antipyrine;[¹⁴C]AP) tracers andon the tracheal vascular response to luminal capsaicin, whichstimulates afferent nerve endings. A tracheal artery was perfused, andtracheal venous blood was collected. H₂O₂exposure (10 mM) reduced tracheal potential difference(42.0 ± 6.4 mV) to zero. It increased arterial andvenous flows (56.7 ± 6.1 and 57.3 ± 10.0%,respectively; n = 5, P < 0.01, paired t-test) but not tracheal lymph flow(unstimulated flow 5.0 ± 1.2 µl ^· min^{1 ·} cm¹,n = 4). DuringH₂O₂exposure, permeability to ^99mTc-DTPA increased from2.6 to 89.7 × 10⁷ cm/s(n = 5, P < 0.05), whereas permeability to[¹⁴C]AP (3,312.6 × 10⁷ cm/s,n = 4) was not altered significantly(2,565 × 10⁷cm/s). Luminal capsaicin (10 µM) increased tracheal blood flow (10.1 ± 4.1%, n = 5)and decreased venous ^99mTc-DTPAconcentration (19.7 ± 4.0, P < 0.01), and these effects weresignificantly greater after epithelial damage (28.1 ± 6.0 and45.7 ± 4.3%, respectively,P < 0.05, unpairedt-test). Thus H₂O₂increases the penetration of a hydrophilic tracer into tracheal bloodand lymph but has less effect on a lipophilic tracer. It also enhancesthe effects of luminal capsaicin on blood flow and tracer uptake.

     

The influence of temperature and light on the upper limit of Microcystis aeruginosa production in a hypertrophic reservoir

Primary production was measured for 7 years, using the in situ¹⁴C-method in hypertrophic Hartbeespoort Dam, South Africa,to examine the influence of light and water temperature on theupper limit of Microcystis aeruginosa production. Water temperaturesvaried from 11 to >25°C and chlorophyll concentrationsreached 6500 mg m^–3. The maximum volumetric rate of production(A_max) was 12->8800 mg C m^–3 h^–1 with areal productions(A) of 69->3300 mg C m^–2 h^–1 for euphotic zonedepths of <0.5–8.4 m. The intrinsic parameters of phytoplanktonproduction (, A_max/B, I_k) indicated that the phytoplankton populationwas adapted to high light levels. Both A_max/B and I_k were correlatedwith temperature. Under optimal conditions, , the theoreticalupper limit of A, was calculated to be 2.8 g Cm^–2 h^–1,while the measured rate was 2.5 g Cm^–2 h^–1. Measuredareal rates exceeding were overestimated due to methodologicalproblems when working with Microcystis scums. Light and watertemperature interacted to yield high production rates: watertemperature through its direct effect on photosynthetic ratesand indirectly in the formation of diurnal mixed layers; lightindirectly through water temperature and directly through itsattenuation and induction of light-adapted physiology in Microcystis.     

Phagotrophy and NH4+ regeneration in a three-member microbial food loop

In a series of batch experiments we compared the efficiencyof nitrogen regeneration of a two- and three-member microbialfood loop consisting of a mixed bacterial assemblage, a small(3–5 µm) heterotrophic flagellate (Paraphysomonassp.), and a large (7–12 µm) heterotrophic flagellate(Paraphysomonas imperforata). In the two-member system the nitrogenregeneration efficiency for NH₄⁺ (R_n) was 41% and the grossgrowth efficiency (GGE) was 57% during active grazing by thesmall flagellate on bacteria. Regeneration of NH₄⁺ continuedduring the stationary phase so that R_n was 75% after 6 daysincubation. When the larger flagellate was introduced at theend of exponential growth of the smaller grazer in the three-membersystem, initially there was rapid regrowth of bacteria, tyingup 15% of the nitrogen originally in the bacteria. The largerflagellate grazed the smaller one with a GGE of 55%. Total nitrogenregeneration efficiency through exponential growth of the largerflagellate was 73%. Because microbial food loops in naturalwaters are far more complicated and with more grazing stepsthan portrayed in this study, we would expect the bulk of nutrientswithin these systems to be recycled with little transfer tohigher trophic levels.     

A Finite-element Model of Radial and Axial Conductivities for Individual Roots: Development and Validation for Two Desert Succulents

A morphologically explicit numerical model for analysing wateruptake by individual roots was developed based on a conductornetwork, with specific conductors representing axial or radialconductivities for discrete root segments. Hydraulic conductivity(L_p; m s^–1 MPa^–1) was measured for roots of Agavedeserti Engelm. and Opuntia ficus-indica (L.) Miller by applyinga partial vacuum to the proximal ends of excised roots in solution.L_p was also measured for 40- to 80-mm segments along a root,followed by measurements of axial conductivity and calculationof radial conductivity. Predicted values of L_p for entire rootsbased on two to ten segments per root averaged 1.04±0.07(mean±s.e. mean for n = 3) of the measured L_p for A.deserti and 1.06±0.10 for O. ficus-indica. The modelalso closely predicted the drop in water potential along theroot xylem (^xylem); when a tension of 50 kPa was applied tothe proximal ends of 0.2 m-long roots of A. deserti and O. ficus-indica,the measured ^xylem to midroot averaged 30 kPa compared witha predicted decrease of 36 kPa. Such steep gradients in ^xylemsuggest that the driving force for water movement from the soilto young distal roots may be relatively small. The model, whichagreed with an analytical solution for a simple hypotheticalsituation, can quantify situations without analytical solutions,such as when root and soil properties vary arbitrarily alonga root. Agave deserti, electrical circuit analog, hydraulic conductivity, Opuntia ficus-indica, water potential