Study of the glow dynamics in a laser-produced plasma jet expanding across the magnetic field

Results are presented from experimental studies of the glow dynamics of a plasma jet generated during the irradiation of a plane aluminum target by an iodine laser pulse with the wavelength 1.315 μm. The laser pulse energy was 330–480 J, the pulse duration was 0.5 ns, and the focal spot diameter was 3 mm, the laser intensity on the target surface being ∼10¹³ W/cm². The jet expanded across an external magnetic field with the strength ∼1 kOe. The residual air pressure in the vacuum chamber was ∼10⁻⁵ Torr. The spatiotemporal behavior of the jet glow was investigated using a nine-frame camera in two mutually perpendicular directions (along and across the magnetic field). The results of measurements indicate azimuthal asymmetry of the jet expansion.     

Diagnostics of plasma produced by femtosecond laser pulse impact upon a target with an internal nanostructure

X-ray diagnostics of the interaction of femtosecond laser pulses with intensities of 10¹⁶–10¹⁸ W/cm² with CO₂ clusters and frozen nanosize water particles is carried out. The stage of cluster expansion and the formation of a plasma channel, which governs the parameters of the formed X-ray radiation source and accelerated ion flows, is studied. The measurements are based on recording spatially resolved X-ray spectra of H- and He-like oxygen ions. Utilization of Rydberg transitions for spectra diagnostics makes it possible to determine plasma parameters on a time scale of t ∼ 10 ps after the beginning of a femtosecond pulse. The role of the rear edge of the laser pulse in sustaining the plasma temperature at a level of ∼100 eV in the stage of a nonadiabatic cluster expansion is shown. The analysis of the profiles and relative intensities of spectral lines allows one to determine the temperature and density of plasma electrons and distinguish the populations of “thermal” ions and ions that are accelerated up to energies of a few tens of kiloelectronvolts. It is shown that the use of solid clusters made of frozen nanoscale water droplets as targets leads to a substantial increase in the number of fast He-like ions. In this case, however, the efficiency of acceleration of H-like ions does not increase, because the time of their ionization in plasma exceeds the time of cluster expansion.     

Non-self-sustained glow discharge with electrostatic confinement of electrons sustained by a fast neutral molecule beam

Experimental study of plasma produced at the nitrogen pressure 0.2–1 Pa in the chamber volume V ≈ 0.12 m³ as a result of injection into the chamber of a broad nitrogen molecule beam with 1–4 keV energy and 0.1–1 A equivalent current is carried out, and the study results are presented. Dependences of the plasma density distribution on the beam equivalent current I _b , energy E _b , and gas pressure p indicate a crucial role of fast molecules in gas ionization, and the probe characteristics reveal two groups of plasma electrons with the temperatures T _e ∼ 0.4 eV and T _e ∼ 16 eV. Immersion in plasma of an electrode isolated from the chamber and application to the electrode of a positive voltage U result in non-self-sustained discharge. When U changes from ∼0.5 to ∼1.5 V, the discharge current I rapidly rises to a certain value I*, and after that the rate of rise dI/dU drops by an order of magnitude. At U ∼ 10 V, the current I rises to I ₀ ≈ 1.5I*, and dI/dU once again drops by an order of magnitude. Current I ₀ specifies the number of electrons produced inside the chamber per second, and it grows up with E _b , I _b , and p. At U > 20 V, due to gas ionization by fast electrons emitted by the chamber and accelerated up to the energy ∼eU in the sheath between the plasma and the chamber walls, the current I rises again. When U grows up to ∼50 V, production of fast electrons with energies exceeding the ionization threshold begins inside the sheath, and the ionization intensity rises dramatically. At U > 150 V, contribution of fast electrons to gas ionization already exceeds the contribution of fast molecules, and the plasma density and its distribution homogeneity inside the chamber both grow up substantially. However, even in this case, the discharge is non-self-sustained, and only at U > 300 V it does not expire when the beam source is switched off.     

Excitation of the isomeric states of silver isotope nuclei in the plasma produced by a subrelativistic picosecond laser pulse

Arrangement and results of experiments on the excitation of the (E _m = 93.125 keV, J ^p = 7/2⁺, T _1/2 = 44.3 s) and (E _m = 88.034 keV, J ^p = 7/2⁺, T _1/2 = 39.6 s) isomeric states of Ag¹⁰⁷ and Ag¹⁰⁹ nuclei under the action of X-ray emission in a hot (T _e ∼0.5 keV) dense plasma produced by a laser pulse with the energy ∼9 J, intensity ∼1.2 × 10¹⁸ W/cm², and duration 0.82 ps on the SOKOL-P facility are described. The experimentally determined half-life of the isomeric states agrees satisfactorily with the half-life of the Ag^107m and Ag^109m isomers, and their number N _m ∼ 6.9 × 10⁴ agrees with the qualitative estimate N _m ∼ 2.8 × 10⁴, obtained within the refined model of the physical processes in laser plasma.     

Runaway electron beams in the gas discharge for UV nitrogen laser excitation

The review of the methods for obtaining the runaway electron beams in the gas discharge is performed. The new method is offered, using which the beam is first formed in a narrow gap (∼1 mm) between the cathode and the grid and then it is accelerated by the field of the plasma column of the anomalous self-sustained discharge in the main gap (10–20 mm long). The electron beams with an energy of about 10 keV and current density of 10³ A/cm² at a molecular nitrogen pressure of up to 100 Torr have been obtained experimentally. The results of research of the UV nitrogen laser with an excitation via runaway electron beam and radiation of energy of ∼1 mJ are given. The UV nitrogen laser generation with the energy of ∼1 mJ has been obtained by the runaway electron beams.     

First results from plasma density measurements in the FTU tokamak by means of a two-frequency pulsed time-of-flight refractometer

A pulsed time-of-flight refractometer was developed and tested to determine the mean plasma density in the T-11M tokamak by measuring the propagation time of nanosecond microwave pulses in plasma. Later, it was also proposed to use such an instrument to measure and control the mean plasma density in the ITER tokamak by probing the plasma with an extraordinary wave, the electric field of which is perpendicular to the magnetic field in plasma, in the transparency window at frequencies of 50–100 GHz. To avoid the effect of the density profile shape on the measurement results in the nonlinear mode of refractometer operation (near the cutoff), a system operating at two different probing frequencies was developed and tested. Such a system provides two values of the time delay, which can be used to estimate the peaking factor of the density distribution α and correctly determine the linear density 〈Nl〉, regardless of the density profile (assuming a smooth density profile of the form of N(ρ) = N(0)(1 − ρ²)^α, where N(0) is the central plasma density and ρ = r/a is the normalized plasma radius). The first experiments on density measurements in the FTU tokamak performed with this refractometer are described, and results from these experiments are presented. The formation of a thin dense plasma layer in the zone of a strong magnetic field (the so-called MARFE layer) at a relatively low (for FTU) plasma density of ∼6 × 10¹⁹ m⁻³ was detected. The thickness of this layer, determined from the refractometry data, agrees well with the data obtained using a digital camera.     

Femtosecond stage of electron transfer in reaction centers of the triple mutant SL178K/GM203D/LM214H of <Emphasis Type="Italic">Rhodobacter sphaeroides</Emphasis>

Coherent processes in an initial phase of charge transfer in reaction centers (RCs) of the triple mutant S(L178)K/G(M203)D/L(M214)H of Rhodobacter sphaeroides were investigated by difference (light — dark) absorption spectroscopy with 18 fsec time resolution. Electron transfer in the B cofactor branch is activated in this mutant, while the A-branch electron transfer is slowed in comparison with native RCs of Rba. sphaeroides. A bulk of absorption difference spectra was analyzed in the 940–1060 nm range (stimulated emission of excited bacteriochlorophyll dimer P* and absorption of bacteriochlorophyll anions B_A⁻ and β⁻, where β is a bacteriochlorophyll substituting the native bacteriopheophytin H_A) and in the 735–775 nm range (bleaching of the absorption band of the bacteriopheophytin H_B in the B-branch) in the −0.1 to 4 psec range of delays with respect to the moment of photoexcitation of P at 870 nm. Spectra were measured at 293 and 90 K. The kinetics of P* stimulated emission at 940 nm shows its decay with a time constant of ∼14 psec at 90 K and ∼18 psec at 293 K, which is accompanied by oscillations with a frequency of ∼150 cm⁻¹. A weak absorption band is found at 1018 nm that is formed ∼100 fsec after excitation of P and reflects the electron transfer from P* to β and/or B_A with accumulation of the P⁺β⁻ and/or P⁺B_A⁻ states. The kinetics of ΔA at 1018 nm contains the oscillations at ∼150 cm⁻¹ and distinct low-frequency oscillations at 20–100 cm⁻¹; also, the amplitude of the oscillations at 150 cm⁻¹ is much smaller at 293 than at 90 K. The oscillations in the kinetics of the 1018 nm band do not contain a 32 cm⁻¹ mode that is characteristic for native Rba. sphaeroides RCs having water molecule HOH55 in their structure. The ΔA kinetics at 751 nm reflects the electron transfer to HB with formation of the P⁺H_B⁻ state. The oscillatory part of this kinetics has the form of a single peak with a maximum at ∼50 fsec completely decaying at ∼200 fsec, which might reflect a reversible electron transfer to the B-branch. The results are analyzed in terms of coherent nuclear wave packet motion induced in the P* excited state by femtosecond light pulses, of an influence of the incorporated mutations on the mutual position of the energy levels of charge separated states, and of the role of water HOH55 in the dynamics of the initial electron transfer.     

Seasonal variations in biomass, abundance and composition of zooplankton in the subarctic waters north of Iceland

The seasonal variations in biomass, abundance and species composition of zooplankton in relation to hydrography and chlorophyll a were studied in the subarctic waters north of Iceland. The sampling was carried out at approximately monthly intervals from February 1993 to February 1994 at eight stations arranged along a transect extending from 66°16′N–18°50′W to 68°00′N–18°50′W. The mean temperature at 50 m depth showed a clear seasonal pattern, with lowest water temperatures in February (∼1.1°C) and the highest in July (∼5.4°C). The spring growth of the phytoplankton began in late March and culminated during mid-April (∼7.0 mg Chl a m⁻³). Both the biomass and the abundance of total zooplankton were low during the winter and peaked once during the summer in late May (∼4 g m⁻² and ∼38,000 individuals m⁻²). A total of 42 species and taxonomic groups were identified in the samples. Eight taxa contributed ∼90% of the total zooplankton number. Of these Calanus finmarchicus was by far the most abundant species (∼60% of the total zooplankton). Less important groups were ophiuroid larvae (∼9%), Pseudocalanus spp. (∼8%), Metridia longa (∼4%), C. hyperboreus (∼3%), Acartia longiremis (∼2%), chaetognaths (∼2%) and euphausiid larvae (∼2%). The dominant copepods showed two main patterns in seasonal abundance: C. finmarchicus, C. hyperboreus and C. glacialis had one annual peak in numbers in late May, while Pseudocalanus spp., M. longa and A. longiremis showed two maxima during the summer (July) and autumn (October/November). Ophiuroid larvae and chaetognaths (mainly Sagitta elegans) peaked during the middle of July, while the number of euphausiid eggs and larvae was greatest from May to July. The succession in population structure of C. finmarchicus indicated its main spawning to be in April and May, coincident with the phytoplankton spring bloom. A minor spawning was also observed sometime between August and October. However, the offspring from this second spawning contributed only insignificantly to the overwintering stock of C. finmarchicus. Received: 12 September 1997 / Accepted: 1 March 1998     

Single anion channels reconstituted from cardiac mitoplasts

Ion channels from sheep cardiac mitoplast (inverted inner mitochondrial membrane vesicle) preparations were incorporated into voltage-clamped planar lipid bilayers. The appearance of anion rather than cation channels could be promoted by exposing the bilayers to osmotic gradients formed by Cl⁻ salts of large, relatively imperment, cations at a pH of 8.8. Two distinct activities were identified. These comprised a multisubstate anion channel of intermediate conductance (∼60 pS in 300vs. 50mm choline Cl, ∼100 pS in symmetric 150mm KCl), and a lower-conductance anion channel (∼25 or ∼50 pS in similar conditions), which only displayed two well-defined substates, at ∼25 and ∼50% of the fully open state. The larger channels were not simple multiples of the lower-conductance channels, but both discriminated poorly, and to a similar extent, between anions and cations (P_Cl ⁻/P_choline ⁺ ∼12, P_Cl ⁻/P_K ⁺∼8). The lower-conductance channel was only minimally selective between different anions (P_{NO 3} ⁻ (1.0)=P_Cl ⁻>P_Br ⁻>P_I ⁻>P_SCN ⁻(0.8)), and its conductance failed to saturate even in high (>1.0 M) activities of KCl. The channels were not obviously voltage dependent, and they were unaffected by 0.5 mM SITS, H₂O₂, propranolol, quinine or amitriptyline, or by 2 mM ATP, or by variations in pH (5.5–8.8). Ca²⁺ and Mg²⁺ did not alter single channel activity, but did modify single current amplitudes in the lower-conductance channel. This effect, together with voltage-dependent substate behavior, is described in the following paper.     

Development of a colorimetric test system for detection of point mutations via ligation of a tandem of short oligonucleotides on methacrylate beads

A new approach for detection of point mutations has been developed. The nonradioactive test system proposed is based on enzymatic ligation of a tandem of three short oligonucleotides B∼pN₈+pN₄+pN′₈ Bio in the presence of a complementary DNA template. The 5′-terminal octanucleotide B∼pN₈ is immobilized on polymer methacrylate beads (B) and the 3′-terminal octanucleotide pN′₈ Bio contains a biotin residue at the 3′-phosphate. Ligation of the tandem produces a 20-mer biotinylated oligonucleotide on a polymer bead, which is then visualized via subsequent treatments with streptavidin-alkaline phosphatase conjugate and chromogenic substrates. Intense staining of the polymer beads is observed when the amount of DNA template (20-mer oligonucleotide) is as low as ∼10⁻¹⁴ mol. It is shown that practically no polymer staining is observed when the complex formed by the tandem and the 20-mer DNA template contains a mismatch either in the tetranucleotide duplex or in the duplex of octanucleotide immobilized on the beads. This suggests a possibility of using the presented approach in test systems for detection of point mutations in PCR-amplified DNA fragments.     

Physical properties of two types of calcium stores and SERCAs in human platelets

The aim of this work was to obtain experimental data depending on the properties of calcium stores and SERCAs, to analyse these data in terms of the models based on simulation of the cellular compartments as communicating vessels, and to relate this way the data to the above properties. The main characteristics of the stores and corresponding SERCAs have been estimated. Calcium content in the DTS is ∼1.5 × 10⁶ ions per cell, that in the acidic stores, ∼0.64 × 10⁶ ions per cell. The rate constant of background calcium leakage from the DTS is ∼0.0033 s⁻¹, that from the acidic stores, ∼0.1 s⁻¹. The background activity of SERCA2b is ∼0.22 × 10⁶ s⁻¹ ions per cell, that of SERCA3, ∼2.5 × 10⁶ s⁻¹ ions per cell. The properties of both calcium stores and the SERCAs and the characteristics found might be related to physiological or pathological state of the cells.     

Turbulence and stratification in Priest Pot,a productive pond in a sheltered environment

Priest Pot is an example of the abundant ponds that, collectively, contribute crucially to species diversity. Despite extensive biological study, little has been reported about the physical framework that supports its ecological richness. This article elucidates the physical character of Priest Pot’s water column and thus that of similar water bodies. Vertical thermal microstructure profiles were recorded during summer 2003 and analyzed alongside concurrent meteorological data. During summer stratification, the thermal structure appeared to be dominated by surface heat fluxes. Surface wind stress, limited by sheltering vegetation, caused turbulent overturns once a surface mixed layer was present but appeared to contribute little to setting up the thermal structure. Variations in full-depth mean stratification occurred predominantly over seasonal and ∼5-day time scales, the passage of atmospheric pressure systems being posited as the cause of the latter. In the uppermost ∼0.5 m, where the stratification varied at subdaily time scales, turbulence was active (sensu Ivey and Imberger 1991) when this layer was mixed, with dissipation values ε ∼ 10⁻⁸ m² s⁻³ and vertical diffusivity K_Z = 10⁻⁴ — 10⁻⁶ m² s⁻¹. Where the water column was stratified, turbulence was strongly damped by both buoyancy and viscosity, and K_Z was an order of magnitude smaller. Vertical transport in the mixed layer occurred via many small overturns (Thorpe scale r.m.s. and maximum values were typically 0.02 m and 0.10 m, respectively), and seston were fully mixed through the water column.     

Inactivation of plasmalemma conductance in alkaline zones of <Emphasis Type="Italic">Chara corallina</Emphasis> after generation of action potential

A microelectrode study with Chara corallina cells has shown that post-excitation changes of membrane potential and plasmalemma resistance, induced by the action potential (AP) generation, differ substantially for cell areas producing zones of high and low external pH. In cell regions producing alkaline zones, the AP generation was followed by post-excitation hyperpolarization by about 50 mV, concomitant with four- to eightfold increase in plasmalemma resistance and a considerable drop of pericellular pH. In the acidic areas the post-excitation hyperpolarization was weak or absent, and the membrane resistance showed no significant increase within 1–2 min after AP. The membrane excitation in the acidic zones was accompanied by a noticeable pH increase near the cell surface, indicative of the inhibition of plasma membrane H⁺ pump. The results suggest that the high local conductance of the plasmalemma is closely related to alkaline zone formation and the depolarized state of illuminated cell under resting conditions. Excitation-induced changes of membrane potential and pH in the cell vicinity were fully reversible, with the recovery period of ∼15 min at a photon flux density of ∼100 μE/(m² s). At shorter intervals between excitatory stimuli, differential membrane properties of nonuniform regions turned smoothed and could be overlooked. It is concluded that the origin of alkaline zones in illuminated Chara cells cannot be ascribed to hypothetical operation of H⁺/HCO₃⁻ symport or OH⁻/HCO₃⁻ antiport.     

Study of the generation of the 13.5-nm EUV radiation from Sn ions in a CO<Subscript>2</Subscript> laser-produced plasma

Results are presented from experimental and theoretical studies of the efficiency of using a CO₂ laser to create a high-power source of 13- to 14-nm EUV radiation for lithography. For a laser intensity of ∼2 × 10¹¹ W/cm², a conversion efficiency of k _EUV ≃ 1.5% was achieved on a plane solid Sn target. The calculated gas dynamics and population kinetics of Sn plasma ions agree qualitatively with experimental results.     

Optimization and control of perfusion cultures using a viable cell probe and cell specific perfusion rates

Consistent perfusion culture production requires reliable cell retention and control of feed rates. An on-line cell probe based on capacitance was used to assay viable biomass concentrations. A constant cell specific perfusion rate controlled medium feed rates with a bioreactor cell concentration of ∼5 × 10⁶ cells mL^-1. Perfusion feeding was automatically adjusted based on the cell concentration signal from the on-line biomass sensor. Cell specific perfusion rates were varied over a range of 0.05 to 0.4 nL cell^-1 day^-1. Pseudo-steady-state bioreactor indices (concentrations, cellular rates and yields) were correlated to cell specific perfusion rates investigated to maximize recombinant protein production from a Chinese hamster ovary cell line. The tissue-type plasminogen activator concentration was maximized (∼40 mg L^-1) at 0.2 nL cell^-1 day^-1. The volumetric protein productivity (∼60 mg L^-1 day^-1 was maximized above 0.3 nL cell^-1 day^-1. The use of cell specific perfusion rates provided a straightforward basis for controlling, modeling and optimizing perfusion cultures. This revised version was published online in July 2006 with corrections to the Cover Date.     

Tracking chromaffin granules on their way through the actin cortex

Quantitative time-lapse evanescent-wave imaging of individual fluorescently labelled chromaffin granules was used for kinetic analysis of granule trafficking through a ∼300-nm (1/e²) optical section beneath the plasma membrane. The mean squared displacement (MSD) was used to estimate the three-dimensional diffusion coefficient (D ⁽³⁾). We calculated the granules' speed, frame-to-frame displacement and direction and their autocorrelation to identify different stages of approach to the membrane. D ⁽³⁾ was about 10,000 times lower than expected for free diffusion. Granules located ∼60 nm beneath the plasma membrane moved on random tracks (D ⁽³⁾≈10⁻¹⁰ cm² s⁻¹) with several reversals in direction before they approached their docking site at angles larger than 45^∘. Docking was observed as a loss of vesicle mobility by two orders of magnitude within <100 ms. For longer observation times the MSD saturated, as if the granules' movement was confined to a volume only slightly larger than the granule. Rarely, the local random motion was superimposed with a directed movement in a plane beneath the membrane. Stimulation of exocytosis selectively depleted the immobile, near-membrane granule population and caused a recruitment of distant granules to sites at the plasma membrane. Their absolute mobility levels were not significantly altered. Application of latrunculin or jasplakinolide to change F-actin polymerisation caused a change in D ⁽³⁾ of the mobile granule population as well as a reduction of the rate of release, suggesting that granule mobility is constrained by the filamentous actin meshwork and that stimulation-dependent changes in actin viscosity propel granules through the actin cortex. Received: 18 November 1999 / Revised version: 26 January 2000 / Accepted: 2 February 2000     

A new method of autotrophic denitrification with spent sulfidic caustic as substrate and alkalinity source

A new method based on sulfide utilizing autotrophic denitrification was adopted to remove nitrate from wastewater and to reuse spent sulfidic caustic containing high sulfide and alkalinity levels. The experiments were performed using a bench-scale upflow anoxic hybrid growth reactor (UAHGR) and an upflow anoxic suspended growth reactor (UASGR) to characterize the stoichiometric relationship between sulfur and nitrate in the process as well as the performance of the reactors. The level of nitrate removal from the UAHGR and UASGR were maintained at over 90% at a nitrate loading rate ranging from 0.15∼0.40 kgNO₃ ⁻/m³·d and no significant nitrite accumulation was observed in either reactor. Although the influent pH values were higher than the optimum range of autotrophic denitrification at 8.7∼10.1, the effluent pH was stable at 7.2∼7.9 due to the production of hydrogen ions during operation. The stoichiometric ratio of sulfate production to nitrate removal was 1.5∼2.1 mgSO₄ ²⁻/mgNO₃ ⁻ in both reactors. A comparison of the reactor performance revealed that the chemical parameters of the UAHGR operation corresponded to a plug flow like type reactor while the chemical parameters of the UASGR operation corresponded to a completely stirred tank reactor like type reactor. UAHGR did not require sludge recycling due to the packed media while UASGR required 300∼700% sludge recycling. Therefore, spent sulfidic caustic could be used in the sulfur utilizing autotrophic denitrification processes as substrate and alkalinity sources.     

Dunaliella salina (Chlorophyta) with small chlorophyll antenna sizes exhibit higher photosynthetic productivities and photon use efficiencies than normally pigmented cells

The photon use efficiencies and maximal rates of photosynthesis in Dunaliella salina (Chlorophyta) cultures acclimated to different light intensities were investigated. Batch cultures were grown to the mid-exponential phase under continuous low-light (LL: 100 μmol photon m^-2 s^-1) or high-light (HL: 2000 μmol photon m^-2 s^-1) conditions. Under LL, cells were normally pigmented (deep green) containing ∼500 chlorophyll (Chl) molecules per photosystem II (PSII) unit and ∼250 Chl molecules per photosystem I (PSI). HL-grown cells were yellow-green, contained only 60 Chl per PSII and 100 Chl per PSI and showed signs of chronic photoinhibition, i.e., accumulation of photodamaged PSII reaction centers in the chloroplast thylakoids. In LL-grown cells, photosynthesis saturated at ∼200 μmol photon m^-2 s^-1 with a rate (P_max) of ∼100 mmol O₂ (mol Chl)^-1 s^-1. In HL-grown cells, photosynthesis saturated at much higher light intensities, i.e. ∼2500 μmol photon m^-2 s^-1, and exhibited a three-fold higher P_max (∼300 mmol O₂ (mol Chl)^-1 s^-1) than the normally pigmented LL-grown cells. Recovery of the HL-grown cells from photoinhibition, occurring prior to a light-harvesting Chl antenna size increase, enhanced P_max to ∼675 mmol O₂ (mol Chl)^-1 s^-1. Extrapolation of these results to outdoor mass culture conditions suggested that algal strains with small Chl antenna size could exhibit 2–3 times higher productivities than currently achieved with normally pigmented cells. This revised version was published online in August 2006 with corrections to the Cover Date.     

Spatial variation in sea urchins, fish predators, and bioerosion rates on coral reefs of Belize

Although sea urchins are critical for controlling macroalgae on heavily fished coral reefs, high densities threaten reefs, as urchins are also prodigous bioeroders. This study examined urchin population characteristics, bioerosion rates, their fish predators (Labridae), and potential competitors (Scaridae) on unprotected reefs and a reef within a marine protected area (MPA) in the lagoonal regions off Belize. Urchin density (<1 m⁻²) and bioerosion rates (∼0.2 kg CaCO₃ m⁻² year⁻¹) were lowest and members of the Labridae were the highest (∼20 fish 200 m⁻³) within the MPA, while several unprotected reefs had higher (∼18–40 m⁻²) urchin densities, lower Labridae abundances (1–3 fish 200 m⁻³), and bioerosion rates ranging from ∼0.3–2.6 kg CaCO₃ m⁻² year⁻¹. Urchin abundances were inversely related to Labridae (wrasses and hogfish) densities; however, on reef ridges, low algal cover (∼15%), small urchin size (∼14 mm), and low proportion of organic material in urchin guts suggested food limitation. Both top–down (predation) and bottom–up factors (food limitation) likely contribute to the control of urchins, predominantly Echinometra viridis, off Belize, thereby potentially diminishing the negative impacts of bioerosion activities by urchins.     

Cardiac Ryanodine Receptor Activity is Altered by Oxidizing Reagents in Either the Luminal or Cytoplasmic Solution

The location of reactive cysteine residues on the ryanodine receptor (RyR) calcium release channel was assessed from the changes in channel activity when oxidizing or reducing reagents were added to the luminal or cytoplasmic solution. Single sheep cardiac RyRs were incorporated into lipid bilayers with 10⁻⁷ m cytoplasmic Ca²⁺. The thiol specific-lipophilic-4,4′-dithiodipyridine (4,4′-DTDP, 1 mm), as well as the hydrophilic thimerosal (1 mm), activated and then inhibited RyRs from either the cis (cytoplasmic) or trans (luminal) solutions. Activation was associated with an increase in the (a) mean channel open time and (b) number of exponential components in the open time distribution from one (∼2 msec) to three (∼1 msec; ∼7 msec; ∼15 msec) in channels activated by trans 4,4′-DTDP or cis or trans thimerosal. A longer component (∼75 msec) appeared with cis 4,4′-DTDP. Activation by either oxidant was reversed by the thiol reducing agent, dithiothreitol. The results suggest that three classes of cysteines are available to 4,4′-DTDP or thimerosal, SHa or SHa* activating the channel and SHi closing the channel. SHa is either distributed over luminal and cytoplasmic RyR domains, or is located within the channel pore. SHi is also located within the transmembrane domain. SHa* is located on the cytoplasmic domain of the protein. Received: 17 March 1998/Revised: 26 October 1998