Fertilisation de communautés phytoplanctoniques. ii. cas d'un milieu eutrophe: upwelling des côtes du sahara espagnol

Experiments with nutrient enrichment of phytoplankton communities have been carried out on surface waters from the upwelling area off Cap Blanc — Cap Corveiro (Spanish Sahara).None of the control flasks showed phytoplankton development despite the large quantities of nutrient available: this may be due to the presence of an inhibitor in the upwelling waters. On the other hand, most of the enrichments stimulated the autotrophic production. On average, the best growth rates were obtained when a full medium minus Fe-EDTA was added; marked phytoplankton development also took place when a medium without vitamins, or silicates, or phosphates, was added. Lack of metallic compounds (other than iron) and nitrates limit the algal growth.The maximum amounts of chlorophyll a (25–30 mg/m³) are close to the maxima usually observed in this area. The productivity rate (biomass produced/nutrients used) is very high.     

Zooplankton distribution in the coastal upwelling system along the Banc d'Arguin,Mauritania

In the framework of the Dutch Mauritania Expedition 1988, zooplankton was sampled in the beginning of the spring upwelling season at 44 stations along the Banc d'Arguin, Mauritania. Distribution patterns of oceanic and neritic copepod species and especially of the upwelling indicator Calanoides carinatus (Kröyer) were analysed in search for direct proof of circulation of ocean upwelling water over the inshore banks. The vertical temperature distribution at the stations pointed to upwelling in two different parts of the shelf area. According to the copepod data the two upwellings were of a different nature. A restricted area at the shelf edge near Cap Timiris appeared to represent a relatively stable coastal upwelling situation. Upwelled water present near Cap Blanc was found to have traveled quite recently over a considerable part of the shelf. In the central area between the two upwellings, Banc-water extended from the shelf into the ocean. The possible impact of the observed circulation of ocean water on productivity of the shelf benthic system is discussed.     

Coastal upwelling, primary production and mussel growth in the Rías Baixas of Galicia

The Rías Baixas are four flooded tectonic valleys located on the northwest Iberian Peninsula that support a mussel production of about 250 × 10⁶ kg y^–1 from 3337 mussel rafts. Mussel production in this region is the highest in Europe and one of the most intensive in the world, giving employment to 9000 people directly and 20000 indirectly. The causes of this high mussel production are discussed through the analysis of published and some unpublished information. The interaction between coastal upwelling and the circulation patterns in the Rías, which channel the 3-dimensional variability of the open ocean into a 2-dimensional system, promotes a massive response in the productivity of phytoplankton populations inside the Rías, even during weak upwelling events along the coast. Coastal upwelling in the area typically occurs between March–April and September–October. The mean value of gross primary production during the whole upwelling season is 1.4 g C m^–2 d^–1, although high sporadic values of 4 g C m^–2 d^–1 may occur during upwelling relaxation events, when phytoplankton export to the coastal shelf is restricted. Mussel growth occurs mainly during the upwelling season. It is estimated that mussel harvest extracts 10% of the primary production. The phytoplankton response to upwelling provides food of high quality (f 0.5) that determines high absorption efficiency (0.6), whereas the characteristics of the Rías maintains the seston concentration at levels (0.5 – 1.3 mg TPM l^–1 and <5 mg Chl a m^–3) below the threshold of pseudo-faeces production. The physiological behaviour of mussels indicates that the high yield of mussel culture in the Rías of Galicia is a consequence of the particular characteristics of the seston.     

Phytoplankton spring bloom in the Tagus coastal waters: hydrological and chemical conditions

Spatial variability of phytoplankton as well as hydrological and chemical conditions in the Tagus coastal waters were studied during the spring of 1994. The highly patchy distribution of phytoplankton and the community structure were related to the specific abiotic conditions prevailing in the area. Two main water masses were distinguished: a tidally mixed one alongshore Lisbon-cape Espichel and a stratified region strongly related to the Tagus plume river. The latter seems to control, to some extent, the spatial phytoplankton variability and the timing and size of the spring bloom. The highest surficial levels of nutrients, specifically PO₄ ^3–=1.00 M, NO₃ ^–=16.0 M and Si(OH)₄=14.6 M, were clearly associated with Tagus river discharges, while the lowest levels were attained offshore in the zone of maximum chlorophyll a concentrations (30 mg m^–3) reflecting the occurrence of high nutrient consumption. Furthermore, the near depletion of PO₄ ^3– indicates P limitation for phytoplankton growth. The diatom Detonula pumila (0.1×10⁶ cells l^–1) was the dominant species of phytoplankton communities in the mixed zone, alongshore Lisbon-cape Espichel, whereas, the small diatom Thalassiosira sp. (3×10⁶ cells l^–1) was blooming offshore, associated with the stratified region.     

Phosphorus and nitrogen excretion rates of zooplankton from the eutrophic Loosdrecht lakes,with notes on other P sources for phytoplankton requirements

Phosphorus and nitrogen excretion rates by zooplankton communities from two eutrophic and shallow Dutch lakes were measured in laboratory. The variations in excretion rates in the lakes (May–October) were caused mainly by fluctuation in zooplankton biomass. Mean summer excretion rates (June–September) were 2.4 and 0.9 µg PO₄P·1^–1·d^–1 in Lake Loosdercht and Lake Breukeleveen, respectively. This difference between the lakes was caused mainly by the lower zooplankton biomass in Lake Breukeleveen. The excretion of 2.4 µg PO₄P·1^–1·d^– compared with the calculated P-demand of phytoplankton of 8.0 µg PO₄P·1^–1·d^–1 is substantial in the summer (June–September) and far more important than the external P-supply of 0.4 µg P·1^–1·d^–1 and sediment release of 0.5 µg P·1^–1·d^–1. Both temperature and composition of zooplankton affected the weight specific excretion rates of the zooplankton community. The weight specific community excretion rates of P and N increased with temperature (exponential model); 1–8 g PO₄P·mg^–1 zooplankton-C·d^–1 and 5–42 µg NH₃N·mg^–1 zooplankton-C·d^–1 (10°C–20°C).     

Build-up and decline of summer phytoplankton biomass in the eastern Weddell Sea,Antarctica

The seasonal development and decline of phytoplankton was investigated in the eastern Weddell Sea during summer and fall 1991. During the first half of the study (15 Jan–13 Feb) in an area off Vestkapp, favourable irradiance/mixing regimes initiated net phytoplankton growth in ice-free waters on the shelf and in stretches of open water over the partially ice-covered deep ocean. Chi a concentrations in the upper water column were moderate (0.2–0.8 g l^–1), but significantly above winter values. Later in the season (16 Feb–11 March), a phytoplankton bloom with surface Chl a concentrations ranging from 1.6–2.3 g l^–1 was encountered in an area further to the east. We suggest that the upper water column must have been stratified in this region for time scales of weeks to faciliate bloom development. Bacterial biomass and productivity generally paralleled the seasonal development of the phytoplankton. Nitrate concentrations in the upper mixed layer were substantially lower than would be expected from the existing phytoplankton standing stock, suggesting that heterotrophic consumption of organic matter by bacteria and zooplankton removed a large fraction of the primary production. The shallow seasonal pycnocline was eventually eroded by the passage of a storm, resulting in a homogeneous distribution of phytoplankton biomass over the entire water column, followed by sedimentation and deposition of phytodetritus on the sea floor. After the storm induced destratification, bacterial productivity was particularly high, amounting to more than half of the primary production (range: 10%–120%) in the upper water column. Subsequently, phytoplankton biomass in the upper water column decreased to values <1 g Chl a l^–1. The combination of low incident irradiances and incessant deep mixing prevented the phytoplankton biomass to increase again. During the last week of the investigation, extensive new-ice formation was observed. A major fraction of the residual surface plankton was incorporated into new sea ice, thus terminating the pelagic growth season of the phytoplankton in the eastern Weddell Sea.     

Calcium binding by spinach stromal proteins

Calcium binding to spinach (Spinacia oleracea L.) stromal proteins was examined by dual-wavelength spectrophotometry using the metallochromic indicator tetramethylmurexide. The data are consistent with the existence of at least two, probably independent, classes of binding sites. The total number of binding sites varied between 90–155 nmol·mg^–1 protein with average binding constants of 1.1–2.7·mM^–1. Both Mg²⁺ and La³⁺ inhibited calcium binding competitively, with average inhibitor constants of 0.26·mM^–1 and 39.4·mM^–1, respectively; an increase in the potassium concentration up to 50 mM had no effect. In a typical experiment a decrease in pH (7.8 to 7.1) resulted in a decrease in the total number of calcium binding sites from 90 to 59 nmol·mg^–1 protein, but in an increase of the average affinity from 2.7 to 4.5·mM^–1. Calculations, using these data and those of Gross and Hess (1974, Biochim. Biophys. Acta 339, 334–346) for binding site I of washed thylakoid membranes, showed that the free-Ca²⁺ concentration in the stroma under dark conditions, pH 7.1, is higher than under light conditions, pH 7.8. The physiological relevance of the observed calcium binding by stromal proteins is discussed.Abbreviations Ca _b ²⁺ bound calcium - Ca _f ²⁺ free calcium     

Phytoplankton around Elephant Island,Antarctica

Phytoplankton studies were conducted in a 4·10⁴ km² grid around Elephant Island, Antarctica, during January–March 1991. In addition to profiling studies at 50 stations during each of two cruises, pumped surface water was used to continuously record in vivo chlorophyll-a (chl-a) fluorescence and beam attenuation coefficients (c_t). Measurements on discrete samples included chl-a, phytoplankton organic carbon (A_c), and total particulate organic carbon (POC). Equations were developed which permitted contour-mapping of chl-a and POC throughout the entire grid based on continuous measurements of in vivo chl-a fluorescence and c_t values. Phytoplankton abundance increased from January–February (cruise I) to February–March (cruise II) as evidenced by mean surface chl-a values (0.8 and 1.34 g l^–1, respectively) and mean A_c values for the upper 20 m of the water column (21.3 and 39.8 g Cl^–1, respectively). Microplankton accounted for 29% (cruise I) and 40% (cruise II) of the total phytoplankton, based on direct measurements of chl-a. Patterns for integrated values of chl-a, A_c and POC were similar to patterns of concentrations in surface waters. The richest areas of phytoplankton were north of Elephant and King George Islands, which coincided with high krill density areas.     

Measurement of apoplasmic and cell-to-cell components of root hydraulic conductance by a pressure-clamp technique

A pressure-clamp technique was devised for the direct measurement of cell-to-cell and apoplasmic components of root hydraulic conductance; the experimental results were analyzed in terms of a theoretical model of water and solute flow, based on a composite membrane model of the root. When water is forced under a constant pressure into a cut root system, an exponential decay of flow is observed, until a constant value is attained; when pressure is released, a reverse water flow out of the root system is observed which shows a similar exponential behavour. The model assumes that the transient flow occurs through a cell-to-cell pathway and the observed decrease is the result of accumulation of solutes in front of the root semi-permeable membrane, whilst the steady-state component results from the movement of water through the parallel apoplasmic pathway. Root conductance components are estimated by fitting the model to experimental data. The technique was applied to the root systems of potted cherry (Prunus avium L.) seedlings; average apoplasmic conductance was 15.5 × 10^–9m³· s^–1· MPa^–1, with values ranging from 12.0 × 10^–9 to 18.5 × 10^–9m³· s^–1· MPa^–1; average cell-to-cell conductance was 11.7 × 10⁹ m³· s^–1· MPa^–1, with values ranging from 8.5 × 10^–9 to 15.3 × 10^–9 m³ · s^–1·MPa^–1. Cell-to-cell conductance amounted on average to 43% of total root conductance, with values between 41 and 45%. Leaf specific conductance (conductance per unit of leaf area supported) of the root systems ranged from 2.7 × 10^–8 to 5.6 × 10^–8 m· s^–1·MPa^–1, with an average of 3.7 × 10^–8 m · s^–1·MPa^–1. The newly developed technique allows the interaction of mass flow of water and of solutes to be explored in the roots of soil-grown plants.Abbreviations and Symbols A L_p root hydraulic conductance - A^aL _p ^a root apoplasmic conductance - A^ccL _p ^cc root cell-to-cell conductance - C_s(t) concentration of solutes in apical root compartment at time t - J_v flow of water through the root - J _v ^a apoplasmic flow of water - J_v/cc cell-to-cell flow of water - LSC leaf specific conductance of the root system - P root hydrostatic pressure - P_appl applied pressure - _s(t) root osmotic pressure at time t - _m osmotic pressure of rooting medium - reflection coefficient of root membrane - time constant of cell-to-cell flow decay This research was funded within the EC Project Long-term effects of CO₂-increase and climate change on European forests (LTEEF) (EV5V-CT94-0468); F.M. was supported by a Ministero dell' Universitá e della Ricerca Scientifica e Tecnologica — British Council agreement (Project The ecological significance of cavitation in woody plants); M.C. was supported by a Consiglio Nazionale delle Ricerche — British Council agreement. We gratefully thank Prof. P.G. Jarvis (University of Edinburgh, UK) for revising an earlier version of this paper and Prof. E. Steudle (University of Bayreuth, Germany) for helpful comments.     

Versatile reaction of the potential host system [(Me3Sn) 4Ru(CN) 6]∞ with 4-thiopyridone/4-mercaptopyridine in water

Single crystals of three derivatives of the structurally still incompletely characterized coordination polymer [(Me₃Sn)₄Ru(CN)₆] 1b have been prepared and subjected to crystallographic studies: [1b · 4H₂O]=2b forms stacks of puckered _∞²[Ru{μ-CNSn(Me₃)NC}₂] sheets interlinked by hydrogen bonds in making use of two additional CNSn(Me₃)OH₂ ligands and quasi-zeolitic water. Mild drying of 2b leads to the “missing link” between 1b and 2b, [1b · 2H₂O], 3b. The structure of [1b · 2tp] (tp=4-thiopyridone) consists of a three-dimensional, negatively charged host framework comprising (via Sn-S bonds) one “aromatic” thione linkage and a [Me₃Sn · tp]⁺guest ion involving a more zwitterionic form of tp. Slow uptake of Me₃SnCl from the gas phase by an aqueous solution of K₄[Ru(CN)₆] and tp afforded the novel assembly [1b · 2H₂O · 0.8pms · 0.2pds] (pms/pds=4,4^′-dipyridylmono-/disulfide), the supramolecular architecture of which resembles that of 2b. Bridging pms or pds molecules occupy equivalent interlayer sites, and the pms/pds ratio is likely to vary. At least three further assemblies containing again 1b and either tp or pds/pms have likewise been isolated, however, not as single crystals.     

Variation among chicken stocks in the fractional rates of muscle protein synthesis and degradation

Fractional rates (% · day^–1) of synthesis and degradation were determined by measuring the output of N-methylhistidine (MeHis) in the excreta at 4 and 8 weeks of age in the chicken. At 4 weeks of age, the fractional rate of synthesis of the meat-type stock was twice that of the egg-type stock (White Leghorn), but the fractional rates of synthesis at 8 weeks of age were similar (4.1–5.1% · day^–1) among stocks. The fractional rate of degradation (1.3–1.5% · day^–1) of the meat-type stock at 8 weeks of age was less than half the rate of the egg-type stock (2.9% · day^–1). The fractional rates of synthesis and degradation at 4 weeks of age in the Satsuma native fowl were relatively high compared with those in the other stocks. In particular, the rate of degradation (8.6% · day^–1) at 4 weeks of age was approximately twice that of other stocks. These results show that fractional rates of synthesis and degradation of muscle protein in the chicken differ among genetically diverse groups. The effect of changes in rates of synthesis and degradation on the change in fractional growth rate also differed. From regression coefficients (b_{K s · FGR} and b_{K d · FGR}) of these rates in skeletal muscle protein on the fractional growth rate, it was recognized that the change in growth rate accompanies the changes in both synthesis and degradation in White Leghorn and commercial broilers but only the change in synthesis in White Plymouth Rock (dw) and Satsuma native fowl.     

Excretion of photosynthetically fixed organic carbon by metalimnetic phytoplankton

The effects of light intensity, oxygen concentration, and pH on the rates of photosynthesis and net excretion by metalimnetic phytoplankton populations of Little Crooked Lake, Indiana, were studied. Photosynthetic rates increased from 1.42 to 3.14 mg C·mg^–1 chlorophylla·hour^–1 within a range of light intensities from 65 to 150E·m^–2·sec^–1, whereas net excretion remained constant at 0.05 mg C·mg^–1 chlorophylla·hour^–1. Bacteria assimilated approximately 50% of the carbon released by the phytoplankton under these conditions. Excreted carbon (organic compounds either assimilated by bacteria or dissolved in the lake water) was produced by phytoplankton at rates of 0.02–0.15 mg C·mg^–1 chlorophylla·hour^–1. These rates were 6%–13% of the photosynthetic rates of the phytoplankton. Both total excretion of carbon and bacterial assimilation of excreted carbon increased at high light intensities whereas net excretion remained fairly constant. Elevated oxygen concentrations in samples incubated at 150E· m^–2·sec^–1 decreased rates of both photosynthesis and net excretion. The photosynthetic rate increased from 3.0 to 5.0 mg C·mg^–1 chlorophylla· hour^–1 as the pH was raised from 7.5 to 8.8. Net excretion within this range decreased slightly. Calculation of total primary production using a numerical model showed that whereas 225.8 g C·m^–2 was photosynthetically fixed between 12 May and 24 August 1982, a maximum of about 9.3 g C·m^–2 was released extracellularly.     

Gross and net primary production in the Scotia-Weddell Sea sector of the Southern Ocean during spring 1988

Summary Daily rates of gross and net primary production were calculated in the Scotia-Weddell Sea sector of the Southern Ocean during spring 1988 (EPOS, Leg 2) on the basis of kinetic experiments, which combine radiotracer technology and classic biochemical procedures, and by taking into account the light regime, the physical structure of the water column, the vertical distribution of chlorophyll a, and the protozoan grazing pressure. From these calculations, three distinct sub-areas were identified: the Closed Pack Ice Zone (CPIZ), characterized by the lowest average gross primary production (0.36 gC · m^–2 · day^–1); the Marginal Ice Zone (MIZ) with a maximum mean value of 1.76 gC · m^–2 · day^–1; and the Open Ocean Zone off the ice edge (OOZ) with an intermediate mean value of 0.87 gC · m^–2 · day^–1. Net primary production fluctuated nearly in the same proportions, averaging 0.55, 0.2 and 1.13 gC · m^–2 · day^–1 in the OOZ, CPIZ and MIZ respectively, representing 53% of the total photo-assimilated carbon under heavy ice cover (CPIZ) and 64% in the two other areas. Available light, strongly dependent on the ice cover, was shown to control the level of primary production in the sea ice associated sub-areas, whilst protozoa grazing on phytoplankton determined the moderate primary production level characteristic of the well illuminated OOZ area.Data presented here were collected during the European Polarstern Study (EPOS) sponsored by the European Science Foundation     

Synthesis, crystal structures and luminescent properties of manganese and zinc complexes with in situ ligand formation of 3-amino-4-(5-tetrazolyl)-1H-pyrazole

Three metal-organic frameworks, [Mn(HL)(N₃)] (1), [Mn(HL)Cl(H₂O)] (2) and [Zn₂(L)₂(H₂O)] (3) where H₂L = 3-amino-4-(5-tetrazolyl)-1H-pyrazole, have been yielded through in situ hydrothermal reactions of manganese(II) or zinc(II) salts, NaN₃ and 3-amino-4-cyano-1H-pyrazole (HACP). The crystal structure analysis reveals that 1-3 have different dense 3D frameworks with Schläfli symbols of {3·4²·5²·6}{3²·4³·5⁴·6⁶} for 1 which is an unprecedented (4,6)-connected framework, {4·12²}2{4²·12⁴} for 2 which is a typical sqc519 structure, and {4²·6}2{4⁴·6²·8⁸·10} for 3 which is a typical ant/anatase structure, respectively. The ligand takes three different coordination modes in 1-3 as 3- or 4-connector. In addition, the photoluminescence of complex 3 was studied in solid state at room temperature, together with its thermal analysis.     

Luminescence properties and solution dynamics of lanthanide complexes composed by a macrocycle hosting site and naphthalene or quinoline appended chromophore

The two-component ligand systems 1 and 2 which contain 1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid (DO3A) as the hosting unit for the lanthanide cations, and naphthalene (which is devoid of any chelating ability) or quinoline units, respectively, as chromophores, were synthesized. The 1:1 complexes with Gd³⁺, Eu³⁺ and Tb³⁺ have been studied in aqueous solution. Relaxometric properties for Gd · 1 indicate that two water molecules (q = 2) are in the first coordination sphere of the metal ion whereas for Gd · 2, q < 2 is found. For Gd · 1, these results indicate that the naphthalene unit is not coordinated to the metal centre; for the case of quinoline, reasons for the lower hydration state are discussed. In case of Eu · 1, Tb · 1, Eu · 2, and Tb · 2 the absorption and luminescence spectra, the overall luminescence efficiencies, and the metal-centred lifetimes, were obtained both in water and deuterated water. The coordination features of these complexes were explored by comparing their luminescence properties, resulting in hydration state q = 2 and 1.4 for the cases of the complexes of 1 and 2, respectively. Use of the photophysical parameters in air-equilibrated water allowed the determination of the ligand-to-cation energy transfer efficiency, Φ_EnT, leading to the overall emission sensitization process. For Eu · 1, and Eu · 2, we found Φ_EnT = 0.034 and 0.078, respectively, supporting that also a non-coordinating chromophore like naphthalene, case of ligand 1, can transfer excitation energy to the metal centre.     

Use of formate gradients for improving biomass yield of Pichia pinus growing continuously on methanol

Summary Investigations were made into the improvement of growth yield (Y) of Pichia pinus MH 4 growing continuously on methanol by feeding formate so as to create an increasing concentration gradient (transient state). Under particular formate supply conditions, Y could be increased from 0.37 g·g^-1 on methanol alone to 0.55 and 0.47 g·g^-1 in the presence of formate at dilution rates (D) of 0.045 and 0.075 h^-1, respectively. These differences could be explained as being due to a limiting formate consumption rate of 50–60 nmol·min^-1·g^-1 dry wt., coupled to a net-energy generation independent of D. Any further formate oxidation proceeded without energy gain. Deviations from optimum conditions of biomass increase are discussed in terms of different formate oxidizing systems and uncoupling properties of formate itself. These results are compared to and confirmed by steady-state considerations.Abbreviations a steepness of the formate gradient (g·l^-1·h^-1) - a acceleration of change of formate concentration in the fermenter (g·l^-1·h^-2) - D dilution rate (h^-1) - Ft formate - S₁ and S₂ initial and final formate concentration of the gradient (g·l^-1) - Y growth yield in g·g^-1 methanol     

The ecology of Lake Nakuru (Kenya)

Summary Abiotic factors, standing crop and photosynthetic production were studied in the equatorial alkaline-saline closed-basin Lake Nakuru (cond. 10,000–160,000 S). Meteorological conditions and abiotic factors offer suppositions for a high primary productivity: mean solar radiation is 450–550 kerg·cm^-2·s^-1, with little seasonal variation, regular winds circulate the lake every day and nutrient concentrations are usually high (>100 g P–PO₄·l^-1). Oxygen concentrations near sediments were <1 gO₂·m^-3 for at least 6 h·d^-1 in 1972/73, resulting in a release of 45 mg P–PO₄·m^-2·d^-1. Attenuation coefficients vary from 3.6–16.5 according to algal densities and mean depth from 0–400 cm. Algal biomass was 200 g·m^-3 (d.w.) in 1972/73, due to a lasting Spirulina platensis bloom (98.5% of algal biomass). In 1974 algal biomass suddenly dropped to 50 g·m^-3 (d.w.). Spirulina and several consumer organisms almost vanished, but coccoid cyanobacteria, Anabaenopsis and diatoms increased. Several causes for this change in ecosystem structure are discussed. The use of the light/dark bottle method to measure photosynthetic production in eutrophic alkaline lakes is discussed and relevant experiments were done. Oxygen tensions of 2–35 gO₂·m^-3 do not influence primary production rates. Net photosynthetic rates (mgO₂·m^-3·h^-1; photosynthetic quotient=1.18) reached 12–17.7 in 1972/73 and 2–3 in 1974, but vertically integrated rates were only 1–1.4 in 1972/73 and 0.8 in 1974, and daily net photosynthetic rates (gO₂·m^-3·24 h^-1) 3.5 in 1972/73 and 1 in 1974. 50% of areal rates were produced within the 10 most productive cm of the depth profile. The disproportion between high algal standing crops and relatively low production rates is due to self-shading of the algae, reducing the euphotic zone to 35 cm in 1972/73 and 77 cm in 1974. Efficiency of light utilization is 0.4–2%, varying with time of day and phytoplankton density. In situ efficiencies show an inverse relationship to light intensities. Photosynthetic rates of L. Nakuru remain within the range of other African lakes (0.1–3 gO₂·m^-2·h^-1). The relation of O₂ produced/Chl a of the euphotic zone is 50% lower then in tropical African freshwater lakes and conforms to lakes of temperate regions.     

Radial transport of water across cortical sleeves of excised roots ofZea mays L.

The stationary radial volume flows across maize (Zea mays L.) root segments without steles (sleeves) were measured under isobaric conditions. The driving force of the volume flow is an osmotic difference between the internal and external compartment of the root preparations. It is generated by differences in the concentrations of sucrose, raffinose or polyethylene glycol. The flows are linear functions of the corresponding osmotic differences ( ) up to osmotic values which cause plasmolysis. The straight lines obtained pass through the origin. No asymmetry of the osmotic barrier could be detected within the range of driving forces applied ( =±0.5 MPa), corresponding to volume-flow densities of j_{v, s}=±7·10^–8 m·s^–1. Using the literature values for the reflection coefficients of sucrose and polyethylene glycol in intact roots (E. Steudle et al. (1987) Plant Physiol.84, 1220–1234), values for the sleeve hydraulic conductivity of about 1·10^–7 m·s^–1 MPa^–1 were calculated. They are of the same order of magnitude as those reported in the literature for the hydraulic conductivity of intact root segments when hydrostatic pressure is applied.Abbreviations and symbols a _s outer surface of sleeve segment - c concentration of osmotically active solute - j _{v, s} radial volume flow density across sleeve segment - Lp_s hydraulic conductivity of sleeves - Lp_r hydraulic conductivity of intact roots - _N thickness of Nernst diffusion layer - reflection coefficient of root for solute - osmotic value of bulk phase - osmotic coefficient     

New polyoxometalate-templated supramolecular networks based on transition metal ions and pyridine-N-oxide ligands

Two new complexes, [Cu(bppdo)(Hbppdo)(CH₃CN)₃][PW₁₂O₄₀]·CH₃CN·0.5H₂O (1) and [Co(2,2′-bpdo)₃]₂[PW^VW^VI₁₁O₄₀]·CH₃CN·2H₂O (2) (bppdo = 1,3-bis(4-pyridyl)propane-N,N′-dioxide, 2,2′-bpdo = 2,2′-bipyridine-N,N′-dioxide), have been synthesized by layering MHPW₁₂O₄₀ salts (M = Cu²⁺ for 1 and Co²⁺ for 2) on the organic bppdo (for 1) or 2,2′-bpdo (for 2) ligands with the mixed media of acetonitrile and water at room temperature. Both complexes were fully characterized by elemental analyses, TG analyses, IR, UV-Vis, cyclic voltammetry and single-crystal X-ray diffraction. Both crystal structures exhibit the new polyoxometalate (POM)-templated supramolecular network arrays. The “host” supramolecular networks with large pores are composed of the [Cu(bppdo)(Hbppdo)(CH₃CN)₃] ring-connected-ring chains for 1 and the [Co(2,2′-bpdo)₃] windstick-type units for 2. The “guest” Keggin-type polyoxoanions are located in the pores and dispersed between two organic layers in both complexes. Electrochemical properties of 1 and 2 were investigated and both compounds show the electrocatalytic activity towards the reduction of nitrite.     

Anion binding studies of tris(2-aminoethyl)amine based amide receptors with nitro functionalized aryl substitutions: A positional isomeric effect

Syntheses and crystal structures of tren-based amide, L¹, N,N′,N″-tris[(2-amino-ethyl)-4-nitro-benzamide] and L², N,N′,N″-tris[(2-amino-ethyl)-2-nitro-benzamide] are reported and compared with previously published tripodal amide receptor L³, N,N′,N″-tris[(2-amino-ethyl)-3-nitro-benzamide]. The crystallographic results show intramolecular and intermolecular hydrogen-bonding interactions between two arms of the tripodal receptor and two other adjacent molecules in cases of L¹ and L² whereas in addition to the above interactions an aromatic π···π stacking among tripodal arms is also observed in L³. Receptors L¹, L² and L³ having electron withdrawing -NO₂ substituted (para, ortho and meta, respectively) phenyl moieties are explored toward their solution state anion binding properties and selectivity studies. The substantial changes in chemical shifts are observed for the amide protons (-NH) and aromatic protons (-CH) with F⁻ and Cl⁻ in cases of L¹ and L³, and only with F⁻ for L², indicating the participation of -NH and -CH protons in the solution state binding events. Binding constants for the above cases are calculated by ¹H NMR titration upon monitoring the -NH signal. Receptor L² shows exclusive selectivity toward F⁻ in dimethyl sulfoxide (DMSO). The structural aspects of binding I⁻, ClO₄⁻ and SiF₆²⁻ with the monoprotonated L¹, L¹H⁺·I⁻·DMF (1), L¹H⁺·ClO₄⁻·DMF (2) and L¹H⁺·0.5SiF₆²⁻·H₂O (3), respectively are examined crystallographically. Anion binding with multiple receptor units is observed via amide N-H···anion as well as aryl C-H···anion hydrogen-bonding interactions in all the complexes as observed in cases of previously reported crystal structures of anionic complexes of protonated L³. The aryl group having nitro functionality that contributes to solution state anion binding with the neutral receptor and solid state coordination in complexes 1-3 through CH···anion interactions is noteworthy.