Détermination par bioessai de la biodisponibilité des ressources azote et phosphore, dans les eaux du Golfe du Morbihan

Des dosages physico-chimiques et des numérations du phytoplancton sont réalisés sur des prélèvements d'eau du Golfe de Morbihan (France), conjointement à des bioessais en milieu non renouvelé :1. Avec le peuplement autochtone, par enrichissement en azote et en phosphore de l'eau filtrée sur 200 µm.2. Avec deux algues tests épuisées en azote et en phosphore, sur l'eau filtrée stérilement. Les quotas cellulaires des algues tests en fin de croissance sont comparés aux quotas cellulaires minimaux et maximaux préalablement établis.Les bioessais avec enrichissement révèlent une limitation de la croissance des algues par l'azote. Les bioessais avec Fragilaria elliptica présentent une relation linéaire croissante simple entre la concentration en azote assimilée par l'algue test et la densité cellulaire du peuplement phytoplanctonique naturel. L'azote total en solution représente la fraction utilisée par ces algues : azote biodisponible. Par contre, la fraction « phosphore soluble » ne représente qu'une partie du phosphore biodisponible, et se trouve consommée de manière très diverses par les algues testées. Cependant, l'analyse des quotas cellulaires en fin de croissance des algues tests montre que le phosphore est l'élément impliqué dans la limitation de leur croissance.Les bioessais exposent des résultats contradictoires de limitation de la croissance. Les algues autochtones utilisent des ressources en phosphore supérieures à celles qui sont mises à disposition des algues tests, par stockage interne ou par consommation du phosphore minéral adsorbé sur les particules en suspension.In estuarine water, the knowledge of the resources bioavailability is indispensable for understanding phytoplankton dynamics. For assessing nitrogen and phosphorus bioavailability, algal bioassays are the most indicated. Nutrient limitations of an alga exhibit a threshold rather than an additive or a multiplicative response (ELRIFI et al., 1985). In contrast, numerous nutrient limitations are described for an algal population (TILMAN,1982).Samples are taken monthly at 1 meter’s depth from two French river estuaries, Noyalo River and Vannes River, in the Morbihan Gulf from April to July. Main physico-chemical measures (temperature, salinity, nitrate, nitrite, ammonia, total nitrogen, reactive phosphorus and total phosphorus) were taken. Total soluble nitrogen and total soluble phosphorus concentrations were determined after filtration en 0,2 µm, for comparison with bioassays. In addition two kinds of batch bioassays with 250 ml erlenmeyers into controlled environment (17 °C and 180~200 µEinstein), were employed :- Estuarine waters filtered on 200 µm, to remove most of the zooplankton, were enriched with sodium nitrate and/or dipotassium hydrogen phosphate. Algal Growth response, was recorded by fluorescence « in vivo », against a blank.- With the total growth on sterilely filtered (0,2 µm) estuarine waters of two test algae, starved simultaneously in phosphorus and nitrogen in artificial sea water, it was possible to determine the bioavailability of these two elements in the soluble traction. The cellular density of the two test algae : Fragilaria elliptica Schumann and Phaeodactylum tricornutum Bohlin, was related to optical density measures at 750 nm. Nitrogen and phosphorus intracellular concentrations, in the whole culture at the end of the growth, gave their bio-availability for the test algae provided. Nitrogen and phosphorus cell quotas, intracellular concentration per cell, were then compared with their higher and lower cell quotas. Higher and lower cell quotas were measured at the end of the test algae growth in an artificial medium without phosphorus or nitrogen resources. They were similar to those described by DAUTA (1982).In 1989, an exceptional drought reduced the river flow. Consequently, nutrient flux decreased and phytoplankton density, mainly diatoms, was reduced to between 100 and 800 cells/ml. Sea water dominated in the Morbihan Gulf with salinities higher Chan 30 %o.First, waters filtered on 200 µm and added to which was nitrate and/or phosphate revealed a nitrogen limitation of natural phytoplankton.Secondly, phosphorus limitation were found in bioassays on sterilely filtered waters, in all the test algae starved for phosphorus and nitrogen. At the end of the growth in bioassays, phosphorus cell quotas of the test algae were generaly similar to the lower ones. The growth of the tests algae was limited by that resource.These contrary results are related to the intracellular storage by the natural phytoplankton and/or the major role of particulate fractions in phosphorus bioavailability.Phosphorus and nitrogen concentrations assimilated by the test alga : Fragilaria elliptica, were inversely related. From May to July 1989 the results showed a simple linear relation between soluble nitrogen source (filtered on 0.2 µm) which was bioavailable for the test alga and natural phytoplankton density (R2 = 0.98). The two measurements of April did not agree with the latter relation, rather because of environmental factors such as temperature (11 °C against 20~21 °C for the next ones), than because of the resources. Moreover, the station B in July exhibited a natural phytoplankton density significantly lower than the station A and independent of the nitrogen bioavailability for the two test algae. The bioavailable phosphorus for Phaeodactylum tricornutum, was also significantly lower at station B. That resource can explain the difference in density monitored in July.The total soluble nitrogen source, inorganic and organic, and just a fraction of the total soluble phosphorus source, were assimilated. The soluble phosphorus assimilated by test algae, after sterile filtration, is not the total bioavailable phosphorus in the water of the Morbihan Gulf and seems to be inversely related to the natural phytoplancton density. A part from mineral phosphorus adsorbed on suspended matter is also bioavailable (BERLAND et al., 1973). However, for the phosphorus resource, bioassays have to be performed on particulate fractions

Shape control of QDs studied by cross-sectional scanning tunneling microscopy

In this cross-sectional scanning tunneling microscopy study we investigated various techniques to control the shape of self-assembled quantum dots (QDs) and wetting layers (WLs). The result shows that application of an indium flush during the growth of strained InGaAs/GaAs QD layers results in flattened QDs and a reduced WL. The height of the QDs and WLs could be controlled by varying the thickness of the first capping layer. Concerning the technique of antimony capping we show that the surfactant properties of Sb result in the preservation of the shape of strained InAs/InP QDs during overgrowth. This could be achieved by both a growth interrupt under Sb flux and capping with a thin GaAsSb layer prior to overgrowth of the uncapped QDs. The technique of droplet epitaxy was investigated by a structural analysis of strain free GaAs/AlGaAs QDs. We show that the QDs have a Gaussian shape, that the WL is less than 1 bilayer thick, and that minor intermixing of Al with the QDs takes place.Comment: 7 pages, 10 figure

Shape control of QDs studied by cross-sectional scanning tunneling microscopy

In this cross-sectional scanning tunneling microscopy study we investigated various techniques to control the shape of self-assembled quantum dots (QDs) and wetting layers (WLs). The result shows that application of an indium flush during the growth of strained InGaAs/GaAs QD layers results in flattened QDs and a reduced WL. The height of the QDs and WLs could be controlled by varying the thickness of the first capping layer. Concerning the technique of antimony capping we show that the surfactant properties of Sb result in the preservation of the shape of strained InAs/InP QDs during overgrowth. This could be achieved by both a growth interrupt under Sb flux and capping with a thin GaAsSb layer prior to overgrowth of the uncapped QDs. The technique of droplet epitaxy was investigated by a structural analysis of strain free GaAs/AlGaAs QDs. We show that the QDs have a Gaussian shape, that the WL is less than 1 bilayer thick, and that minor intermixing of Al with the QDs takes place.Comment: 7 pages, 10 figure

Première évaluation du risque toxique lié aux cyanobactéries d'eau douce en France : le programme " EFFLOCYA "

Une enquête menée à l'aide d'un questionnaire couvrant l'ensemble du territoire français ainsi que l'étude spécifique d'écosystèmes aquatiques de type lacs et réservoirs ont permis de montrer que les proliférations de cyanobactéries toxiques pouvaient a priori affecter n'importe quel plan d'eau de l'Héxagone. Les genres rencontrés responsables de la production de toxines, le plus souvent hépatiques, sont Microcystis, Planktothrix, Anabœna et Cylindrospermopsis.Les efflorescences peuvent se produire tout au long de l'année car certaines espèces sont adaptées aux eaux froides et elles ne sont pas liées de manière univoque à un état eutrophe de l'écosystème.La production toxinique est la plus élevée lorsque les populations cyanobactériennes sont peu ou pas diversifiées et à la lumière de travaux récents, cette production serait favorisée, en ce qui concerne les microcystines, par un milieu riche en nitrate, pauvre en ammonium et fortement carencé en fer.La suite à donner à ce travail pourrait être la création d'un observatoire national des efflorescences toxiques.An inquiry covering the whole French territory together with specific studies of natural and artificial reservoirs has been made to assess the human health risk related to the prolifération of toxin producing blue green algae (cyanobacteria). The conclusions show that any lentic ecosystem can be affected like it has been shown already in other countries all over the world and more specifically within the EU. The main genus concemed are: Microcystis, Planktothrix, Anabœna and Cylindrospermopsis.Blooms can occur juring the whole year since some species are well adapted to cold water and are not clearly linked with the trophic status of the aquatic ecosystem.The toxinic production is the largest when the cyanobacteria populations contain only a few number of species. The results of recent works are well correlated with our findings which show that the production of toxic heptapeptides (microcystins) could be correlated with high levels of nitrate and depletions of ammonium and iron.The follow up of this work should now include the set up of a national observatory of toxic algal blooms

Direct estimate of active bacteria: CTC use and limitations

During the last 10 years, the dye 5-cyano-2,3-ditolyl tetrazolium chloride (CTC) has been used to determine the in situ number of "active" bacteria in different ecosystems. A part of this success is due to a simple protocol, which does not require sophisticated equipment. However, it has not been established whether the method determines viable cells, e.g. those capable of growth and cell division, as opposed to cells that are active in the sense of having some detectable metabolic activity. In this study, the number of CTC-positive cells through the growth stages of Escherichia coli was estimated and compared to counts of the total number of bacteria, the culturability (CFU counts) and respiratory activity (CO2 evolution). There was a good correlation between the number of CTC-positive cells and the CFU count, regardless of the growth phase. However, CTC could still be reduced by a large part of the population during the first hours of stationary phase even if the bacteria were no longer releasing CO2. Thus, the reduction of CTC is a good estimator for cell viability, rather than cell activity. Additionally, a review of the literature showed that there is presently no standardized protocol for using CTC, which makes difficult at present the comparison of active bacterial numbers in different samples from different sites. [KEYWORDS: Active bacteria; 5-Cyano-2,3-ditolyl tetrazolium chloride (CTC); Method]

The importance of avian-contributed nitrogen (N) and phosphorus (P) to Lake Grand-lieu, France

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Dépôts et remises en suspension dans le lac de Grandlieu (France)

This paper looks at the nitrogen and phosphorus supplies, at the sedimentation and wind effect on currents in the Grandlieu (44) lake, as well as at the technical means used to save this lake. / Les apports en azote et phosphore, la sédimentation et les effets des vents sur la courantologie du lac de Grandlieu (44), France, sont examinés, ainsi que les moyens techniques de sauvegarde du lac

Combination of biomolecular and stable isotope techniques to determine the origin of organic matter used by bacterial communities: application to sediment

Natural isotopic composition is a good tool to trace organic matter in ecosystems. Recent studies used a combination of molecular and stable isotope techniques to determine the origin of the organic carbon used by bacteria in the water column. In our study, we show that this procedure can be used for analysis of sediment bacterial communities with few modifications. In the water column, bacterial recovery is done before DNA extraction. In the sediment, we tested qualitatively and quantitatively a direct and indirect extraction of DNA. The direct extraction was the most efficient. It recovered between 3.1 and 15.8 mu g DNA g(-1). dry sediment and the contamination of field samples by eucaryotic DNA was less than 13%. In this preliminary study of the salt marsh ecosystem, the delta(13)C values of DNA (-26 to -24 parts per thousand) recovered from the sediment were close to the delta(13)C values of halophytic plants (-26.4 and -25.3 parts per thousand) showing a relationship between plants and microorganisms, Thus, this procedure can be used to trace the flow of carbon through the sediment microbial biomass and to understand the variation of bacterial activity according to the inputs of allocthonous and autochtonous organic matter. [KEYWORDS: DNA; delta C-13 values; bacteria; sediment Direct extraction; carbon; dna; phytoplankton; purification; probes; bloom; soil; rna]

Observation of room-temperature laser emission from type III InAs/GaSb multiple quantum well structures

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