Joint field experiments for comparisons of measuring methods of photosynthetic production

During the 1st GAP Workshop at Konstanz in April 1982 comparative measurements of phytoplankton primary production by several techniques were conducted simultaneously at an offshore station in Lake Konstanz and an experimental algal pond. Suspended glass bottle exposure techniques using 14C and 13C uptake gave Pz (mg C m−3 h−1) values which varied considerably near-surface, but estimates of areal rates for the euphotic zone ΣPcu(mg C m−3 h−1) which were reasonably close. In the lake, ΣPz, from a vertical tube exposure (with 14C uptake) was greater than rates derived for integrated bottle samples. The oxygen bottle method permitted a good estimate of compensation depth, corresponding to in situ growth studies. There were difficulties in direct comparison between O2 and carbon methods. Correlation between them for Pz was good in the lake but poor in the pond, both for suspended bottle and vertical tube methods. This series demonstrates that despite reasonable overall estimates, comparatively minor methodological differences in experimental technique can cause large variatio

Interactions between ammonium and urea uptake by five strains of Alexandrium catenella (Dinophyceae) in culture

10 pages, 3 figures, 2 tablesShort-term experiments were carried out to investigate whether interactions between ammonium (NH4+) and urea uptake regulate the total nitrogen assimilation of the toxic dinoflagellate Alexandrium catenella. To test for strain variability, 5 strains of A. catenellafrom the NW Mediterranean were used: 3 strains from the Thau lagoon (southern France) and 2 strains from the Catalonia basin (Spain). For each strain, the uptake rate of 1 nutrient (NH4+or urea) at a reference concentration (10 μgatN l–1) was measured as a function of the increasing concentration of the other nutrient (0to 10 μgatN l–1). Simultaneous N uptake rates of the distinct nitrogen sources were obtained from 15 N-NH4+ and 15N-urea incorporation measurements. A strong inhibition of urea uptake by NH4+ (maximum inhibition, Imax> 55%) was observed exclusively for the French strains. No influence of urea on the NH4+-uptake rate was noted for any strain. Estimation of total N uptake rates revealed that the N-urea uptake inhibition was not a competitive disadvantage for A. catenellacells considering that the reduced N-urea uptake was more than compensated for by NH4+uptake. Furthermore, the computation of composite kinetic parameters from total N uptake data suggested that French strains were more competitive than the Spanish ones in an environment characterized by low NH4+ concentrations (≤5 μgatN l–1) and high urea concentrations (as 10 μgatN l–1). These N uptake characteristics may reflect particular metabolic adaptations by the strains to their respective environment.We thank R. Ventosa for the nutrient analyses. This study was financed by the Centre National de la Recherche Scientifique (CNRS) and Institut Français pour l’Exploitation de la Mer (Ifremer) ALTOX program, as well as the EU-funded Research Project SEED (Life cycle transformations among HAB species, and the environmental and physiological factors that regulate them, GOCE-CT 2005-003875). S.L. was supported by a Fundação para a Ciência e para a Tecnologia, Portugal (FCT) grant within the III Quadro Comunitário de Apoio by the Fundo Social Europeu. E.G. was supported by the Ramon y Cajal contract of the Spanish Ministry of Education and Science. C.J. was supported by a scholarship from the Région Languedoc-Roussillon and IfremPeer reviewe

Etude et paramétrisation de la nutrition azotée et phosphorée d Alexandrium catenella, microalgue toxique responsable d efflorescences dans la lagune de Thau

L'activité ostréicole de l'Etang de Thau subit, depuis une dizaine d'années, des pertes de production régulières dues à des contaminations des parcs conchylicoles par des microalgues toxiques de l'espèce Alexandrium catenella. Cette thèse a été menée dans le but d'améliorer la compréhension des processus de contrôle environnementaux régulant ces efflorescences toxiques, en étudiant plus particulièrement l'influence des éléments nutritifs azotés et phosphorés sur la croissance d'A. catenella. Ces travaux ont permis de mettre en évidence une grande variété de sources potentielles nutritives pour cette espèce, dont la majorité peut être régénérée au niveau de la colonne d'eau. Différentes sources organiques contribueraient à la croissance d'A. catenella grâce à divers processus de nutrition : absorption direct à travers la membrane plasmique (pour l'urée), dégradation enzymatique dans le milieu extracellulaire (pour le phosphore organique dissous) ou phagocytose (pour internaliser des cyanobactéries). Ces capacités mixotrophes représenteraient un des avantages compétitif permettant à cette espèce de réaliser des efflorescences de grande ampleur ; un autre correspondrait à l'utilisation potentielle de nutriments azotés la nuit. Les expérimentations réalisées ont de plus révélé une grande complexité dans les processus de nutrition de cette espèce, par la mise en évidence de variations temporelles dans les capacités d'absorption, de phénomènes d'excrétion/réabsorption et d'interactions entre processus d'assimilation. Cette complexité a pu être en partie paramétrée afin de définir des relations mathématiques pouvant être intégrées à un futur modèle de croissanceIn the Thau lagoon, blooms of the toxic microalgae Alexandrium catenella have induced regular losses of shellfish production since 1998 due to bioaccumulation of toxins in oysters and mussels. This thesis was conducted to enhance knowledge of toxic bloom development regulation by environmental factors, focusing on nitrogen and phosphorus nutrients influence on A. catenella growth. These works highlighted a large variety of potential nutritive sources for this species and a major part of these nutrients appeared to be potentially regenerated in the water column. Different organic sources may contribute to the growth of A. catenella from diverse nutrition processes: direct absorption through the cell membrane (for urea), enzymatic degradation in the extracellular medium (for dissolved organic phosphorus) or phagocytosis (for ingestion of cyanobacteria). These mixotrophic capacities may represent competitive advantages for this species allowing the development of huge blooms; another competitive advantage may correspond to the potential use of nitrogen nutrients at night. A high complexity in nutrition processes of this species was revealed through observation of temporal variations in uptake capacities, excretion/absorption phenomena and interactions between nutrient uptake processes. The parametrization of a part of this complexity allowed the definition of mathematical formulations which can be integrated in a future growth modelMONTPELLIER-BU Sciences (341722106) / SudocPLOUZANE-Bibl.La Pérouse (290195209) / SudocSudocFranceF

Short-term temporal variability of ammonium and urea uptake by Alexandrium Catenella (Dinophyta) in cultures

10 pages, 4 figures, 3 tablesIn batch cultures of four Mediterranean strains (from France, Italy, and Spain) of Alexandrium catenella (Whedon et Kof.) Balech growing on a daily light cycle, ammonium and urea uptake were estimated by the 15N tracer technique. Ammonium uptake could be described by Michaelis-Menten kinetics along a substrate gradient of 0.1-10 μgat N · L−1 for the four strains, while two different patterns were observed for urea uptake with Michaelis-Menten kinetics for one strain and linear kinetics for the others. In all cases, an increase in uptake rates with time was noted over the daylight period. This trend led to a net increase in the maximum uptake rate (Vmax; for saturable kinetics) and in the initial slope α. For ammonium, Vmax increased by a factor of 2-10 depending on the strain, and, for urea, the maximal uptake rates measured increased by a factor of 2-18. Temporal variations of half-saturation constants (Ks) for both nutrients did not show a clear trend. Increases in Vmax and α showed an acclimation of the cells' uptake system over time to a N pulse, which may be explained by the light periodicity. For two strains, extensive ammonium release was observed during urea assimilation. This mechanism removes urea from the medium, so it is no longer available to other potential competitors, but supplies N back to the medium in the form of ammonium. From a methodological point of view, the phenomenon leads to considerable underestimates of the contribution of urea to phytoplankton growthThe authors wish to thank R. Ventosa for the nutrient analyses. We thank S. Fraga for the strain ACATA4. This study was financed by the CNRS (Centre National de la Recherche cientifique) and Ifremer (Institut Français pour l’Exploitation de la Mer), as well as the EU-funded Research Project SEED (Life cycle transformations among HAB species, and the environmental and physiological factors that regulate them, GOCE-CT-2005-003875). E. G.’s work was sustained by a Ramon y Cajal contract from the Spanish Ministry of Science and Education. This work benefited from correspondence with Dr. T. A. V. ReesPeer reviewe

Mass of particulate carbon and nitrogen in suspended matter as a function of volume filtered in several oceanic areas

We address two issues in the determination of particulate carbon and nitrogen in suspended matter of aquatic environments. One is the adsorption of dissolved organic matter on filters, leading to overestimate particulate matter. The second is the material loss during filtration due to fragile algal cells breaking up. Examples from both laboratory cultures and natural samples are presented. We recommend using stacked filters in order to estimate thefirst and filtering different volumes of water in order to evaluate the second

Grazing affects carbon fixation pathways by phytoplankton in coastal marine ecosystems.

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The globally distributed genus Alexandrium: Multifaceted roles in marine ecosystems and impacts on human health

The dinoflagellate genus Alexandrium is one of the major harmful algal bloom (HAB) genera with respect to the diversity, magnitude and consequences of blooms. The ability of Alexandrium to colonize multiple habitats and to persist over large regions through time is testimony to the adaptability and resilience of this group of species. Three different families of toxins, as well as an as yet incompletely characterized suite of allelochemicals are produced among Alexandrium species. Nutritional strategies are equally diverse, including the ability to utilize a range of inorganic and organic nutrient sources, and feeding by ingestion of other organisms. Many Alexandrium species have complex life histories that include sexuality and often, but not always, cyst formation, which is characteristic of a meroplanktonic life strategy and offers considerable ecological advantages. Due to the public health and ecosystem impacts of Alexandrium blooms, the genus has been extensively studied, and there exists a broad knowledge base that ranges from taxonomy and phylogeny through genomics and toxin biosynthesis to bloom dynamics and modeling. Here we present a review of the genus Alexandrium, focusing on the major toxic and otherwise harmful species

Perverse impacts of eutrophication's remediation actions in a Mediterranean coastal lagoon.

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