Origins of carbon sustaining the growth of whitefish Coregonus lavaretus early larval stages in Lake Annecy: insights from fatty-acid biomarkers.

International audienceThe hypothesis that diatom carbon (C) produced during the spring peak supported spring zooplankton production and, ultimately, the growth of Coregonus lavaretus early larval stages from March to May 2006 in Lake Annecy, France, was tested using gut content analyses and fatty acid biomarkers. Gut content results showed that C. lavaretus larvae from stages 1 to 4 preferentially fed on copepods with Daphnia sp. only a minor proportion of larval diet. The levels of diatom-marker fatty acids (C16:1n-7 and C20:5n-3) were high in Daphnia sp., but lower in both copepods and C. lavaretus larvae from stages 0 to 4. These results indicated that the spring diatom biomass was actually grazed by Daphnia sp., but, contrary to what was expected, the spring bloom was not the only C source supporting copepods secondary production and, consequently, the growth of C. lavaretus early larval stages. In contrast, levels of terrestrial fatty acid marker (C24:0) were low in Daphnia sp. but high in copepods and C. lavaretus larvae, indicating a significant contribution of terrestrial carbon to copepods and, ultimately, to the growth of C. lavaretus early larval stages

Fatty acid composition of the heterotrophic nanoflagellate Paraphysomonas sp. : influence of diet and de novo synthesis

International audienceWe compared the relative importance of dietary factors versus de novo synthesis in determining the polyunsaturated fatty acid (PUFA) composition of the heterotrophic nanoflagellate Paraphysomonas sp. The flagellate was fed with different mutants of the picocyanobacterial strain Synechocystis PCC6803, which differ in their capability to synthesize specific PUFAs. The desA, desB, and desD genes of Synechocystis PCC6803 encode lipid desaturases at the Δ12, Δ15, and Δ6 positions of 18C fatty acids (FAs), respectively. Thus, the use of desA–, desB–, desD–, and desA–/des D– mutants of Synechocystis PCC6803 as food sources permitted us to provide the heterotrophic flagellate with decreasing levels of unsaturated FAs. In each treatment, Paraphysomonas sp. exhibited the same FA composition pattern, i.e. high levels of 16:0 and 18:1, and significant amounts of 18C PUFAs and long-chain PUFAs such as 20:4(n-6), 20:5(n-3), and 22:6(n-3), which indicated that Paraphysomonas sp. is capable of synthesizing these PUFAs de novo. Results also showed that dietary 18C PUFAs seem to be preferentially accumulated in Paraphysomonas sp. lipids. This demonstrates that heterotrophic protists could play a key role in transferring essential compounds from primary producers to metazoan consumers

Questioning the roles of resources nutritional quality in ecology

International audienceOur understanding of ecosystem functioning is strongly linked to the study ofpredator–prey relationships and food web structures. However, trophic ecology hasoften focused on identifying taxonomic relationships and quantifying the biomassor energy ingested by consumers, but has often failed to integrate the importanceof the nutritional quality of resources in ecological dynamics. Underlying this gap isthe multi-dimensional nature of resource quality which has hampered any consensuson the definition of resource nutritional quality. In this special issue, we aimed atgathering a subset of articles exemplifying the diversity of variables by which resourcesquality is quantified, the diversity of research topics that can be tackled in ecology– from physiological or evolutionary aspects to ecosystem processes – and proposesome perspectives on the integration of nutritional quality within broader ecologicalconcepts. Using a semi-automated literature analysis, we map the current landscapeof the ‘resources nutritional quality’ research of the last 30 years. We depict how ithas been quantified through physical, biological or chemical indicators, the use ofthese parameters being largely dependent on the type of ecosystem studied and on theinvestigated ecological process. We then position the articles published in this specialissue of Oikos within this landscape, showing they cover a small but relatively wellrepresentative subset of the domains of resources quality-related issues. Articles in thisspecial issue browse a range of individual and population-level approaches (embracingevolutionary questions) to community related questions, include methodologicalissues and ecosystem-wide approaches using trophic quality indicators as tracers ofresources origin. Based on these studies and on the literature review, we identify a nonexhaustivelist of challenges and perspectives of research that we consider of highestpriority in the large topic of trophic ecology

Turbulence for the generalised Burgers equation

In this survey, we review the results on turbulence for the generalised Burgers equation on the circle: u_t+f'(u)u_x=\nu u_{xx}+\eta,\ x \in S^1=\R/\Z, obtained by A.Biryuk and the author in \cite{Bir01,BorK,BorW,BorD}. Here, f is smooth and strongly convex, whereas the constant 0<\nu << 1 corresponds to a viscosity coefficient. We will consider both the case \eta=0 and the case when \eta is a random force which is smooth in x and irregular (kick or white noise) in t. In both cases, sharp bounds for Sobolev norms of u averaged in time and in ensemble of the type C \nu^{-\delta}, \delta>=0, with the same value of \delta for upper and lower bounds, are obtained. These results yield sharp bounds for small-scale quantities characterising turbulence, confirming the physical predictions \cite{BK07}.Comment: arXiv admin note: substantial text overlap with arXiv:1201.5567, arXiv:1107.4866, arXiv:1208.524

Temperature and nutrient effects on the relative importance of brown and green pathways for stream ecosystem functioning: A mesocosm approach

In addition to global warming, aquatic ecosystems are currently facing multiple global changes among which include changes in nitrogen (N) loads. While several studies have investigated both temperature and N impacts on aquatic ecosystems independently, knowledge on their interactive effects remains scarce. In forested headwater streams, decomposition of leaf litter represents the main process ensuring the transfer of nutrients and energy to higher trophic levels, followed by autochthonous primary production, mainly ensured by phototrophic biofilms. The main aim of this study was to disentangle the independent and combined effects of temperature increase and nutrient availability on the relative importance of brown and green processes involved in stream functioning. We hypothesised that water temperature and nutrients would lead to a general increase in leaf‐litter decomposition and primary production, but that the intensity of these effects would be largely modulated by competitive interactions arising between microorganisms as well as by the top‐down control of microorganisms by macro‐invertebrates. Macro‐invertebrates would, in turn, be bottom‐up controlled by microbial resources quality. To test these hypotheses, we conducted a 56‐day experiment in artificial streams containing leaf litter, microbial decomposers and biofilm inoculum, and an assemblage of macro‐invertebrates. Two water inorganic N:phosphorus (P) ratios (33 and 100, molar ratios) and two temperatures (ambient, +2°C) were manipulated, each treatment being replicated three times. Fungal and biofilm growth as well as leaf‐litter decomposition and primary production were quantified. Top‐down impacts of invertebrate primary consumers on brown and green compartments were evaluated using exclosures while bottom‐up control was evaluated through the measurement of resource stoichiometry and fatty acid profiles, as well as quantification of macro‐invertebrate growth and survival. Contrary to expectations, microbial decomposition was not significantly stimulated by nutrient or temperature manipulations, while primary production was only improved under ambient temperature. In the + 2°C treatment with high N:P, greater biofilm biomass was associated with lower fungal development, which indicates competition for nutrients in these conditions. Temperature increased macro‐invertebrate growth and leaf‐litter consumption, but this effect was independent of any improvement of basal resource quality, suggesting that temperature mediated changes in consumer metabolism and activity was the main mechanism involved. Most of our hypotheses that were based on simplified laboratory observations have been rejected in our semi‐controlled mesocosms. Our study suggests that the complexity of biological communities might greatly affect the response of ecosystems to multiple stressors, and that interactions between organisms must be explicitly taken into account when investigating the impacts of global change on ecosystem functioning

Interactive Impacts of Silver and Phosphorus on Autotrophic Biofilm Elemental and Biochemical Quality for a Macroinvertebrate Consumer

Autotrophic biofilms are complex and fundamental biological compartments of many aquatic ecosystems. In particular, these biofilms represent a major resource for many invertebrate consumers and the first ecological barrier against toxic metals. To date, very few studies have investigated the indirect effects of stressors on upper trophic levels through alterations of the quality of biofilms for their consumers. In a laboratory study, we investigated the single and combined effects of phosphorus (P) availability and silver, a re-emerging contaminant, on the elemental [carbon (C):nitrogen (N):P ratios] and biochemical (fatty acid profiles) compositions of a diatom-dominated biofilm initially collected in a shallow lake. We hypothesized that (1) P and silver, through the replacement of diatoms by more tolerant primary producer species, reduce the biochemical quality of biofilms for their consumers while (2) P enhances biofilm elemental quality and (3) silver contamination of biofilm has negative effects on consumers life history traits. The quality of biofilms for consumers was assessed for a common crustacean species, Gammarus fossarum, by measuring organisms’ survival and growth rates during a 42-days feeding experiment. Results mainly showed that species replacement induced by both stressors affected biofilm fatty acid compositions, and that P immobilization permitted to achieve low C:P biofilms, whatever the level of silver contamination. Gammarids growth and survival rates were not significantly impacted by the ingestion of silver-contaminated resource. On the contrary, we found a significant positive relationship between the biofilm P-content and gammarids growth. This study underlines the large indirect consequences stressors could play on the quality of microbial biomass for consumers, and, in turn, on the whole food web

Formation and Transfer of Fatty Acids in Aquatic Microbial Food Webs: Role of Heterotrophic Protists

The term protist was first coined by Haeckel in 1866 for diverse microorganisms including bacteria (Haeckel 1866). However, in 1925 in a paper on an amoeboid parasite of Daphnia, Chatton (1925) highlighted for the first time the fundamental difference between prokaryotic and eukaryotic organisms and the term protist to be now used to describe unicellular eukaryotes, which do not differentiate into tissues (see Adl et al. 2005)

Flux de matière dans le réseau trophique planctonique du lac d'Annecy (approche expérimentale et in situ par les acides gras marqueurs et leurs rapports isotopiques ( 13C))

Ce travail est principalement consacré aux interactions trophiques qui s'établissent entre les organismes microbiens et les microcrustacés zooplanctoniques du lac d'Annecy. Des expériences de nutrition ont permis d'apprécier la qualité nutritionnelle des protistes hétérotrophes pour le zooplancton. Les teneurs en Acides Gras Polyinsaturés (AGPI) de la série (n-3) de la nourriture semble être le principal facteur déterminant sa valeur nutritive. Cette étude montre en outre que l'addition d'un flagellé algivore comme intermédiaire trophique dans la chaîne alimentaire microalgue-zooplancton favorise le développement de Daphnia hyalina. L'amélioration trophique de la qualité nutritionnelle par les protistes phagotrophes (trophic upgrading) serait principalement liée aux capacités biosynthétiques de ces derniers. L'approche en milieu naturel s'est principalement focalisée sur le transfert des AG entre les communautés microbiennes et les microcrustacés zooplanctoniques prélevés en zone pélagique de mars à septembre 2000. Afin de révéler l'association AG biomarqueur-groupe taxonomique, le matériel sestonique a été divisé en classes de taille par filtrations successives. L'évolution des teneurs en 20:5(n-3) dans les lipides de Daphnia hyalina a pu être corrélée positivement avec les variations de sa fécondité. Les cryptophycées et les diatomées centriques apparaissent comme les principales sources d'AGPI(n-3) pour le cladocère. Enfin, il a été envisagé de caractériser les AG par leur rapport isotopique 13C afin d'évaluer l'intérêt de l'utilisation des rapports isotopiques 13C d'AG individuels comme biomarqueurs des relations trophiques.This study is mostly dedicated to microorganisms-crustacean zooplankton trophic relationships occurring in Lake Annecy. Nutrition experiments were performed to evaluate food quality of heterotrophic protists for zooplankton. Food contents in (n-3) Polyunsaturated Fatty Acids (PUFA) seem to be the main factor determining food quality. Moreover this study shows that the addition of an algivorous flagellate as an intermediary trophic level in the microalga-zooplancton food chain enhance Daphnia hyalina development. Trophic upgrading of food quality may be surely associated to biosynthetic capacities of heterotrophic protists. The field study mainly focused on FA transfer between microbial communities and crustacean zooplankton sampled in the pelagic zone between march and september 2000. To facilitate the FA biomarker approach, seston has been fractionated in different size classes. The evolution of 20:5(n-3) contents in Daphnia hyalina lipids has been positively correlated to its fecundity. Cryptophytes and centric diatoms appears as the main sources in (n-3) PUFA for the cladoceran. Endly the reliability of fatty acids-specific stable isotope analysis ( 13C) for trophic studies has been assessed.CHAMBERY -BU Bourget (730512101) / SudocSudocFranceF

Trophic partitioning among three littoral microcrustaceans: relative importance of periphyton as food resource

The high species richness of zooplankton communities in macrophytes littoral zones could result from the diversity of potential trophic niches found in such environment. In macrophytes littoral zones, in addition to phytoplankton, neustonic, benthic and epiphytic biofilms can also be potential components of the microcrustacean diet. Here, we investigated the ability of three large cladocerans: Daphnia longispina, Simocephalus vetulus and Eurycercus lamellatus, to develop on periphyton as their only food source or as a complement to a phytoplankton resource in scarce supply. D. longispina exhibited a very low growth and reproduction rates on the periphytic resource and as S. vetulus seems unable to scrape on periphyton. In contrast, E. lamellatus could not grow on phytoplankton, and appears to be an obligatory periphyton scraper. This latter finding contrasts with previous studies suggesting that E. lamellatus could be able to scrap periphyton as well as filter-feed on suspended matter. These differences in feeding strategy probably reflect the different trophic niches occupied by these three species in macrophytes littoral zones, and may explain at least in part their ability to coexist in the same environment