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)

Lipid class and fatty acid composition of planktivorous larval pike

This study was undertaken in a pond used for natural spawning of pike. Zooplankton and pike larvae were sampled using a horizontal haul net (120 μm or 1 mm mesh size) through the aquatic vegetation. Among the different class sizes of larvae, one cohort was isolated (11.1 mm < L < 46 mm), in which larval pike first fed on small Cyclopoids then switched to Cladocerans until they fed on insect larvae. Length (1.2 mm/day) and weight growth were found to be fast. Between day 8 and day 13 of exogenous feeding there was a marked decrease of reserve lipids of larvae. This decrease coincided with a lower growth and seemed to be ascribable to changes from Cyclopoid (60.4% of ingested prey) to 2 species of Cladoceran (42.4% and 36.4% of ingested prey), in the feeding sequence of larvae. Pike probably found difficulties in capturing Cyclopoid and digesting Cladocerans as their alimentary canal was not completely developed. After day 13, Cladoceran digestibility was good as the S-bend gut was formed, when triacylglycerol content of larvae increased until the experiment ceased. As indicators, of good growth, we detected increasing amounts of phospholipids throughout the whole study. The fatty acid composition of larval triacylglycerols appeared to be very similar to the composition of zooplankton lipids. Dietary fatty acids could have been incorporated whithout modification into triacylglycerols and into phospholipids possibly with limited elongation/desaturation. The results suggest that pike larvae require both (n-3) FA and (n-6) FA

Croissance et régime alimentaire des juvéniles de Saumon atlantique (Salmo salar L.) dans la rivière Allier (origine et transferts des acides gras dans la chaîne trophique)

Ce projet a été conduit dans le contexte général du renforcement de la population naturelle de Saumon atlantique de l'Allier et s'est focalisé sur les modalités de croissance des juvéniles et tacons en grand cours d'eau. Nous nous sommes intéréssés au développement des juvéniles relâchés dans le milieu naturel, avant de considérer ensuite la chaîne trophique dans son ensemble. Notre travail avait plusieurs objectifs : 1-Effectuer une analyse de l'alimentation, de la croissance et du statut nutritionnel des tacons issus d'une salmoniculture, relâchés et recapturés le long du continuum fluvial de l'Allier ; 2-Evaluer si, en raison de compositions en acides gras différentes, les macroinvertébrés pouvaient affecter la croissance des tacons, ainsi que leur survie hivernale ; 3-Déterminer, grâce à une approche par les AG biomarqueurs, l'origine des composés lipidiques retrouvés dans les macroinvertébrés et identifier les sources de C assimilées par ces derniers le long du continuum fluvialCLERMONT FD-BCIU Sci.et Tech. (630142101) / SudocSudocFranceF

Interspecific competition between restocked 0+ salmon parr (Salmo salar) and native fish species in a large European river

International audienceAtlantic salmon populations have experienced strong declines in most large European rivers. In the Loire Basin, France, a major restocking effort has been carried out on the Allier River to reduce the wild population decline. This study aimed at investigating the trophic insertion of hatchery-reared salmon parr and whether they compete for resources with native fish species. For this purpose, two riffles located in the upstream and downstream parts of the restocked area have been surveyed monthly following the initial 0+ salmon fry release. 0+ parr densities remained high upstream, with autumnal values greater than those recommended to maximise smolt production. In contrast, downstream densities declined to weak values despite a faster increase in size, probably mainly due to summer temperatures rising above the discomfort threshold. By analysing the vulnerability to predation of ingested invertebrate prey, we highlighted the role of habitat constraints in driving most of the different fish species to a likely opportunistic utilisation of available food resources. As a result, species such as stone loaches and barbel juveniles exhibited narrow trophic niches and high overlaps with 0+ parr. Nonetheless, both abundant prey resources and spatial or temporal habitat segregation have limited interspecific competition, enabling competitive coexistence of sympatric species, except for barbel juveniles in certain circumstances. Therefore, our findings suggest that future restocking programmes should carefully consider the thermal and hydraulic conditions regarding further smolt production as well as their general trophic context, according to the potential inconvenience of restocked salmon fry arrival relative to native species

Nutritional value of different food sources for the benthic Daphnidae Simocephalus vetulus: role of fatty acids

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Does sterol availability in a forested headwater stream constitute a nutritional constraint for macroinvertebrates?

International audienceHeadwater streams are characterised by predominantly heterotrophic functioning resulting from leaf litter input, but autochthonous primary production can also contribute to energy fluxes and the supply of nutrients. While much work has focused on the role of polyunsaturated fatty acids as essential nutrients, less attention has been paid to the significance of sterols in stream food webs. Yet these molecules are essential to arthropods that can only synthesise cholesterol from a limited range of dietary sterols. In a headwater stream, we tracked the transfer of dominant sterols from allochthonous and autochthonous resources to 5 benthic macroinvertebrates. Despite their formal functional feeding group, all the taxa tended to be opportunistic omnivores that relied on all available resources. These behaviours could be attributed to nutritional constraints in the stream food web, but these constraints were not related to the sterol supply from the different basal sources. Dominant sterols from detrital sources (leaf litter, fine benthic organic matter [FBOM]) and primary producers (epilithic biofilms, bryophytes) were all Δ5-sterols (β-sitosterol, campesterol, stigmasterol, epibrassicasterol). The levels of cholesterol detected in macroinvertebrates, as well as the presence of desmosterol, indicate efficient dealkylation activity of these Δ5-sterols. Leaf litter has consistently shown high β-sitosterol/stigmasterol ratios, giving it greater nutritional value than usually accepted. But bryophytes, and especially epilithic biofilms, contained cholesterol, providing a direct supply to invertebrates, thereby promoting their growth. Detrital particles (FBOM) colonised by microalgae could be the best-balanced diet to avoid sterol deficiencies due to their cholesterol content and high β-sitosterol/stigmasterol ratios

How well can the fatty acid content of lake seston be predicted from its taxonomic composition ?

International audience1. Results from the few field studies that have tried to relate seston taxonomic and fatty acid (FA) composition suggest that phytoplankton composition only partially explains seston FA composition. However, in these studies, the heterotrophic components of seston (i.e. bacteria and heterotrophic protists) have not been accounted for. 2. The general premise of this article was that including the contribution of heterotrophs to seston biomass can improve understanding of the variability in seston FA composition. This was tested for an oligotrophic clearwater lake, in which the taxonomic and FA compositions of seston, fractionated into three size classes, were monitored every 2 weeks over a growth season. The relationship between seston taxonomic and FA composition was studied using canonical correlation analyses. 3. Because of their relative richness in branched FA and lack of highly unsaturated FAs (HUFA) compared to autotrophs and other protists, the contribution of bacteria to seston biomass was shown to explain an important part of the differences in FA composition between the different seston size classes. Phytoplankton seasonal succession also affected the FA composition of seston but only for size classes that were dominated by autotrophs. 4. The results also indicated that heterotrophic protists such as ciliates and heterotrophic nanoflagellates might substantially influence the seston FA, and especially, HUFA, composition. 5. The per cent of variability in seston FA composition that was explained by its taxonomic composition was still relatively low, even when taking account of heterotrophs. Hence, other possible influences, such as phytoplankton species composition, physiological state and the contribution of terrestrial detritus, need investigation

Influence of natural food on growth and fatty acid composition of restocked salmon parr along the longitudinal gradient of a large temperate river : the Allier

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Caractérisation des particules virales planctoniques dans un lac du Massif Central français : aspects méthodologiques et premiers résultats

International audienceDans cette note, nous décrivons une méthode simple, permettant l'ultracentrifugation de particules virales planctoniques et leur observation en microscopie électronique à transmission. Une description de différents morphotypes de particules assimilables à des virus est faite à partir d'échantillons prélevés dans la couche euphotique d'un lac eutrophe (Aydat), au cours des périodes printanière et estivale. Ces particules présentent chacune une capside icosaédrique régulière à 6 faces et une queue, dont les dimensions sont similaires à celles des phages décrits en milieu marin. La diversité des formes et le nombre de virions (10 fois supérieur à celui des bactéries), permettent d'émettre l'hypothèse selon laquelle les virus interviennent de façon significative dans le contrôle des abondances des communautés bactériennes en milieu lacustre

Combined effects of food quality and temperature on somatic growth and reproduction of two freshwater cladocerans

International audienceWe investigated the effect of food quality on somatic growth and reproduction of zooplankton at different temperatures (12°C, 15°C, 20°C, and 25°C). Standardized growth experiments of two cladocerans, Daphnia magna and Simocephalus vetulus, were performed on (1) high-quality food (Cryptomonas sp.), (2) relatively low-quality food (Scenedesmus obliquus), and (3) intermediate-quality food (Cryptomonas: Scenedesmus mixture). Food quality constraints on somatic growth and reproduction of the two cladocerans decreased with increasing temperature. For D. magna and for S. vetulus, differences between clutch size and growth rate of individuals fed on the three food sources were highly pronounced when they were reared at 12°C and 15°C; however, such differences decreased at 20°C and were negligible at 25°C. Variations in food quality constraints with temperature can be explained by the variability of dietary polyunsaturated fatty acids, such as eicosapentaenoic acid and stearidonic acid requirements of these cladocerans. We conclude that dietary constraints exerted by food quality for zooplankton development vary as a function of different temperature conditions