The Nature of the Dietary Protein Impacts the Tissue-to-Diet 15N Discrimination Factors in Laboratory Rats

Due to the existence of isotope effects on some metabolic pathways of amino acid and protein metabolism, animal tissues are 15N-enriched relative to their dietary nitrogen sources and this 15N enrichment varies among different tissues and metabolic pools. The magnitude of the tissue-to-diet discrimination (Δ15N) has also been shown to depend on dietary factors. Since dietary protein sources affect amino acid and protein metabolism, we hypothesized that they would impact this discrimination factor, with selective effects at the tissue level. To test this hypothesis, we investigated in rats the influence of a milk or soy protein-based diet on Δ15N in various nitrogen fractions (urea, protein and non-protein fractions) of blood and tissues, focusing on visceral tissues. Regardless of the diet, the different protein fractions of blood and tissues were generally 15N-enriched relative to their non-protein fraction and to the diet (Δ15N>0), with large variations in the Δ15N between tissue proteins. Δ15N values were markedly lower in tissue proteins of rats fed milk proteins compared to those fed soy proteins, in all sampled tissues except in the intestine, and the amplitude of Δ15N differences between diets differed between tissues. Both between-tissue and between-diet Δ15N differences are probably related to modulations of the relative orientation of dietary and endogenous amino acids in the different metabolic pathways. More specifically, the smaller Δ15N values observed in tissue proteins with milk than soy dietary protein may be due to a slightly more direct channeling of dietary amino acids for tissue protein renewal and to a lower recycling of amino acids through fractionating pathways. In conclusion, the present data indicate that natural Δ15N of tissue are sensitive markers of the specific subtle regional modifications of the protein and amino acid metabolism induced by the protein dietary source

Interannual variability of methane storage and emission during autumn overturn in a small lake

Methane emissions from freshwater systems, and especially from small lakes source a significant proportion of naturally produced atmospheric methane. In small temperate lakes, storage flux, i.e. the diffusive emission of methane that was stored in anoxic waters during the seasonal overturn of the water column, can contribute a large fraction of annual methane emissions. Here we use an extensive field dataset to quantify methane storage in anoxic deep water and identify as well as quantify the sources and losses of this methane. The comparison of measurements from four years in a small temperate lake (Illmensee) shows that methane storage can differ substantially between years. In 2012 and 2018, the annual maximum of the methane stored in the entire water column was around twice as high (5350 kg and 5822 kg) as in 2013 and 2017 (2722 kg and 2295 kg). A methane mass balance approach suggests that the variability of methane storage in the anoxic water between the years was not caused by the methane flux at the anoxic-oxic water interface, but rather was related to changes in the methane source from the anoxic sediments. The interannual differences in sediment flux could not be explained by sediment temperature, but rather by the differing supply of organic matter. Our findings suggest that phytoplankton blooms promote methane storage within the same year and thus cause interannual variability in emissions during autumn overturn.publishe

Interspecific and nutrient-dependent variations in stable isotope fractionation: experimental studies simulating pelagic multitrophic systems

Stable isotope signatures of primary producers display high inter- and intraspecific variation. This is assigned to species-specific differences in isotope fractionation and variable abiotic conditions, e.g., temperature, and nutrient and light availability. As consumers reflect the isotopic signature of their food source, such variations have direct impacts on the ecological interpretation of stable isotope data. To elucidate the variability of isotope fractionation at the primary producer level and the transfer of the signal through food webs, we used a standardised marine tri-trophic system in which the primary producers were manipulated while the two consumer levels were kept constant. These manipulations were (1) different algal species grown under identical conditions to address interspecific variability and (2) a single algal species cultivated under different nutrient regimes to address nutrient-dependent variability. Our experiments resulted in strong interspecific variation between different algal species (Thalassiosira weissflogii, Dunaliella salina, and Rhodomonas salina) and nutrient-dependent shifts in stable isotope signatures in response to nutrient limitation of R. salina. The trophic enrichment in 15N and 13C of primary and secondary consumers (nauplii of Acartia tonsa and larval herring) showed strong deviations from the postulated degree of 1.0‰ enrichment in δ13C and 3.4‰ enrichment in δ15N. Surprisingly, nauplii of A. tonsa tended to keep “isotopic homeostasis” in terms of δ15N, a pattern not described in the literature so far. Our results suggest that the diets’ nutritional composition and food quality as well as the stoichiometric needs of consumers significantly affect the degree of trophic enrichment and that these mechanisms must be considered in ecological studies, especially when lower trophic levels, where variability is highest, are concerned

Gestion et conservation des ceintures de végétation lacustre = Management and conservation of lake littoral vegetation

National audienceFaire le point le plus complet sur les connaissances et les expériences en terme de conservation et de gestion des ceintures de végétation lacustre, tel était l'objectif de ce séminaire de restitution du programme LIFE Nature "Lac du Bourget". Les 6 sessions ont permis de mettre en exergue : la valeur des ceintures de végétation lacustre sur le plan écologique et de la diversité biologique ; le rôle social que jouent ces formations végétales et les représentations différentes qui en découlent ; le caractère complexe de leur fonctionnement, très sensible au contexte géomorphologique et aux facteurs physiques ; le fait que la plupart des écosystèmes correspondants n'évoluent plus de façon naturelle en Europe, la variation naturelle des niveaux d'eau étant aujourd'hui trop souvent contrariée dans ces systèmes ; la nécessité de comprendre les contextes locaux, les représentations et les usages avant de proposer des mesures de préservation et de restauration