Protozoans as a food source for Antarctic krill, Euphausia superba: complementary insights from stomach content, fatty acids, and stable isotopes

We studied the diet of Antarctic krill, Euphausia superba, at five stations across the southwest Atlantic sector in summer 2003 by analyzing stomach content, fatty acids, and stable isotopes on the same individuals. Our aim was to examine what each method could contribute to our understanding of krill nutrition and whether differences seen in growth rates were linked to their food. All three methods indicated clear regional differences in diet, but small ontogenetic and sex-related differences. Overall, diatoms were the most abundant item in the stomach, but at three of the stations, tintinnids, large dinoflagellates, and other armored flagellates dominated the identifiable biomass. Copepod remains were rare. Fatty acids profiles gave additional information about feeding on weakly silicified diatoms and athecate heterotrophic dinoflagellates, with the latter being the main food source at one of the stations. Two independent indices of carnivory, d15N and the fatty acid ratio 18:1(n-9)/18:1(n-7), were correlated among krill from the same swarm, suggesting consistent differences in diet between individuals. An internal index of trophic position, (i.e., d15Nglutamic acid-d15Nphenylalanine) underlined the importance of heterotrophic food for the nutrition of krill, even in summer. Highest growth rates of krill were found during a diatom bloom and coincided with a mixed diet, large digestive gland, and fast stomach passage. However, even in a nonbloom, flagellate-dominated system, krill were able to sustain medium growth rates when feeding on heterotrophic dinoflagellates. Each method supplied specific information on krill nutrition, and the true picture is only revealed when the various methods are used together

Natural abundance of nitrogen isotopes in amino acids are biomarkers of the metabolic syndrome onset

International audienceIntroduction: The isotope effects associated with nitrogen metabolic pathways result in differences in the nitrogen stable isotopecompositions (δ15N) of metabolic pools within individuals. Thus, inter-individual differences in some δ15N values can reflect differencesin nitrogen metabolismObjectives: Here, we evaluated in rats whether the metabolic syndrome (MS) onset involve some dysregulations of nitrogen metabolismwhich can be traced with δ15N values of individual amino acids in plasma and tissue proteins.Method / Design: Sixty male Wistar rats were fed a high fat diet for 10 weeks and 2 groups were selected in either the upper (MS group,n=10) or the lower (Lean, control group, L, n=10) ends of body weight and fasting insulin distributions and killed for tissue collection. Plasma and tissue proteins were precipitated and hydrolyzed and δ15N values of Ala, Asx, Glx, Gly, Leu, Pro and Ser were determined usinggas chromatograph combustion isotope ratio mass spectrometry.Results: Body weight and fasting insulin were 1.3 and 8.2 fold higher in the MS compared with the L group, respectively. No differencewas observed between groups for δ15N values of Asx, Glx, Leu, Pro and Ser, whereas δ15N values of Ala and Gly were significantly lowerin plasma and muscle proteins of the MS group. A discriminant analysis showed that the difference between δ15N of Gly and Leu in plasmaproteins is the main contributor to group separation. On the basis of this variable alone, 95% of the rats were correctly classified as MS or L.Conclusions: These results evidence that nitrogen metabolism is affected during the onset of insulin resistance in a tissue-specificmanner. They also suggest that the δ15N of some plasma amino acids represent promising early biomarkers of these metabolic changes.Keywords: (maximum 5): compound-specific amino acid δ15N values, early biomarkers, metabolic syndrome, protein and amino acidmetabolis