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Dynamics of bacteria–substrate stable isotope separation: dependence on substrate availability and implications for aquatic food web studies

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Heterotrophic bacteria growing without substrate limitation discriminate against the heavier stable isotopes of C and N, which is reflected in the isotopic signatures of consumers of bacteria. We measured bacteria and substrate isotopic separation (C and N) during glucose and ammonium uptake by three strains of bacteria (Pseudomonas putida, Bacillus megaterium, and Enterobacter aerogenes) grown in batch culture at 4 and 30°C. Isotopic separation between bacteria and substrate was dependent on substrate availability. Higher discrimination against the heavier isotopes, and therefore more depleted 13C and 15N values of bacterial biomass, were observed during the exponential growth phase when the nutrient supply was in excess of the demand. We also compared the isotopic ratios of Chironomus tentans grown on aged macrophyte detritus and commercial fish food. Isotopic signatures of Chironomus larvae grown on fish food were within the ranges typical of one trophic step, whereas larvae grown on detritus were strongly depleted in 13C, suggesting assimilation of bacteria that had grown on the C-rich detritus. Our results are consistent with the very few other studies of bacterial fractionation of nonlimiting nutrients and may also explain the frequent observation of unexpectedly depleted isotopic signatures in aquatic food web studies.

Les bactéries hétérotrophes qui croissent sans limitation de substrat sont capables de rejeter les isotopes stables plus lourds de carbone et d’azote, un phénomène qui se reflète dans les signatures isotopiques des consommateurs de bactéries. Nous avons mesuré la séparation des isotopes (de C et de N) dans les bactéries et le substrat durant l'absorption de glucose et d'ammonium par trois souches de bactéries (Pseudomonas putida, Bacillus megaterium et Enterobacter aerogenes) élevées en culture discontinue à 4 °C et à 30 °C. La séparation isotopique entre les bactéries et le substrat dépend de la disponibilité du substrat. On observe un plus fort rejet des isotopes plus lourds, et ainsi des valeurs réduites en 13C et en 15N de la biomasse bactérienne, durant la phase de croissance exponentielle lorsque l'apport de nutriments dépasse la demande. Nous avons aussi comparé les rapports isotopiques chez des Chironomus tentans Fabricius élevés sur du détritus de macrophytes vieilli et sur des aliments commerciaux pour poissons. Les signatures isotopiques des larves de Chironomus élevées sur des aliments pour poissons se situent dans la gamme des valeurs typiques d'un échelon trophique, alors que les larves élevées sur le détritus ont une forte carence en 13C, ce indique une assimilation des bactéries qui se sont développées sur le détritus riche en carbone. Nos résultats sont en accord avec les très rares études sur la fractionation bactérienne des nutriments non limitants et ils peuvent aussi expliquer l'observation fréquente de signatures isotopiques à carence inattendue dans les études des réseaux alimentaires aquatiques.

Document Type: Research Article

Publication date: 2008-09-01

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  • Published continuously since 1901 (under various titles), this monthly journal is the primary publishing vehicle for the multidisciplinary field of aquatic sciences. It publishes perspectives (syntheses, critiques, and re-evaluations), discussions (comments and replies), articles, and rapid communications, relating to current research on cells, organisms, populations, ecosystems, or processes that affect aquatic systems. The journal seeks to amplify, modify, question, or redirect accumulated knowledge in the field of fisheries and aquatic science. Occasional supplements are dedicated to single topics or to proceedings of international symposia.
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