Dissolved nitrogen uptake versus nitrogen fixation: Mode of nitrogen acquisition affects stable isotope signatures of a diazotrophic cyanobacterium and its grazer.

Publisher: Public Library of Science Country of Publication: United States NLM ID: 101285081 Publication Model: eCollection Cited Medium: Internet ISSN: 1932-6203 (Electronic) Linking ISSN: 19326203 NLM ISO Abbreviation: PLoS One Subsets: MEDLINE

Original Publication: San Francisco, CA : Public Library of Science

Nitrogen Fixation* ; Nitrogen Isotopes*/metabolism ; Nitrogen Isotopes*/analysis ; Nitrogen*/metabolism ; Daphnia*/metabolism ; Cyanobacteria*/metabolism ; Cyanobacteria*/growth & development; Animals ; Carbon Isotopes/analysis ; Carbon Isotopes/metabolism ; Nitrates/metabolism ; Nitrates/analysis ; Phytoplankton/metabolism ; Phytoplankton/growth & development

Field studies suggest that changes in the stable isotope ratios of phytoplankton communities can be used to track changes in the utilization of different nitrogen sources, i.e., to detect shifts from dissolved inorganic nitrogen (DIN) uptake to atmospheric nitrogen (N2) fixation by diazotrophic cyanobacteria as an indication of nitrogen limitation. We explored changes in the stable isotope signature of the diazotrophic cyanobacterium Trichormus variabilis in response to increasing nitrate (NO3-) concentrations (0 to 170 mg L-1) under controlled laboratory conditions. In addition, we explored the influence of nitrogen utilization at the primary producer level on trophic fractionation by studying potential changes in isotope ratios in the freshwater model Daphnia magna feeding on the differently grown cyanobacteria. We show that δ 15N values of the cyanobacterium increase asymptotically with DIN availability, from -0.7 ‰ in the absence of DIN (suggesting N2 fixation) to 2.9 ‰ at the highest DIN concentration (exclusive DIN uptake). In contrast, δ 13C values of the cyanobacterium did not show a clear relationship with DIN availability. The stable isotope ratios of the consumer reflected those of the differently grown cyanobacteria but also revealed significant trophic fractionation in response to nitrogen utilization at the primary producer level. Nitrogen isotope turnover rates of Daphnia were highest in the absence of DIN as a consequence of N2 fixation and resulting depletion in 15N at the primary producer level. Our results highlight the potential of stable isotopes to assess nitrogen limitation and to explore diazotrophy in aquatic food webs.
Competing Interests: The authors have declared that no competing interests exist.
(Copyright: © 2024 Helmer et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.)

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0 (Nitrogen Isotopes)
N762921K75 (Nitrogen)
0 (Carbon Isotopes)
0 (Nitrates)

Date Created: 20240801 Date Completed: 20240801 Latest Revision: 20240803

20250114

PMC11293670

10.1371/journal.pone.0306173

39088456