Contribution of Heterotrophic Diazotrophs to N2 Fixation in a Eutrophic River: Free-Living vs. Aggregate-Associated

Eyal Geisler, Eyal Rahav, Edo Bar-Zeev

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8 Scopus citations


Recent studies have indicated that heterotrophic diazotrophs are highly diverse and fix N2 in aquatic environments with potentially adverse conditions for diazotrophy, such as oxic and rich in total nitrogen. In this study, we compared the activity and diversity of heterotrophic diazotrophs associated with aggregates (>12 μm) to free-living cells in the eutrophic Qishon River during the winter and summer seasons. Overall, measured heterotrophic N2 fixation rates in the Qishon River ranged between 2.6–3.5 nmol N L–1 d–1. Heterotrophic N2 fixation was mainly associated with aggregates in the summer samples (74 ± 24%), whereas during the winter the bulk diazotrophic activity was mostly ascribed to the free-living fraction (90 ± 6%). In addition, immunolabeled micrographs indicated the presence of aggregate-associated heterotrophic diazotrophs in both seasons, while phototrophic diazotrophs were also captured during the winter. The richness of free-living and aggregate-associated heterotrophic diazotrophs were overall similar, yet the evenness of the later was significantly smaller, suggesting that few of the species gained advantage from particle lifestyle. The differences in the activity, micro-localization and diversity of the diazotrophic community were mostly attributed to spatiotemporal changes in the ambient C:N ratios (total organic carbon, TOC: total nitrogen) and the TOC concentrations. Taken together, our results shed new light on the contribution of heterotrophic diazotroph associated with aggregates to total heterotrophic N2 fixation in oxic, highly eutrophic aquatic environments.

Original languageEnglish
Article number779820
JournalFrontiers in Microbiology
StatePublished - 14 Feb 2022
Externally publishedYes

Bibliographical note

Publisher Copyright:
Copyright © 2022 Geisler, Rahav and Bar-Zeev.


EG was supported by the Yohai Ben-Nun (IOLR) and the Negev (BGU) Scholarships. EB-Z was supported by the Israeli Science Foundation (Grant No. 944\21) and the Roy J Zuckerberg Career Development Katerdra.

FundersFunder number
Yohai Ben-Nun
Israel Science Foundation944\21


    • aggregates
    • diazotrophs
    • eutrophic river
    • free-living cells
    • heterotrophic N fixation


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