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Fungal parasites infecting N2-fixing cyanobacteria reshape carbon and N2 fixation and trophic transfer

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Abstract

Fungal parasites are associated with bloom-forming algae, yet their impact on N2 fixation and the fate of newly fixed nitrogen during cyanobacterial blooms is poorly understood. We report infections on the ecologically important N2-fixing cyanobacterium Dolichospermum (formerly Anabaena) in the Baltic Sea. Using single-cell isotope probing, microscopy, and biogeochemical analyses, we examine how infections affect carbon and N2 fixation and elemental transfer within a natural community. Fungal sporangia infect up to 80% of filaments, mostly targeting storage cells (akinetes, 82% prevalence) and N2-fixing cells (heterocytes, 44%), but rarely vegetative cells (5%). Infections at akinete–heterocyte junctions extract 4- and 10-fold more carbon and nitrogen than those on vegetative cells, reducing host storage by 28% and 56%. Overall, 22% of newly fixed nitrogen is transferred to fungi, comparable to heterotrophic bacteria. Infections also occur in Nodularia and Aphanizomenon, suggesting fungi-like parasitism broadly affects bloom dynamics and the fate of new nitrogen.

Data availability

Sequence data have been deposited in ENA (European Nucleotide Archive) under project accession no. PRJEB96922. Accession numbers used to construct the phylogenetic tree in Supplementary Fig. S5 are listed in Supplementary Table S5, as accessed on the NCBI database. The raw mass spectrometer output can be found in Supplementary Dataset 1. Source data are provided with this paper.

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Acknowledgements

We sincerely thank Christian Burmeister for nutrient analyses, Jenny Jeschek for DOC/DN analyses, as well as Christin Fechtel and the entire IOW monitoring team for mycoplankton sampling during the Baltic Sea monitoring cruises and at Heiligendamm. We further acknowledge the valuable support of Susanne Busch during phytoplankton identification, Heejin Jeon during IMS-1280 analyses, Sascha Plewe, as well as Karoline Schulz, Armin Springer, and Marcus Frank from the Medical Biology and Electron Microscopic Centre (Rostock University Medical Center, Rostock) during SEM imaging, Annett Grüttmüller during nanoSIMS analyses, and Quentin Devresse and Stanislav Jabinski during proofreading. A.F., C.D.L., and I.K. were funded by the German Research Foundation (DFG, Emmy Noether grant KL 3332/1-1 to IK). Sequencing was funded by the DFG (KL 3332/3-1 to IK) and conducted at the DFG Research Infrastructure NGS Competence Center (DcGC, project 407482635) as part of the Next Generation Sequencing Competence Network (DFG project 423957469). NGS library preparation, data production and analyses were carried out at the DcGC Dresden-concept Genome Center, core facility of the CMCB and Technology Platform of the Dresden University of Technology (TU Dresden). The NordSIMS facility is operated under a Swedish Research Council infrastructure grant 2021-00276. This is NordSIMS publication #814. The NanoSIMS at the Leibnitz-Institute for Baltic Sea Research, Warnemünde (IOW) was funded by the German Federal Ministry of Education and Research (BMBF, grant 03F0626A). The IOW long-term observations were financially supported by the IOW, the Federal Maritime and Hydrographic Agency (BSH), and the state and federal government.

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Authors and Affiliations

  1. Department of Biological Oceanography, Leibniz Institute for Baltic Sea Research, Rostock/Warnemuende, Germany

    Anna Feuring, Connor D. Lawrence, Jessica Salcedo, Angela Vogts & Isabell Klawonn

  2. Department of Geosciences, Swedish Museum of Natural History, Stockholm, Sweden

    Martin J. Whitehouse

  3. Core Facility Bioinformatics, BOKU University, Vienna, Austria

    Luca Zoccarato

  4. Institute of Computational Biology, BOKU University, Vienna, Austria

    Luca Zoccarato

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  1. Anna Feuring
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  2. Connor D. Lawrence
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  7. Isabell Klawonn
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Contributions

I.K. acquired funding and designed research; A.F., C.D.L., J.S., M.J.W., A.V., and I.K. performed sampling and sample analyses; A.F., L.Z., and I.K. analyzed data; and A.F. and I.K. wrote the paper with input and approval from all coauthors.

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Isabell Klawonn.

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Feuring, A., Lawrence, C.D., Salcedo, J. et al. Fungal parasites infecting N2-fixing cyanobacteria reshape carbon and N2 fixation and trophic transfer.
Nat Commun (2026). https://doi.org/10.1038/s41467-025-67818-x

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