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

Sep 30, 2026

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Fungal parasites infect N2-fixing cyanobacteria in Baltic Sea blooms, but their effects on nitrogen fixation and the fate of newly fixed nitrogen were unclear. The study reports infections in Dolichospermum using single-cell isotope probing, microscopy, and biogeochemical analyses. Fungal sporangia infect up to 80% of filaments, mainly storage cells and N2-fixing heterocytes, extracting substantially more carbon and nitrogen at specific junctions. About 22% of newly fixed nitrogen is transferred to fungi, comparable to heterotrophic bacteria, and similar parasitism is observed in Nodularia and Aphanizomenon.

LanguageEnglish

CategoryResearch & Report

FormatPDF

Pages13

Reading Time33 min

AccessFree to download

Introduction

Background on algal blooms and N2-fixing cyanobacteria

Underexplored role of fungal parasites

Research gap and study objectives

Results

Dolichospermum long-term observations

Article [https://doi.org/10.1038/s41467-025-67818-x](https://doi.org/10.1038/s41467-025-67818-x) Fungal parasites infecting N2-ﬁxing cyanobacteria reshape carbon and N2 ﬁxation and trophic transfer Received: 11 June 2025 | Accepted: 9 December 2025 | | --- | | | | Check for updates | Anna Feuring 1, Connor D. Lawrence 1, Jessica Salcedo 1, Martin J. Whitehouse 2, Angela Vogts 1, Luca Zoccarato 3,4 & Isabell Klawonn 1 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 atakinete–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 andAphanizomenon, suggesting fungi-like parasitism broadly affects bloom dynamics and the fate of new nitrogen. Algal blooms have signiﬁcant impacts on aquatic ecosystems and human activities, with both positive and negative consequences. On the one hand, they account for nearly half of global primary production, forming the base of aquatic food webs and supporting ﬁsheries1–3. On the other hand, they can cause oxygen depletion, toxin release, and water quality degradation, threatening ecosystem functioning, biodiversity, ﬁsheries, and recreational activities4,5. Under current climate change, phytoplankton blooms are expanding and intensifying in both coastal and freshwater systems6,7, with harmful effects often outweighing the beneﬁts. For example, in the Baltic Sea, blooms of N2-ﬁxing, partially toxin-producing cyanobacteria have increased in frequency, biomass, and duration since the 1970s8–10. Due to their ability to ﬁx atmospheric N2, diazotrophic cyanobacteria thrive in nitrogendepleted waters, introducing hundreds of kilotons (180–430 kt) of new nitrogen per year into the central Baltic Sea11,12. This internal nitrogen loading is nearly equivalent to the external nitrogen load from river discharge (480 kt N yr–1)11, exacerbating the vicious cycle that fuels further algal growth, oxygen depletion, and eutrophication13,14. The integration of algal blooms into food webs is typically linked to interactions with zooplankton grazers, heterotrophic bacteria, protists, and viruses15, while cyanobacterial blooms are commonly considered to be regulated by temperature and nutrient availability16,17. In contrast, the role of fungi—particularly parasitic fungi—has largely been overlooked. Mounting evidence, however, suggests that parasitic fungi, belonging mostly to the order Chytridiomycota (hereafter referred to as chytrids), are widespread in coastal18–20 and freshwater ecosystems21,22. While thriving on major phytoplankton groups, including diatoms, cyanobacteria, and dinoﬂagellates, those fungal parasites infect up to 90% of the phytoplankton host population23–25. Parasitism, such as fungal parasitism, can thereby alter carbon transfer 1Department of Biological Oceanography, Leibniz Institute for Baltic Sea Research, Rostock/Warnemuende, Germany. 2Department of Geosciences, Swedish Museum of Natural History, Stockholm, Sweden. 3Core Facility Bioinformatics, BOKU University, Vienna, Austria. 4Institute of Computational Biology, BOKU University, Vienna, Austria. e-mail: [klawonn@iow.de](klawonn@iow.de) pathways and food web dynamics26,27, whereas its quant

FAQs

Which organism and ecosystem are studied to examine fungal parasitism effects?

The work focuses on the Baltic Sea and the N2-fixing cyanobacterium Dolichospermum (formerly Anabaena), also examining Nodularia and Aphanizomenon for broader fungal-like parasitism patterns.

How were infections measured and how did the study quantify carbon and nitrogen effects?

Stable isotope tracing, single-cell SIMS, partial 18S rRNA-gene sequencing, environmental monitoring, microscopy, and biogeochemical analyses were used to assess prevalence and to quantify carbon and nitrogen transfer from cyanobacterial cells to fungal sporangia.

What fraction of filaments and newly fixed nitrogen is associated with fungal infection and transfer?

Fungal sporangia infect up to 80% of filaments, and about 22% of newly fixed nitrogen is transferred to fungi, comparable to heterotrophic bacteria.

Sep 30, 2026

Upload by Levi