Shifts in carbon and N2 fixation-based incorporation rates and nitrogen transfer during fungal epidemics. Credit: Nature Communications (2026). DOI: 10.1038/s41467-025-67818-x

Under the lead of the Leibniz Institute for Baltic Sea Research Warnemünde (IOW) the influence of parasitic fungi on the physiology and survival of cyanobacteria in the Baltic Sea was investigated. Such infections are known from lakes. Due to the high nutrient load in the Baltic Sea, there are high levels of cyanobacteria, some of which are toxic (algal blooms).

In addition, the decomposition of algal blooms leads to oxygen depletion. Cyanobacteria are important for the nitrogen cycle, as some fix nitrogen and thus further increase nutrient concentrations in the Baltic Sea. The findings were published in the journal Nature Communications.

Parasitic fungi alter the nitrogen cycle

The IOW researchers analyzed the phytoplankton Dolichospermum, an ecologically important nitrogen-fixing cyanobacterium, and examined its susceptibility to infection by parasitic fungi (Chytridiomycota). They conducted the study together with colleagues from the Swedish Museum of Natural History in Stockholm and the University of Natural Resources and Life Sciences in Vienna (BOKU).

Cyanobacteria are part of the phytoplankton, which drives primary production in the ocean by converting carbon dioxide into biomass through photosynthesis. The researchers found that the fungi primarily infect specialized cells within the cyanobacterial filaments—namely, storage cells and nitrogen-fixing cells—while largely sparing the vegetative cells responsible for photosynthesis.

By targeting these specialized cells, the fungi gain access to the cyanobacterium’s carbon and nitrogen reserves. Extrapolations showed that up to one-fourth of the newly fixed nitrogen was transferred from Dolichospermum to the fungal parasite. This substantial loss of carbon and nitrogen may directly impair the survival of the cyanobacteria and influence the intensity and persistence of algal blooms in the Baltic Sea.

Impact of fungal infection on the Baltic Sea food web

Phytoplankton, including cyanobacteria, form the base of the aquatic food webs and serve as food for zooplankton. However, filamentous cyanobacteria such as Dolichospermum are difficult for zooplankton to ingest. Fungal infection changes this dynamic: infected filaments tend to fragment, making them more accessible as a food source.

Moreover, the fungi transform the nutritionally poor and partly toxic cyanobacterial biomass into fungal zoospores. These zoospores are rich in sterols and polyunsaturated fatty acids, making them a high-quality, energy-rich food source for other organisms. In this way, parasitic fungi may enhance the transfer of energy and nutrients through the Baltic Sea food web.

Frequency of fungal infections

Between June and September from 2022 to 2024, Dolichospermum was detected in 33 out of 52 weekly water samples from the Baltic Sea. More than half of these samples contained Dolichospermum populations infected with fungi. Due to living as a parasite, the fungi thrive during times with high algal cell mass which increases with rising nutrient loads in the Baltic Sea. This demonstrates that fungal infections of Dolichospermum are common and that it is therefore essential to investigate the consequences of this infection in more detail.

The present study provides an important data basis for this. According to IOW scientist Isabell Klawonn, the lead of the Emmy-Noether-Working group and corresponding author of the study, it is very important to investigate the so far unexplored influences of marine fungi on algal blooms. "Previous research has mainly focused on nutrient availability and water temperature as drivers of nitrogen-fixing cyanobacteria, some of which are toxic. Our findings show that parasitic fungi can also significantly influence cyanobacterial growth and activity, and therefore deserve much more attention."

In addition to Dolichospermum, the researchers also detected fungal infections in other bloom-forming cyanobacteria in the Baltic Sea, namely Nodularia and Aphanizomenon. The study provides an important foundation for future research into how fungal parasites shape algal blooms, nutrient cycling, and food-web interactions in coastal ecosystems. Algal blooms are most intense in coastal areas, which means that high rates of fungal infections can be expected here.

Publication details

Anna Feuring et al, Fungal parasites infecting N2-fixing cyanobacteria reshape carbon and N2 fixation and trophic transfer, Nature Communications (2026). DOI: 10.1038/s41467-025-67818-x

Citation: Parasitic fungi infect nitrogen-fixing cyanobacteria, altering Baltic Sea nutrient cycles (2026, January 21) retrieved 21 January 2026 from https://phys.org/news/2026-01-parasitic-fungi-infect-nitrogen-cyanobacteria.html

This document is subject to copyright. Apart from any fair dealing for the purpose of private study or research, no part may be reproduced without the written permission. The content is provided for information purposes only.