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

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.