Klawonn, Isabell, Van den Wyngaert, Silke, Iversen, Morten H., Walles, Tim J. W., Flintrop, Clara M., Cisternas-Novoa, Carolina, Nejstgaard, Jens C., Kagami, Maiko and Grossart, Hans-Peter (2023) Fungal parasitism on diatoms alters formation and bio–physical properties of sinking aggregates. Communications Biology, 6 (1). Art.Nr. 206. DOI 10.1038/s42003-023-04453-6.

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Abstract

Phytoplankton forms the base of aquatic food webs and element cycling in diverse aquatic systems. The fate of phytoplankton-derived organic matter, however, often remains unresolved as it is controlled by complex, interlinked remineralization and sedimentation processes. We here investigate a rarely considered control mechanism on sinking organic matter fluxes: fungal parasites infecting phytoplankton. We demonstrate that bacterial colonization is promoted 3.5-fold on fungal-infected phytoplankton cells in comparison to non-infected cells in a cultured model pathosystem (diatom Synedra, fungal microparasite Zygophlyctis, and co-growing bacteria), and even ≥17-fold in field-sampled populations (Planktothrix, Synedra, and Fragilaria). Additional data obtained using the Synedra–Zygophlyctis model system reveals that fungal infections reduce the formation of aggregates. Moreover, carbon respiration is 2-fold higher and settling velocities are 11–48% lower for similar-sized fungal-infected vs. non-infected aggregates. Our data imply that parasites can effectively control the fate of phytoplankton-derived organic matter on a single-cell to single-aggregate scale, potentially enhancing remineralization and reducing sedimentation in freshwater and coastal systems.

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