Microbial players involved in the decline of filamentous and colonial cyanobacterial blooms with a focus on fungal parasitism

In the forthcoming decades, it is widely believed that the dominance of colonial and filamentous bloom-forming cyanobacteria (e.g. Microcystis, Planktothrix, Anabaena and Cylindrospermopsis) will increase in freshwater systems as a combined result of anthropogenic nutrient input into freshwater bodies and climate change. While the physicochemical parameters controlling bloom dynamics are well known, the role of biotic factors remains comparatively poorly studied. Morphology and toxicity often – but not always – limit the availability of cyanobacteria to filter feeding zooplankton (e.g. cladocerans). Filamentous and colonial cyanobacteria are widely regarded as trophic dead-ends mostly inedible for zooplankton, but substantial evidence shows that some grazers (e.g. copepods) can bypass this size constraint by breaking down filaments, making the bloom biomass available to other zooplankton species. A wide range of algicidal bacteria (mostly from the Alcaligenes, Flavobacterium/Cytophaga group and Pseudomonas) and viruses (Podoviridae, Siphoviridae and Myoviridae) may also contribute to bloom control, via their lytic activity underpinned by a diverse array of mechanisms. Fungal parasitism by the Chytridiomycota remains the least studied. While each of these biotic factors has traditionally been studied in isolation, emerging research consistently point to complex interwoven interactions between biotic and environmental factors.