Unveiling the role of the cyanosphere: interactions between cyanobacteria and bacterial associates during blooms

Abstract Global change severely increases the recurrence of freshwater cyanobacterial proliferations, with known deleterious consequences for ecosystem structure and functioning. A few studies highlighted the existence of bacterial communities associated to cyanobacteria (i.e., Cyanosphere), comparable to the well-known phycosphere. While phycosphere in general is expected to exchange nutrients with their associated phytoplankton communities, specific functions and dynamic of the cyanosphere remain unknown. Here, based on metabarcoding approaches, we showed that during bloom episodes, cyanosphere accounts for up to 30% of total bacterial abundance. More importantly, we demonstrated for the first-time the dynamics aspect of cyanosphere community composition at crucial bloom phases. In our study, these community changes imply variations in cyanosphere ecological functions over time. Our results highlight the ability of cyanosphere to regulate nutrient and energy exchange with cyanobacteria during the entire bloom period, modulating bloom intensity and dynamics. In the latter bloom phases, cyanosphere additionally tends to create microhabitats favoring cyanobacterial persistence in deleterious environmental conditions. At wider scale, as blooms constitute major turnovers of matter and energy in aquatic ecosystems, we argue that cyanosphere can have an important, yet underestimated, role in mediating C and nutrient cycles in environments.