Reproducible spatial structure formation and stable community composition in the cyanosphere predicts metabolic interactions

Cyanobacterial granules and aggregates can readily form in aquatic environments. The microbial communities found within and around these structures can be referred to as the cyanosphere, and can enable collective metabolic activities relevant to biogeochemical cycles. Cyanosphere communities are suggested to have different composition to that in the surrounding environment, but studies to date are mostly based on single time point samples. Here, we retrieved samples containing cyanobacterial granules from a freshwater reservoir and maintained a culture through sub-culture passages under laboratory conditions for over a year. We show that cyanobacteria-dominated granules form readily and repeatedly in this system over passages, and that this structure formation process seems to be associated with cyanobacterial motility. Performing longitudinal short-read sequencing over several culture passages, we identified a cyanosphere community comprising of 17 species with maintained population structure. Using long-read sequencing from two different time point samples, we have re-constructed full, circular genomes for 15 of these species and annotated metabolic functions within. This predicts several metabolic interactions among community members, including sulfur cycling and carbon and vitamin exchange. Using three individual species isolated from this cyanosphere, we provide experimental support for growth on carbon sources predicted to be secreted by the cyanobacterium in the system. These findings reinforce the view that the cyanosphere can recruit and maintain a specific microbial community with specific functionalities embedded in a spatially-organised microenvironment. The presented community will act as a key model system for further understanding the formation of the structured cyanosphere, its function and stability, and its metabolic contribution to biogeochemical cycles.