Coelosphaerium aerugineum is a colonial planktonic cyanobacterium in the family Merismopediaceae, forming spherical to irregularly shaped gelatinous colonies in which individual cells are arranged in a peripheral layer surrounding a central mucilaginous matrix. Like other members of the genus, C. aerugineum is found in freshwater lakes, ponds, and slow-flowing waters of the temperate zone, including Scandinavia. Cyanobacteria are among the oldest life forms on Earth, contributing to oxygen evolution in ancient oceans, and continue to play fundamental roles in modern aquatic biogeochemistry through nitrogen fixation and primary production. Coelosphaerium species can form blooms under nutrient-enriched eutrophic conditions, although they are generally less notorious for bloom formation than genera such as Microcystis or Dolichospermum. The spherical colonial architecture provides some protection against grazing by zooplankton, while the cells contain gas vesicles that assist in vertical positioning within the water column. Distribution of Coelosphaerium aerugineum spans temperate freshwater habitats across Europe, including northern countries such as Norway and Sweden, where it occurs in lakes and reservoirs during warmer months. No formal conservation assessment has been undertaken for this species, as freshwater cyanobacteria at this taxonomic level are generally not evaluated by the IUCN.