Coelosphaerium dubium is a colonial planktonic cyanobacterium in the family Merismopediaceae, named for the taxonomic uncertainty surrounding its morphological boundaries with related species in the genus. Like its congeners, it forms spherical, gelatinous mucilaginous colonies in which cells are distributed at the periphery of a central hyaline matrix. The genus Coelosphaerium has historically been defined by gross morphological characteristics—colony shape, cell arrangement, and mucilage properties—but molecular phylogenetic analyses have substantially revised understanding of cyanobacterial diversity and systematics within this group. Coelosphaerium dubium has a relatively broad geographic distribution compared to some congeners, with records from freshwater bodies in Denmark, Norway, Sweden, and Taiwan, suggesting either a wide ecological tolerance or cryptic species diversity within the taxon. It inhabits lentic and slow-flowing freshwater environments, particularly stratified lakes and ponds where thermal and chemical gradients allow cyanobacterial cells equipped with gas vesicles to adjust their buoyancy and optimise light exposure. As a primary producer in these aquatic ecosystems, it contributes to nutrient cycling and forms part of the phytoplankton community grazed by zooplankton. No formal conservation assessment has been undertaken for this species by the IUCN.