Genome-resolved taxonomy, global distribution, and EPS potential of Zoogloea , the canonical floc-former in wastewater treatment systems

The bacterial genus Zoogloea is the fourth most abundant genus in wastewater treatment plants (WWTPs) globally, where it plays key roles in floc formation and nutrient removal but can also contribute to viscous bulking when highly abundant. The taxonomic assignment of Zoogloea has historically been inconsistent, as several taxa were classified based on phenotypic characteristics and later reassigned to other families or orders. This has obscured both the identity and distribution of Zoogloea and hindered interpretation of its floc-forming mechanisms, which have been attributed to polysaccharides that may not originate from phylogenetically defined Zoogloea . Using our collection of more than 22,000 high-quality genomes from global activated sludge systems (MiDAS global genome catalog), we identified 47 genomes affiliated with the phylogenetically defined Zoogloea . In combination with abundance data, this enabled resolution of the genus’ taxonomy, biogeographical distribution, and metabolic diversity. Building on this framework, we mapped the phylogenetic distribution of a recently identified Zoogloea -associated floc-forming polysaccharide biosynthesis gene cluster by screening the MiDAS global genome catalog and GTDB. The gene cluster was identified across multiple genera within the Burkholderiales order, including Zoogloea , Thauera , Azoarcus , and Ca . Accumulibacter, as well as several previously undescribed taxa. The gene cluster is broadly conserved, with minor variations in gene composition and synteny, suggesting corresponding differences in polysaccharide structure. Genomes encoding this cluster also consistently contained co-occurring floc-regulating genes, indicating a common and widespread mechanism underlying floc formation in activated sludge systems.