ABSTRACT
Candidatus
Saccharimonadia is a class‐level lineage of ultrasmall bacteria within the phylum
Minisyncoccota
(formerly Candidate Phyla Radiation or
Ca
. Patescibacteria), commonly found in activated sludge processes treating municipal wastewater. In this study, we aimed to elucidate the metabolic potential of
Ca
. Saccharimonadia by using shotgun metagenomic sequencing combined with a filtration‐based size‐fractionation approach for activated sludge from five wastewater treatment plants. A total of 65 high‐quality metagenomic bins were recovered, belonging to four orders and 19 families of
Ca
. Saccharimonadia, including previously unreported lineages in activated sludge. These bins had small genomes (approximately 0.46–1.73 Mbp) with limited metabolic capabilities, indicating dependency on other microorganisms. Notably, the order
Ca
. Saccharimonadales retained a type IV secretion system and effector gene cluster for parasitic interactions with the hosts, suggesting that
Ca
. Saccharimonadales bacteria may exhibit a parasitic lifestyle. Co‐occurrence network analysis showed that members of the order
Ca
. Saccharimonadales were significantly correlated with multiple lineages, including
Actinobacteriota
, for which a parasitic relationship has been previously demonstrated. Our results shed light on the potential ecophysiology of the diverse members of
Ca
. Saccharimonadia, providing a comprehensive understanding of
Ca
. Saccharimonadia in activated sludge.