Abundance and ecophysiology of MiDAS 4 family f37-13 filamentous bacteria in Japan possessing the Eikelboom morphotype 1863

Abstract Members of the Bacteroidota family MiDAS 4 f37-13 are plentiful in activated sludge microbial communities, although most are uncultured. This study focused on their identification and influence on activated sludge bulking. Fluorescence in situ hybridization (FISH) analysis of samples from a Japanese enhanced biological nutrient removal (EBNR) plant showed they possessed a filamentous morphology, sharing an Eikelboom 1863 morphotype, previously described for the Gamma-proteobacteria Acinetobacter. Members of two genera, midas_g_535 and midas_g_543, exhibited higher relative abundances than the other genera in the family MiDAS 4 f37-13. Neither were members of the Gamma - proteobacteria . Both showed a statistically significant correlation with plant sludge volume index, suggesting they were responsible for activated sludge bulking in this WWTP. Metagenome analyses revealed that existing metagenome-assembled genomes within the MiDAS 4 f37-13 family possessed genes associated with denitrification, polyphosphate accumulation, and polyhydroxyalkanoate synthesis, and their potential role in nitrogen and phosphate removal in EBNR WWTPs is discussed.