Epiphytic Saprospiraceae: Ecology and physiology of novel and globally abundant members in wastewater treatment plants

The Saprospiraceae family within the phylum Bacteroidota is commonly present in high abundance in wastewater treatment plants worldwide. The family contains many genera, but little is known about their function and importance. Usually, members are described as rod-shaped or having filamentous morphology, aerobic heterotrophic metabolism, and a potential to be involved in nutrient removal. Some members are epiphytic bacteria and likely involved in symbiotic relationship with filamentous bacteria, often from the Chloroflexota phylum. However, most of the epiphytic Saprospiraceae are currently undescribed, so their function is unknown. In our study, using fluorescence in situ hybridization based on a comprehensive set of full-length 16S rRNA gene sequences, we were able to identify multiple genera of epiphytic bacteria from the Saprospiraceae. We also analyzed their global distribution in >700 treatment plants worldwide. Additionally, high-quality metagenome assembled genomes retrieved from Danish wastewater treatment plants allowed us to analyze their potential role in nutrient removal. Metabolic reconstruction predicted different storage polymers, such as glycogen, polyphosphate, and polyhydroxyalkanoates, and their presence was confirmed by fluorescence in situ hybridization combined with Raman microspectroscopy. This study provides the first deep insight into epiphytic Saprospiraceae and their potential role and involvement in wastewater treatment processes.