Development of bipolar prosthecae by candidate phylum Acetothermia bacteria

Members of the candidate phylum Acetothermia are globally distributed and detected in various habitats. However, little is known about their physiology and ecological importance. We recently discovered an operational taxonomic unit (OTU) belonging to Acetothermia that were at high abundance in four full-scale anaerobic digesters by 16S rRNA gene amplicon sequencing. Using differential coverage binning of metagenomes and scaffolding with long nanopore reads we were able to close the first genome from this phylum. Genome annotation and metabolic reconstruction suggested an anaerobic chemoheterotrophic lifestyle in which the bacterium obtain energy and carbon via fermentation of peptides, amino acids, and simple sugars to acetate, formate, and hydrogen. Fluorescence in situ hybridization combined with confocal laser scanning microscopy, Raman microspectroscopy and atomic force microscopy revealed an unusual morphology composed of a central rod-shaped cell with bipolar prosthecae. We hypothesize that these prosthecae allow for increased nutrient uptake by greatly expanding the cell surface area, providing a competitive advantage under nutrient-limited conditions.