Unusual ecophysiology of Candidate phylum Acetothermia bacteria associated with anaerobic digesters revealed by complete genome sequencing and advanced in situ microscopy

Members of the candidate phylum Acetothermia are globally distributed and have been detected in diverse natural habitats as well as engineered systems. However, little is known about their physiology and ecological importance. In this study, an OTU belonging to Acetothermia was identified as one of the most abundant genus-level bacterial taxa in full-scale mesophilic anaerobic digesters at two Danish wastewater treatment plants. A closed genome was obtained by differential coverage binning of metagenomes and long Nanopore reads. Genome annotation and metabolic reconstruction revealed an anaerobic chemoheterotrophic lifestyle in which the bacterium obtain energy and carbon from the fermentation of peptides, amino acids, and simple sugars to acetate, formate and hydrogen. In contrast to previously studied Acetothermia, the genome did not encode a functional acetyl-CoA pathway required for acetogenesis. Fluorescent in situ hybridization probes targeting Acetothermia were designed and used in combination with confocal laser scanning microscopy and atomic force microscopy. This revealed an unusual morphology composed of a central rod cell with bipolar prosthecae similar to those observed in Caulobacter and Asticcacaulis. These prosthecae greatly expand the cell surface to volume ratio and allow for increased nutrient uptake, providing the bacteria with a competitive advantage under nutrient limited conditions.