a, Transmission electron micrograph of a spiral-shaped cell with a flagellum. The size of Ca. N. inopinata cells is 0.18 to 0.3 μm in width and 0.7 to 1.6 μm in length. Scale bar represents 200 nm. b, Transmission electron micrograph of a thin section preparation. Microcolony showing the wide periplasmic space (PS), which is a characteristic feature of Nitrospira15. Scale bar represents 200 nm. c, Fluorescence image of cells from enrichment ENR4 after hybridization with oligonucleotide probes targeting Nitrospira (Ntspa662 and Ntspa712 both labelled with Cy3, red), the betaproteobacterium (Nmir1009 labelled with Cy5, blue), and Bacteria (EUB338 probe mix labelled with FLUOS, green). Ca. N. inopinata cells and microcolonies appear yellow and the betaproteobacterial cells appear cyan due to simultaneous hybridization to the respective specific probe and the EUB338 probe mix. Scale bar represents 2 μm. d, Cell metabolic cartoon constructed from the annotation of the Ca. N. inopinata genome. Enzyme complexes of the electron transport chain are labelled by Roman numerals.