Fig. 3: Elemental analysis shows that the fibers are Ni and S rich.

Representative STEM-EDX spectra from A intact cable bacteria and B fiber sheaths shows a detectable Ni signal and lower Fe and Cu levels in the fiber sheath. Elemental compositions are found in Supplementary Table 1. Representative synchrotron LEXRF maps for C intact cable bacteria (10 μm × 25 μm) and D fiber sheaths (11 μm × 27 μm). SP + C denotes Scatter Peak plus Compton and L denotes low-energy L-band. E Average counts per pixel from LEXRF maps showing that Ni is mainly found in the fiber sheath. Shown are the average of the maps ± SE (intact cable bacteria N = 5 and fiber sheaths N = 6, data are background corrected) and the data points for the individual maps. Data given for the detected transition metals and SP + C. The latter data were scaled to fit into the graph by setting the average of the intact cable bacteria (CB) counts to 100 (original counts 4290 ± 2640). F NanoSIMS images of fiber sheaths. Mapping of ³²S/¹²C ion count ratio (first 100 planes) shows the sulfur rich fibers. The Ni (⁵⁸Ni + ⁶⁰Ni) ion count has a lower signal/noise ratio and its mapping (first 50 planes) only shows visible fibers in restricted regions (as indicated by the rectangle). The complete set of NanoSIMS images is given in Supplementary Fig. 5. Independent replicas (N = 2) showed similar results.