Extended Data Fig. 6: Morphology and CO₂ production of Candidatus M. methanotrophicum.

(a–b) Electron micrographs of Candidatus M. methanotrophicum (a) and M. smegmatis (a). Inset in panel a shows vesicles in the periplasm of Candidatus M. methanotrophicum at a higher magnification. Schematic representations of the micrographs in panels a’ and b’ show the cytosol (grey), the capsular layer (green), the plasma membrane (black line), the periplasm (yellow), the DNA (blue), the elucent areas (white), and the vesicles (red). (c–d) Evolution of CO₂ during incubations of Candidatus M. methanotrophicum enrichment cultures with CH₄. Incubations were conducted under aerobic (c) or anaerobic (d) conditions using either live cultures (green symbols) or a sterile NMS medium (black symbols). Different symbols correspond to replicate cultures (rep 1–3), and the values correspond to the total amount of CO₂ per incubation bottle. Green lines show fits of the experimental data within the time interval of 4–22 days (c) and 0–22 days (d) with a linear model. For the replicate culture 1 incubated under aerobic conditions (rep 1), the slope of this linear model is significantly different from the corresponding negative slope of the linear model characterizing the removal of CH₄ during the incubation (two-sided ANOVA, F = 6.79, p = 0.03; CH₄ data shown in Fig. 3b). In contrast, the corresponding slopes are not significantly different for replicate cultures 2 (F = 0.0118, p = 0.92) and 3 (F = 3.38, p = 0.10). For both the aerobic and anaerobic incubations, the abiotic controls showed no significant variation in the CO₂ amounts over time in comparison to the live cultures.