Extended Data Fig. 7: Additional nanoSIMS images of cells from Candidatus M. methanotrophicum enrichment cultures and cell biomass dynamics.

(a–d) Images of the ¹³C atom fraction, which is a measure of carbon assimilation from methane provided during the incubation. (e–h) Images of an overlay between the ¹²C¹⁴N ion counts intensity (blue), which is a proxy for biomass, and the ¹³C atom fraction (green). Cells shown in panels a–c and e–g were grown on ¹³C-labelled methane for 110 days, while cells in panels d and h were grown on unlabelled methane (control cells). Some cells in panels e–g appear blue because their ¹³C labeling is significantly lower compared to cells that appear cyan, although it was still significant compared to the control cells (see Fig. 4d). Note the filament in panels c and g, which belongs to a fungus from the genus Acidomyces. (i) Cell biomass as a function of time in two parallel subcultures of Candidatus M. methanotrophicum grown on CH₄ as the sole carbon and energy source. One culture used ¹³C-labelled CH₄ (¹³C atom fraction of 0.5), the other one used unlabelled CH₄. Lines show the modelled biomass assuming an exponential growth with a doubling time of 94 days (solid line) and 98 days (dashed line) and a lag phase of 21 days. The decrease in the doubling time from 150–200 in the original culture to 94–98 days in these subcultures was possibly due to small changes in the culturing conditions combined with improved growth properties of Candidatus M. methanotrophicum. (j)¹³C atom fractions in the cells of Candidatus M. methanotrophicum grown on ¹³C-labelled CH₄. Symbol shows the mean value, error bar corresponds to the standard deviation (calculated based on the measurement of N = 110 cells, where the cells were not treated by any post-incubation chemical procedure prior to the nanoSIMS analysis). Solid line shows the ¹³C atom fraction modelled based on the assumption that the ¹³C-labelled CH₄ was the sole carbon and energy source and the growth characteristics were as shown in panel i.