Data were acquired with the prototype (4 detectors) and a second-generation instrument (NanoSIMS 50L, Cameca: 7 detectors). Quantitative mass images were analysed with “Open MIMS” software, a plug-in to ImageJ (http://www.nrims.harvard.edu/software.php). Stable isotopes were obtained from Cambridge Isotopes, Inc. (15N-thymidine, 13C-thymidine) and Sigma-Aldrich, Inc. (13C-palmitate).
Full Methods and any associated references are available in the online version of the paper at www.nature.com/nature.