1 Max Planck Institute for Terrestrial Microbiology & LOEWE Research Center for Synthetic Microbiology (SYNMIKRO), Marburg, Germany. Electronic address: victor.sourjik@synmikro.mpi-marburg.mpg.de.
2 Department of Bioengineering, University of California, San Diego, USA. Electronic address: cmetallo@eng.ucsd.edu.