\subsection{Acknowledgements} The authors would like to acknowledge the
assistance of John Christian Gaby and Mallory Choudoir in developing the method
used to produce $^{13}$C-labeled cellulose. We would also like to thank
Ruth Ley, Steve Zinder, Nelson Hairston, and Nick Youngblut for providing comments that
were helpful in the development of this manuscript. Sander Hunter assisted with
the microcosm set up. This material is based upon
work supported by the Department of Energy Office of Science, Office of
Biological \& Environmental Research Genomic Science Program under Award Numbers
DE-SC0004486 and DE-SC0010558. Manuscript preparation by Ashley N. Campbell was
performed under the auspices of the U.S. Department of Energy by LLNL under
Contract DE-AC52-07NA27344.

