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DNA was extracted using a modified Griffiths procotol \cite{Griffiths_2000}. To prepare nucleic acid extracts for isopycnic centrifugation as presviously previously  described \cite{Buckley_2007}, DNA was size selected (\textgreater4kb) using 1\% low melt agarose gel and $\beta$- agarase I enzyme extraction per manufacturers protocol (New England Biolab, M0392S). Isopycnic gradients were established as described previously (35, 48) setup using a modified protocol \cite{Neufeld_2007}  for five \textsuperscript{12}C-control, five \textsuperscript{13}C-xylose, and four \textsuperscript{13}C-cellulose microcosms; one per day harvested. A density gradient solution of 1.762 g cesium chloride (CsCl) ml\textsuperscript{-1} in gradient buffer solution (pH 8.0 15mM Tris-HCl, 15mM EDTA, 15mM KCl) was used to separate \textsuperscript{13}C-enriched and \textsuperscript{12}C-nonenriched DNA. Each gradient was loaded with approximately 5$\mu$g of DNA and utlracentrifuged for 66 h at 55,000 rpm and room temperature (RT). Fractions of $\sim$100 $\mu$L were collected into Acroprep\textsuperscript{TM} 96 filter plate (part no. 5035, Pall Life Sciences) by displacing the DNA-CsCl-gradient buffer solution in the centrifugation tube with water. The refractive index of each fraction was measured using a Reichart AR200 digital refractometer. Then buoyant density was calculated from the refractive index using the equation $\rho$=\textit{a}$\eta$-\textit{b}, where $\rho$ is the density of the CsCl (g ml\textsuperscript{-1}), $\eta$ is the measured refractive index, and \textit{a} and \textit{b} are coefficient values of 10.9276 and 13.593, respectively, for CsCl at 20{\textdegree}C \cite{9780408708036}. The collected DNA fractions were purified by repetitive washing with TE followed by centrifugation. Finally, 50$\mu$L TE was added to each fraction, incubated on filter for 5min at RT then resuspended DNA was pipetted off the filter into a new microfuge tube. For every gradient, 20 fractions were chosen for sequencing between the density range 1.67-1.75g/mL.