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DNA was extracted using a modified Griffiths procotol \cite{Griffiths_2000}. To prepare nucleic acid extracts for isopycnic centrifugation as 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 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 each time point. 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 ultracentrifuged  for 66 h at 55,000 rpm and room temperature (RT). Fractions of $\sim$100 $\mu$L were collected from below by displacing the DNA-CsCl-gradient buffer solution in the centrifugation tube with water using a syringe pump at a flow rate of x $\mu$L s\textsuperscript{-1} \cite{Manefield_2002} into Acroprep\textsuperscript{TM} 96 filter plate (part no. 5035, Pall Life Sciences). The refractive index of each fraction was measured using a Reichart AR200 digital refractometer modified as previously described \cite{Buckley_2007} to measure a volume of 5$\mu$L. Then buoyant density was calculated from the refractive index as previously described \cite{Buckley_2007} (see also suppl methods). The collected DNA fractions were purified by repetitive washing of Acroprep filter wells with TE. Finally, 50$\mu$L TE was added to each fraction 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. Barcoded 454 primers were designed using 454-specific adapter B, 10bp barcodes \cite{Hamady_2008}, a 2bp linker (put those bp in here), and 806R primer for reverse primer (BA806R); and 454-specific adapter A, a 2bp linker, and 515F primer for forward primer (BA515F). Each fraction was PCR amplified using 0.25$\mu$L 5U/$\mu$l AmpliTaq Gold (N8080243, Life Technologies, Grand Island, NY), 2.5$\mu$L 10X Buffer II (100 mM Tris-HCl, pH 8.3, 500 mM KCl), 2.5$\mu$L 25mM MgCl_{2}, 4$\mu$L 5mM dNTP, 1.25$\mu$L 10mg/mL BSA, 0.5$\mu$L 10$\mu$M BA515F, 1$\mu$L 5$\mu$M BA806R,3$\mu$L H_{2}O, 10$\mu$L 1:30 DNA template) in triplicate. Samples were normalized either using Pico green quantification and manual calculation or by SequalPrep\textsuperscript{TM} normalization plates (A10510, Invitrogen, Carlsbad, CA), then pooled in equimolar concentrations. Pooled DNA was gel extracted from a 1\% agarose gel using Wizard SV gel and PCR clean-up system (A9281, Promega, Madison, WI) per manufacturer's protocol. Amplicons were sequenced on Roche 454 FLX system using titanium chemistry at Selah Genomics (formerly EnGenCore, Columbia, SC)