98%, Cambridge Isotopes). Twelve replicate bottles were established for all universally-labeled acetate-amended microcosms, allowing for three triplicate sets to be sacrificed for protein extraction at 24h, 144 hr, and 408 hr, and a single triplicate set for liquid sampling throughout for VFA analysis. Biomass was pelleted from 10 ml liquid samples via centrifugation (10,000xg) and stored at minus 20C until protein extraction. Protein from cell pellet samples were extracted in an 8M urea solution, then reduced and alkylated. Proteins were then digested with trypsin and subsequently desalted using C18 solid phase extraction. MS analysis was performed using 0.1 ug/ul of peptide solution injected into a Q Exactive HF X mass spectrometer (Thermo Scientific). Data processing Mass spectrometry (MS) data for each biological replicate at all time points (n=18) were analyzed using an implementation of OpenMS implemented in KNIME. Briefly, MS/MS spectra were searched using the MS-GF+ tool against a protein database consisting of all ORFs from the de-replicated set of MAGs, concatenated with reversed (decoy) sequences of all protein entries. Peptide spectra matches (PSMs) were filtered at a 5% false discovery rate (FDR) with Percolator. ]]>