Activity-targeted metaproteomics uncovers rare syntrophic bacteria central to anaerobic community metabolism.

Syntrophic microbial consortia can contribute substantially to the activity of anoxic ecosystems but are often too rare to allow the study of their in situ physiologies using traditional molecular methods. Here we combined bioorthogonal non-canonical amino acid tagging (BONCAT), stable isotope probing and metaproteomics to improve the recovery of proteins from active members and track isotope incorporation in an anaerobic digestion community. Click-chemistry-enabled cell sorting and direct protein pull-down coupled to metaproteomics improved recovery of isotopically labelled proteins during anaerobic acetate oxidation. BONCAT-enabled protein profiles revealed elevated activity and labelling of a rare and so-far uncharacterized syntrophic bacterium belonging to the family Natronincolaceae that expressed a previously hypothesized oxidative glycine pathway for syntrophic acetate oxidation. Stable-isotope-probing-informed metabolic modelling predicted that this organism accounted for a majority of acetate flux, suggesting that the oxidative glycine pathway is an important route for anaerobic carbon transformation and is probably central to community metabolism in natural and engineered ecosystems.